Question 1

Bioenergetics and redox states Which pair represents reduced coenzymes as they participate in cellular metabolism and electron transport?

Accepted Answer

Introduction / Context: Many metabolic questions test your ability to recognize oxidized versus reduced forms of electron carriers. Reduced coenzymes donate electrons to the respiratory chain or to biosynthetic reactions. Given Data / Assumptions: NADH is the reduced form of NAD+. FADH2 is the reduced form of FAD. ATP/GTP are phosphoryl transfer molecules, not redox pairs. CoA carries acyl groups; ubiquinone/ubiquinol interconvert but only ubiquinol is the reduced form. Concept / Approach: Identify reduced species: the suffixes “H” or “H2” indicate additional hydride/electrons. These carry reducing equivalents to the electron transport chain, enabling proton pumping and ATP synthesis. Step-by-Step Solution: Recall redox pairs: NAD+/NADH and FAD/FADH2.Select the option listing both reduced forms.Exclude options listing oxidized forms or unrelated cofactors. Verification / Alternative check: Stoichiometric coupling in oxidative phosphorylation: oxidation of NADH and FADH2 correlates with proton pumping and ATP generation, confirming their role as reduced carriers. Why Other Options Are Wrong: NAD+ and FAD: oxidized, not reduced.ATP and GTP: energy currency via phosphate transfer, not electron carriers.CoA and ubiquinone: CoA is acyl-carrier; ubiquinone is the oxidized form (reduced form is ubiquinol).FMN/Cytochrome c option as stated refers to oxidized states or mixed roles, not both reduced carriers. Common Pitfalls: Equating high-energy phosphates with redox carriers; overlooking that names ending in “-one” are often oxidized (e.g., ubiquinone). Final Answer: NADH and FADH2.

Question 2

Structure of vitamin K1 (phylloquinone) Vitamin K1 was first isolated from alfalfa and contains a phytyl side chain composed of how many isoprene units?

Accepted Answer

Introduction / Context: Vitamin K1 (phylloquinone) is a naphthoquinone with a characteristic phytyl side chain. Recognizing its structural features helps in understanding its lipid solubility and biological handling in membranes. Given Data / Assumptions: The phytyl group is derived from isoprenoid units (C5 each). Phylloquinone has a C20 phytyl side chain. Counting isoprene units connects structure to biosynthetic origin (mevalonate/isoprenoid pathway). Concept / Approach: Convert chain length to isoprene count: C20 corresponds to 4 × C5 units. Therefore, the phytyl side chain contains four isoprene units. Step-by-Step Solution: Note: phytyl length ≈ C20.Isoprene unit = C5.Calculate units: C20 / C5 = 4 units.Select “Four isoprene units.” Verification / Alternative check: Chemical depictions of phylloquinone show a long saturated isoprenoid tail consistent with four isoprenoid repeats, explaining its hydrophobicity and membrane localization. Why Other Options Are Wrong: Three or five or six or nine units do not match the C20 phytyl chain of vitamin K1. Common Pitfalls: Confusing vitamin K1 (phylloquinone) with vitamin K2 (menaquinones), which have multiple isoprenoid units that vary by species and length. Final Answer: Four isoprene units.

Question 3

Biotin biochemistry Biotin occurs mainly in combined forms bound to proteins through which specific linkage (often called biocytin when released)? Choose the most accurate biochemical description.

Accepted Answer

Introduction / Context: Biotin functions as a CO2 carrier in carboxylation reactions and is covalently attached to carboxylase enzymes. Understanding how biotin is linked to proteins is vital for biochemistry and nutrition. Given Data / Assumptions: Biotin is attached via an amide bond to the ε-amino group of a lysine residue in apocarboxylase proteins. The biotin–lysine conjugate released during proteolysis is called biocytin (ε-N-biotinyl-L-lysine). Lipoic acid forms a different linkage (lipoyl-lysine) in dehydrogenase complexes. Concept / Approach: Identify the precise covalent linkage: an amide bond between biotin’s carboxylate and lysine’s ε-amino group, producing ε-N-biotinyl-L-lysine. Step-by-Step Solution: Recognize that biotinylation targets lysine side chains via an amide bond.Recall the name “biocytin” for ε-N-biotinyl-L-lysine.Select the option that explicitly states ε-N-biotinyl-L-lysine. Verification / Alternative check: Biotinidase deficiency causes impaired recycling of biocytin, underscoring that biotin normally exists as ε-N-biotinyl-L-lysine in proteins. Why Other Options Are Wrong: Generic ε-N-lysine moiety lacks the essential biotinyl group.ε-S-lysine is not a standard biochemical linkage.Lipoyl-lysine involves lipoic acid, not biotin.Disulfide to cysteine is characteristic of other modifications, not biotin. Common Pitfalls: Confusing biotin–lysine with lipoyl–lysine; overlooking the amide bond nature (ε-N- linkage) rather than sulfur-based linkage. Final Answer: ε-N-biotinyl-L-lysine.

Question 4

Vitamin A (retinol) deficiency: earliest clinical sign In human nutrition and ophthalmology, what is typically the earliest recognizable sign of retinol (vitamin A) deficiency?

Accepted Answer

Introduction / Context: Vitamin A is essential for vision, epithelial integrity, and immune function. The earliest ocular manifestation of deficiency is frequently tested because early recognition prevents permanent damage. Given Data / Assumptions: Retinal (derived from retinol) is a component of rhodopsin in rod cells, mediating scotopic (low-light) vision. Progression of deficiency signs: impaired dark adaptation (night blindness) → conjunctival xerosis → Bitot’s spots → corneal xerosis/keratomalacia (xerophthalmia). Skin keratinization occurs later or in more severe deficiency. Concept / Approach: Identify the first functional deficit: decreased sensitivity to dim light (nyctalopia) due to inadequate 11-cis-retinal availability in photoreceptors. Step-by-Step Solution: Relate vitamin A to rhodopsin regeneration in rods.Understand that reduced rhodopsin impairs dark adaptation first.Select “Night blindness” as the earliest sign. Verification / Alternative check: Clinical assessments of dark adaptation and patient history (difficulty driving at night) commonly reveal early deficiency; supplementation improves symptoms if instituted promptly. Why Other Options Are Wrong: Keratinization and xerophthalmia represent later epithelial changes; they are not typically the earliest sign.“None of these” contradicts well-established clinical progression.Photophobia with normal dark adaptation does not match retinol deficiency physiology. Common Pitfalls: Confusing xerophthalmia (a later, structural change) with the earliest functional deficit of night blindness. Final Answer: Night blindness.

Question 5

Vitamin A deficiency outcomes Which of the following is a well-known clinical consequence of vitamin A deficiency in humans?

Accepted Answer

Introduction / Context: Vitamin A deficiency is a major cause of preventable childhood blindness worldwide. Recognizing the hallmark outcome helps prioritize nutritional interventions and public health measures. Given Data / Assumptions: Vitamin A is required for vision (rhodopsin cycle) and epithelial health in the eye. Severe deficiency leads to xerophthalmia, corneal ulceration, keratomalacia, and ultimately blindness. Other listed conditions are associated with different vitamins or etiologies (e.g., vitamin K for clotting). Concept / Approach: Connect vitamin A’s role in ocular physiology to clinical outcomes. While early deficiency causes night blindness, advanced disease can culminate in irreversible blindness if untreated. Step-by-Step Solution: Identify the vitamin: A (retinol/retinal/retinoic acid).Recall the progression: night blindness → xerosis → Bitot’s spots → corneal involvement → blindness.Select “Blindness” as a recognized consequence. Verification / Alternative check: WHO public health data highlight vitamin A supplementation as a key strategy to prevent blindness and reduce childhood mortality in deficiency-prone regions. Why Other Options Are Wrong: Osteoporosis: more closely linked to calcium/vitamin D imbalance (although hypervitaminosis A can affect bone, that is not deficiency).Impaired taste perception: nonspecific and not a defining vitamin A deficiency hallmark.Impaired blood clotting: classically due to vitamin K deficiency.Gingival bleeding: characteristic of vitamin C deficiency (scurvy). Common Pitfalls: Mixing vitamin deficiency manifestations (A vs C vs K); assuming all fat-soluble vitamin deficiencies share identical outcomes. Final Answer: Blindness.

Question 6

Provitamin A identification Which of the following is a dietary precursor (provitamin) that the human body can convert into vitamin A (retinol)?

Accepted Answer

Introduction / Context: Provitamin A carotenoids, particularly beta-carotene, are important contributors to vitamin A nutrition from plant sources. Recognizing which nutrients are precursors versus active vitamins is essential for dietary planning. Given Data / Assumptions: Beta-carotene can be cleaved enzymatically to retinal, which is reduced/oxidized to retinol/retinoic acid. Cobalamin (B12), pyridoxine (B6), thiamine (B1), and ascorbic acid (vitamin C) are not precursors of vitamin A. Carotenoids are abundant in orange-colored and dark green leafy vegetables. Concept / Approach: Select the carotenoid capable of yielding retinoids upon intestinal and hepatic metabolism. Beta-carotene is the canonical example with provitamin A activity. Step-by-Step Solution: Identify the compound class with provitamin A activity: carotenoids.Match the specific carotenoid: beta-carotene.Exclude unrelated water-soluble vitamins. Verification / Alternative check: Nutrient databases assign retinol activity equivalents (RAE) to beta-carotene intake, reflecting conversion efficiency to vitamin A. Why Other Options Are Wrong: Cobalamin, pyridoxine, thiamine, ascorbic acid are distinct vitamins with unrelated functions, not vitamin A precursors. Common Pitfalls: Assuming all carotenoids are equal; note that only specific carotenoids (e.g., beta-carotene, alpha-carotene, beta-cryptoxanthin) have provitamin A activity, whereas lutein and zeaxanthin do not. Final Answer: Beta-Carotene.

Question 7

In biochemistry, lipoic acid (lipoamide) functions as a swinging-arm cofactor in multienzyme dehydrogenase complexes. It serves as a cofactor for which enzyme system(s)?

Accepted Answer

Introduction / Context: Lipoic acid (often attached to proteins as lipoamide) is a classic cofactor in oxidative decarboxylation reactions of central metabolism. Knowing which enzyme complexes require lipoic acid helps connect carbohydrate oxidation to the citric acid cycle. Given Data / Assumptions: Lipoic acid participates in acyl-group transfer and redox chemistry. Multienzyme complexes use swinging prosthetic arms to channel intermediates efficiently. We are comparing dehydrogenase complexes in core metabolism. Concept / Approach: Lipoic acid is covalently linked to a lysine on the E2 subunit (dihydrolipoyl transacetylase or transsuccinylase) of 2-oxoacid dehydrogenase complexes. It accepts an acyl group from the decarboxylated 2-oxoacid and is then reoxidized, coupling acyl transfer to NADH production. Step-by-Step Solution: Identify complexes using lipoamide: pyruvate dehydrogenase (PDH) and α-ketoglutarate dehydrogenase (OGDH).Recall shared architecture: E1 (decarboxylase, TPP-dependent), E2 (lipoamide arm), E3 (dihydrolipoamide dehydrogenase, FAD-dependent).Conclude that both PDH and OGDH require lipoic acid as an essential cofactor.Di-hydroorotate dehydrogenase is a different enzyme (pyrimidine biosynthesis) and does not use lipoic acid. Verification / Alternative check: Standard biochemistry texts describe identical lipoamide-centered mechanisms for PDH and OGDH, and similarly for branched-chain α-ketoacid dehydrogenase. Why Other Options Are Wrong: Pyruvic dehydrogenase or α-ketoglutaric dehydrogenase alone are incomplete; both use lipoamide. Di-hydroorotate dehydrogenase uses flavins and quinones, not lipoic acid. Common Pitfalls: Confusing lipoic acid (E2 cofactor) with thiamine pyrophosphate (E1 cofactor) or FAD/NAD+ (E3 cofactor). Each plays a distinct role. Final Answer: both (a) and (b)

Question 8

Folate metabolism: The enzyme folate reductase first reduces folic acid to which specific intermediate before further reduction to the fully active form?

Accepted Answer

Introduction / Context: Folate coenzymes carry one-carbon units in amino acid and nucleotide biosynthesis. Understanding the reduction steps from folic acid to active tetrahydrofolate underlies mechanisms of action for drugs like methotrexate and trimethoprim. Given Data / Assumptions: Folic acid is the oxidized vitamin form. Dihydrofolate reductase (folate reductase) catalyzes sequential reductions using NADPH. Active coenzyme form is tetrahydrofolate (THF). Concept / Approach: The pathway is folic acid → dihydrofolate (DHF) → tetrahydrofolate (THF). The first reduction yields DHF; the second reduction yields THF, which accepts and donates one-carbon units at various oxidation states. Step-by-Step Solution: Identify enzyme: dihydrofolate reductase (often termed folate reductase).First product after reduction of folic acid: dihydrofolic acid (DHF).Second reduction: DHF → THF (tetrahydrofolic acid), the biologically active cofactor. Verification / Alternative check: Antifolate drugs block DHFR, causing DHF accumulation and THF depletion, validating the order of intermediates. Why Other Options Are Wrong: Hydrofolic/trihydrofolic acids are not standard biochemical entities. Tetrahydrofolic acid is the final product after a second reduction, not the first intermediate. Common Pitfalls: Assuming folic acid is directly active; it must be reduced first to DHF and then to THF. Final Answer: dihydrofolic acid

Question 9

Clinical biochemistry: A deficiency of vitamin K leads most directly to decreased levels of which coagulation factor in plasma?

Accepted Answer

Introduction / Context: Vitamin K is essential for hepatic γ-carboxylation of specific glutamate residues on several clotting factors, enabling calcium binding and normal coagulation. Identifying the most directly affected factor is a common exam point. Given Data / Assumptions: Vitamin K–dependent factors include II (prothrombin), VII, IX, and X, plus proteins C and S. Carboxylation occurs in the liver, enabling interaction with phospholipid surfaces. Deficiency leads to prolonged bleeding and abnormal coagulation tests. Concept / Approach: Prothrombin (factor II) is the direct zymogen precursor of thrombin. Its synthesis and activation depend on vitamin K–mediated γ-carboxylation. Without vitamin K, prothrombin levels and activity decrease, impairing thrombin generation and fibrin formation. Step-by-Step Solution: Vitamin K deficiency → impaired γ-carboxylation in factors II, VII, IX, X.Among options, prothrombin (II) is the vitamin K–dependent zymogen listed.Therefore, decreased prothrombin is the most direct effect. Verification / Alternative check: Correction of coagulopathy with vitamin K administration in deficiency states demonstrates the dependence of prothrombin activation on vitamin K. Why Other Options Are Wrong: Thrombin and fibrin are downstream products; their reduction is secondary. Fibrinogen (factor I) synthesis is not vitamin K–dependent. Common Pitfalls: Confusing synthesis (zymogens) with activated enzymes; vitamin K acts earlier at the zymogen modification stage. Final Answer: prothrombin

Question 10

Nutrition and deficiency diseases: A lack of niacin (vitamin B3) most classically causes which condition characterized by dermatitis, diarrhea, and dementia?

Accepted Answer

Introduction / Context: Niacin (vitamin B3) is a precursor for NAD+ and NADP+, essential cofactors in redox metabolism. Deficiency leads to a characteristic clinical triad and historical public health relevance. Given Data / Assumptions: Niacin deficiency arises from poor diet, alcoholism, malabsorption, or Hartnup disease. Classical triad: dermatitis, diarrhea, dementia; severe cases may be fatal. Contrast with other vitamin deficiencies to avoid confusion. Concept / Approach: NAD+/NADP+ participate in glycolysis, TCA cycle, fatty acid metabolism, and biosynthesis. Inadequate niacin disrupts cellular energy and repair, especially in high-turnover tissues (skin, gut, brain), explaining the pellagra triad. Step-by-Step Solution: Identify the deficiency: niacin (B3) or tryptophan (a precursor).Relate systemic manifestations to impaired NAD(H)/NADP(H)-dependent reactions.Match with condition: pellagra is the hallmark disease of niacin deficiency. Verification / Alternative check: Supplementation with niacin or tryptophan improves symptoms; historical elimination of pellagra followed dietary fortification. Why Other Options Are Wrong: Scurvy: vitamin C deficiency. Cataract: multifactorial; not a specific vitamin B3 deficiency label. Anemia: may occur in other deficiencies (e.g., folate, B12, iron). Common Pitfalls: Confusing pellagra (B3) with beriberi (B1) or pellagra-like photosensitive rash in other metabolic disorders; always check the triad. Final Answer: pellagra

Question 11

History of vitamins: Who is credited with the discovery and isolation of vitamin C (ascorbic acid) from biological sources?

Accepted Answer

Introduction / Context: Vitamin C (ascorbic acid) is a key dietary antioxidant and cofactor in collagen biosynthesis. Knowing the scientists behind its discovery helps anchor historical progress in nutrition and biochemistry. Given Data / Assumptions: Timeframe: early 20th century biochemical discoveries. Isolation from adrenal glands and paprika led to ascorbic acid characterization. Different scientists popularized or expanded vitamin research later. Concept / Approach: Albert Szent-Györgyi isolated and identified ascorbic acid, earning the Nobel Prize in Physiology or Medicine (1937) for his work on biological combustion and vitamin C. Others in the options contributed to different fields. Step-by-Step Solution: Match scientist to contribution: Szent-Györgyi → discovery and isolation of vitamin C.Differentiate from Linus Pauling, who advocated high-dose vitamin C later, but did not discover it.Paul Berg: recombinant DNA pioneer; Kary Mullis (misspelled) invented PCR. Verification / Alternative check: Historical accounts and Nobel citations attribute discovery/isolation directly to Szent-Györgyi. Why Other Options Are Wrong: Paul Berg: DNA research, not vitamin C discovery. Linus Pauling: promotion of vitamin C; not its discoverer. Kary Mullis: PCR, unrelated to vitamin C discovery. Common Pitfalls: Equating advocacy or later research with original discovery; distinguish discovery from popularization. Final Answer: Albert Szent-Gyorgyi

Question 12

Digestive physiology: Which of the following is an example of a digestive hormone (not an enzyme) that regulates gastric function?

Accepted Answer

Introduction / Context: Digestion involves both enzymes that catalyze hydrolysis of food and hormones that regulate secretion and motility. Recognizing the difference is fundamental in physiology. Given Data / Assumptions: Lipase, pepsin, and amylase are hydrolytic enzymes. Gastrin is a peptide hormone released from G cells of the stomach. We are asked to pick the hormone among enzymes. Concept / Approach: Gastrin stimulates gastric acid secretion by parietal cells and promotes gastric mucosal growth and motility. Enzymes like pepsin (protease), lipase (fat hydrolysis), and amylase (starch digestion) are produced in zymogen or active forms and act directly on substrates. Step-by-Step Solution: Classify each option: enzymes vs hormone.Identify gastrin as the hormone controlling gastric physiology.Conclude gastrin is the correct choice. Verification / Alternative check: Clinical tests measuring serum gastrin (e.g., in Zollinger–Ellison syndrome) confirm its endocrine role distinct from digestive enzymes. Why Other Options Are Wrong: Lipase, pepsin, amylase: all are enzymes, not hormones. Common Pitfalls: Confusing naming conventions; many digestive factors are peptides, but only some are secreted as hormones that act on receptors to regulate function. Final Answer: gastrin

Question 13

Vitamin nomenclature: Which molecule is known as vitamin H and serves as a coenzyme for carboxylase reactions?

Accepted Answer

Introduction / Context: Vitamins often have multiple names. Vitamin H is the historical name for a coenzyme that transfers CO2 in carboxylation reactions critical for gluconeogenesis and fatty acid metabolism. Given Data / Assumptions: Biotin is covalently attached to carboxylases via a lysine (biocytin linkage). Key biotin-dependent enzymes: pyruvate carboxylase, acetyl-CoA carboxylase, propionyl-CoA carboxylase. Alternative molecules listed have different roles. Concept / Approach: Vitamin H = biotin. As a swinging-arm CO2 carrier, biotin shuttles activated CO2 between active sites, enabling anaplerotic and biosynthetic reactions. Step-by-Step Solution: Map old name to modern: vitamin H → biotin.Confirm function: coenzyme for ATP-dependent carboxylases.Select biotin as the correct answer. Verification / Alternative check: Biotin deficiency (e.g., raw egg white avidin binding) impairs carboxylase activity, aligning with vitamin H identity. Why Other Options Are Wrong: Carnitine transports acyl groups into mitochondria; not a vitamin in this context. Folic acid is vitamin B9, a one-carbon carrier of different chemistry. None of these: incorrect since biotin fits exactly. Common Pitfalls: Mixing up biotin (vitamin H) with pantothenic acid (B5) or folate (B9); each has distinct coenzymatic roles. Final Answer: Biotin

Question 14

Riboflavin (vitamin B2) behavior: The yellow pigment in egg yolk and milk becomes what on reduction with zinc in acid, and what upon re-oxidation?

Accepted Answer

Introduction / Context: Riboflavin (vitamin B2) forms flavin coenzymes (FMN, FAD) that are yellow due to their conjugated isoalloxazine ring. The color changes with oxidation state provide a simple chemical demonstration of redox behavior. Given Data / Assumptions: Reduction performed with zinc in acidic solution. Observation of visible color changes tied to electronic structure. Re-oxidation occurs on exposure to air or oxidants. Concept / Approach: Oxidized flavins are yellow; the fully reduced form (leucoflavin) is colorless. Thus, reduction bleaches the pigment; re-oxidation restores the yellow color by re-establishing the chromophore's conjugation. Step-by-Step Solution: Start: oxidized riboflavin → yellow.Reduce with Zn/H+: riboflavin → leucoflavin (colorless).Expose to oxygen: leucoflavin → oxidized riboflavin (yellow returns). Verification / Alternative check: Laboratory demonstrations and spectroscopic data show diminished absorbance in the visible range upon reduction, recovering on re-oxidation. Why Other Options Are Wrong: Claiming colorless on oxidation reverses the real chemistry. Suggestions of deeper color on reduction are contrary to the bleaching effect of forming leucoflavin. Common Pitfalls: Confusing semiquinone (blue/green radical) with fully reduced leucoflavin; complete reduction under the stated conditions yields colorless leuco species. Final Answer: colorless on reduction with Zn in acid and regained its color on re-oxidation

Question 15

Redox chemistry of vitamins: Ascorbic acid (vitamin C) primarily acts in cells as which type of agent?

Accepted Answer

Introduction / Context: Ascorbic acid is a water-soluble antioxidant. Its biochemical importance lies in its ability to donate electrons, protecting biomolecules and enabling specific enzyme chemistry. Given Data / Assumptions: Ascorbate cycles between reduced (ascorbate) and oxidized forms (dehydroascorbate). Hydroxylation reactions (e.g., collagen proline/lysine hydroxylases) require a reducing environment. Cellular antioxidants often operate in redox pairs. Concept / Approach: As a reductant, ascorbate donates electrons to neutralize reactive oxygen species and to regenerate metal centers in monooxygenases. It is then regenerated by cellular systems (e.g., glutathione). Step-by-Step Solution: Identify role: electron donor in enzymatic hydroxylations and antioxidant defense.Note redox couple: ascorbate ↔ dehydroascorbate.Conclude it primarily acts as a reducing agent. Verification / Alternative check: Clinical scurvy (vitamin C deficiency) reflects impaired collagen hydroxylation that requires reducing equivalents, confirming its reductant role. Why Other Options Are Wrong: Oxidizing agent: contrary to its main biological function. Both: while it forms an oxidized product, its physiological action is reducing. None: incorrect given its well-defined role. Common Pitfalls: Confusing the existence of an oxidized form with acting as an oxidant; the direction of electron flow in physiological reactions is the key. Final Answer: reducing agent

Question 16

Metalloflavoproteins: In at least one known iron-containing flavoprotein, the iron is present in which form?

Accepted Answer

Introduction / Context: Many electron-transfer proteins combine flavin cofactors (FMN/FAD) with iron centers. The nature of the iron (heme vs nonheme) influences redox properties and spectroscopic signatures. Given Data / Assumptions: Flavoproteins contain flavin prosthetic groups. Some also contain iron as iron–sulfur clusters (nonheme iron). Heme iron is coordinated within porphyrin rings; nonheme iron is not. Concept / Approach: Examples like NADH dehydrogenase (Complex I) and other dehydrogenases are flavoproteins that also house multiple iron–sulfur centers. These iron centers are nonheme; they participate in stepwise single-electron transfers complementary to flavin chemistry. Step-by-Step Solution: Identify iron type in representative flavoproteins: Fe–S clusters (e.g., [2Fe–2S], [4Fe–4S]).Classify as nonheme iron since no porphyrin is present.Conclude that the iron is present as a nonheme type in such metalloflavoproteins. Verification / Alternative check: EPR/Mössbauer and UV–Vis spectra distinguish Fe–S cluster signals from heme-type absorbance bands, confirming nonheme identity. Why Other Options Are Wrong: Heme-protein: not universal for flavoproteins; the cited example specifically features nonheme iron. Both: not applicable to the specific case asked. Flavin moiety: the iron is not part of the flavin ring. Common Pitfalls: Assuming all iron in electron transport is heme-based; many key carriers are nonheme Fe–S proteins. Final Answer: nonheme type

Question 17

Nutrition and food chemistry — Which specific compound present in raw egg whites binds biotin and can cause a syndrome resembling vitamin B (biotin) deficiency if consumed in large amounts without cooking?

Accepted Answer

Introduction / Context: Raw egg white contains several proteins with distinct biochemical effects. One of them, avidin, has a high affinity for the B-complex vitamin biotin. Understanding this interaction is important in nutrition and food safety because persistent, heavy intake of raw egg whites can induce signs mimicking a vitamin B (biotin) deficiency. Given Data / Assumptions: The question addresses a compound in raw egg white. Biotin is a water-soluble B vitamin (vitamin B7) required for carboxylase enzymes. Cooking can denature many egg white proteins. Concept / Approach: Avidin is a tetrameric glycoprotein in egg white that binds biotin extremely tightly (sub-nanomolar dissociation). When avidin is intact (as in raw egg whites), it sequesters dietary biotin in the gut, preventing absorption. Over time, in extreme dietary patterns, this can cause clinical signs of biotin deficiency, such as dermatitis, alopecia, and neurological symptoms. Heat from cooking denatures avidin, releasing biotin and preventing this effect. Step-by-Step Solution: Identify proteins present in egg white (e.g., ovalbumin, ovotransferrin, avidin).Recall which protein specifically interacts with a B vitamin: avidin binds biotin.Connect the biochemical binding to physiology: strong avidin–biotin binding prevents intestinal absorption of biotin.Conclude that the culprit protein causing a biotin-deficiency-like syndrome is avidin. Verification / Alternative check: Food science texts and clinical nutrition references consistently cite avidin–biotin binding as the mechanistic basis for the so-called “egg-white injury,” resolved by cooking, which denatures avidin. Why Other Options Are Wrong: Betabindin: not an established egg-white protein linked to biotin binding. Ovalbumin: major storage protein; does not tightly bind biotin. Albumin (serum albumin): not the specific egg white factor responsible for biotin sequestration. Common Pitfalls: Assuming any egg white protein causes the deficiency or overlooking the effect of cooking. Only avidin’s extreme affinity for biotin is responsible, and heat inactivation prevents the problem. Final Answer: Avidin

Question 18

Vitamin B12 (cobalamin) biosynthesis — Which domain of life is the sole source of de novo vitamin B12 synthesis?

Accepted Answer

Introduction / Context: Vitamin B12 (cobalamin) is a complex corrinoid cofactor essential for methionine synthase and methylmalonyl-CoA mutase in humans. Understanding its biosynthetic origin helps explain dietary requirements and why certain diets need supplementation. Given Data / Assumptions: B12 is required in trace amounts in humans. Its structure includes a cobalt ion within a corrin ring. Only certain organisms possess the full pathway to synthesize B12. Concept / Approach: De novo biosynthesis of cobalamin is carried out exclusively by micro-organisms (bacteria and some archaea). Plants and animals do not synthesize B12; animals obtain it indirectly by consuming foods harboring microbial B12 or via fortified sources. Therefore, “micro-organisms” is the correct answer. Step-by-Step Solution: Identify biosynthetic capability: only prokaryotes (and some archaea) possess complete cobalamin pathways.Recognize that animals and plants lack these genes and rely on diet or symbiosis.Conclude the source domain is micro-organisms. Verification / Alternative check: Microbial ecology and nutrition literature consistently attribute cobalamin production to bacteria/archaea, explaining B12 presence in ruminant products and fermented foods. Why Other Options Are Wrong: Fishes/Animals: they accumulate B12 but do not synthesize it de novo. Plants: generally lack B12 synthesis; plant foods need fortification to supply B12. Common Pitfalls: Confusing “present in animal foods” with “made by animals.” Animals are consumers, not producers, of B12. Final Answer: Micro-organisms

Question 19

Redox biochemistry — In what forms does lipoic acid (lipoamide cofactor) normally exist during catalytic cycling?

Accepted Answer

Introduction / Context: Lipoic acid (often as protein-bound lipoamide) is a swinging-arm redox cofactor in multienzyme complexes such as pyruvate dehydrogenase and α-ketoglutarate dehydrogenase. Appreciating its redox cycling explains how acyl-group transfer and electron transfer are coupled. Given Data / Assumptions: Lipoic acid participates in oxidative decarboxylation of α-ketoacids. It has a disulfide (S-S) that can be reduced to dithiol (SH, SH). Enzymatic cycles involve alternating oxidation states. Concept / Approach: During catalysis, lipoamide cycles between an oxidized disulfide and a reduced dithiol. The reduced form accepts acyl groups and transfers reducing equivalents to FAD/NAD+, regenerating the oxidized form for subsequent catalytic rounds. Therefore, it exists in both oxidized and reduced states in vivo. Step-by-Step Solution: Oxidized lipoamide (S-S) is attached to E2 of the dehydrogenase complex.Reduction to dihydrolipoamide (SH, SH) occurs during acyl transfer from E1.Re-oxidation via E3 transfers electrons to FAD and then to NAD+, closing the redox loop.Thus both oxidation states are essential and interconvert during catalysis. Verification / Alternative check: Spectroscopic and structural studies of pyruvate dehydrogenase confirm lipoamide’s redox swing between disulfide and dithiol states with concurrent electron flow. Why Other Options Are Wrong: Only oxidized or only reduced cannot support catalytic cycling. “None” contradicts established enzymology. Common Pitfalls: Forgetting that lipoamide is covalently tethered to the enzyme and serves both as an acyl carrier and redox carrier, requiring both forms. Final Answer: Oxidized and reduced form both

Question 20

Cobalamin redox chemistry — The NADH/flavoprotein system reduces a disulfide (S–S) to a dithiol (SH, SH) that then converts vitamin B12 from which state to which state?

Accepted Answer

Introduction / Context: Vitamin B12 (cobalamin) functions through multiple oxidation states of cobalt: Co(III), Co(II), and Co(I). In certain B12-dependent reactions, a cellular reducing system (NADH via a flavoprotein and a disulfide/dithiol protein) must generate the highly nucleophilic cob(I)alamin state. Given Data / Assumptions: NADH donates electrons through a flavoprotein. A specific protein disulfide is reduced to a dithiol (SH, SH). This reduced protein further reduces cobalamin. Concept / Approach: Cobalamin interconverts among Co(III) (oxidized), Co(II) (one-electron reduced), and Co(I) (two-electron reduced, supernucleophilic). The NADH/flavoprotein system ultimately yields cob(I)alamin. Thus, the physiological step relevant here is reduction of B12 from Co(II) to Co(I) mediated by a dithiol protein. Step-by-Step Solution: Start from B12 in the Co(II) (cob(II)alamin) state.NADH → flavoprotein → reduces a protein disulfide to a dithiol.The resulting dithiol donates electrons to B12.B12(Co2+) → B12(Co+) (cob(I)alamin), enabling catalysis (e.g., in methyltransferase chemistry). Verification / Alternative check: Biochemical studies of methionine synthase activation show requirement for cob(I)alamin regeneration via NADPH-dependent reductive systems, consistent with Co(II) → Co(I) conversion. Why Other Options Are Wrong: Co2+ → Co (neutral) is not the conventional notation/state used for cobalamin in biochemistry. Co → Co2+ or Co+ → Co2+ represent oxidation, not the required reduction to active Co(I). Common Pitfalls: Confusing cobalamin oxidation states and symbols; in biochemical notation Co(III) = Co3+, Co(II) = Co2+, Co(I) = Co+. Final Answer: B12(Co2+) to B12(Co+)

Vitamins and Coenzymes Questions