Question 1

Cofactor identity — Which metal ion is specifically coordinated at the center of vitamin B12 (cobalamin)?

Accepted Answer

Introduction / Context: Vitamin B12 (cobalamin) is defined structurally by a corrin ring that coordinates a central metal ion. Identifying this metal explains both its name and its unique redox chemistry in enzymatic reactions. Given Data / Assumptions: The corrin macrocycle binds a single transition metal ion. Cobalamin participates in redox and radical reactions. Naming often reflects the metal (e.g., cobalamin, hemoglobin's heme iron). Concept / Approach: The center of cobalamin contains cobalt in variable oxidation states (Co(III), Co(II), Co(I)). The cobalt–carbon bond in adenosylcobalamin is among the few biologically relevant organometallic bonds, enabling rearrangements and methyl transfers. Step-by-Step Solution: Identify vitamin structure: corrin ring + central metal.Recall the name “cobalamin” derives from “cobalt.”Select cobalt as the specific metal ion. Verification / Alternative check: Spectroscopic and crystallographic data show cobalt at the core with axial ligands (5,6-dimethylbenzimidazole below and variable upper ligands above). Why Other Options Are Wrong: Copper, zinc, iron are key in other metalloproteins but not in the corrin ring of B12. Common Pitfalls: Confusing B12 with heme (iron) or zinc fingers; the defining metal of cobalamin is cobalt. Final Answer: Cobalt

Question 2

Vitamin classification — Vitamin C (ascorbic acid) is best classified as which type of vitamin based on solubility?

Accepted Answer

Introduction / Context: Vitamins are often grouped by solubility because it influences absorption, storage, toxicity, and deficiency risk. Vitamin C is a classic example of a water-soluble vitamin with distinct physiological roles. Given Data / Assumptions: Ascorbic acid is hydrophilic and dissolves readily in aqueous solutions. Water-soluble vitamins are not stored in large quantities in the body. Fat-soluble vitamins (A, D, E, K) require lipid absorption pathways. Concept / Approach: Vitamin C acts as a reducing agent (antioxidant) and cofactor for enzymes such as prolyl and lysyl hydroxylases in collagen synthesis. Its water solubility means excess is excreted in urine and daily intake is important. Therefore, it is categorized as water-soluble. Step-by-Step Solution: Identify vitamin: ascorbic acid (Vitamin C).Check solubility class: hydrophilic → water soluble.Differentiate from fat-soluble group (A, D, E, K).Select “Water soluble.” Verification / Alternative check: Nutrition guidelines list Vitamin C with B-complex as water-soluble vitamins. Why Other Options Are Wrong: Fat soluble or “fat and water soluble” do not describe ascorbic acid. “None” is incorrect because a clear category exists. Common Pitfalls: Assuming antioxidant role implies fat solubility; many antioxidants, including Vitamin C, are water soluble. Final Answer: Water soluble

Question 3

Endocrine physiology — What is the principal metabolic effect of insulin on blood glucose and hepatic glycogen?

Accepted Answer

Introduction / Context: Insulin is a peptide hormone from pancreatic β-cells that coordinates fed-state metabolism. Its primary clinical relevance is the reduction of blood glucose concentrations after a carbohydrate-containing meal. Given Data / Assumptions: Insulin promotes glucose uptake in insulin-sensitive tissues (muscle, adipose). Insulin stimulates hepatic glycogenesis and inhibits gluconeogenesis. Hyperglycemia follows insulin deficiency or resistance. Concept / Approach: Insulin lowers plasma glucose by increasing GLUT4-mediated uptake in muscle and adipose tissue, enhancing glycolysis and glycogenesis, and suppressing hepatic glucose output. Therefore, the most direct and overarching description of insulin’s “main function” is to decrease blood glucose. It also increases liver glycogen, but the systemic hallmark is the fall in blood sugar. Step-by-Step Solution: Insulin binding → receptor tyrosine kinase activation.Signal transduction → increased GLUT4 translocation and glycogen synthase activation.Outcomes: more glucose uptake and storage → reduced blood glucose.Net systemic effect: decreased blood sugar post-prandially. Verification / Alternative check: Clinical practice monitors insulin effect via blood glucose measurements; insulin therapy aims to lower hyperglycemia in diabetes mellitus. Why Other Options Are Wrong: Increase glycogen in liver occurs, but the question asks for the main effect; decreased blood sugar is broader and primary. Decrease glycogen in liver is the opposite of insulin’s anabolic action. Increase blood sugar describes glucagon, epinephrine, and cortisol effects, not insulin. Common Pitfalls: Equating a single tissue effect (hepatic glycogen) with the systemic role; prioritize the whole-body effect on blood glucose. Final Answer: Decrease blood sugar

Vitamins and Coenzymes Questions