Carbohydrate biochemistry — glycogen structure: Which specific glycosidic linkages are present in glycogen (the animal storage polysaccharide), considering its linear chains and branch points?

Introduction / Context:
Glycogen is the principal storage form of glucose in animals and many microbes. Understanding its covalent architecture is essential for explaining rapid mobilization and compact storage of glucose. This question tests recognition of the exact glycosidic bonds that construct glycogen’s backbone and branches.

Given Data / Assumptions:

• Glycogen is a highly branched homopolymer of glucose.
• Two types of glycosidic linkages are possible in branched glucans: those that extend the chain and those that create branch points.
• We focus on the anomeric configuration (α or β) and the carbon positions involved (1→4, 1→6).

Concept / Approach:
Linear elongation in glycogen occurs by forming α-1,4 glycosidic bonds between glucose units. Branches are introduced by the branching enzyme, which creates α-1,6 linkages at defined intervals, enabling multiple nonreducing ends for rapid synthesis and breakdown.

Step-by-Step Solution:
Identify the main-chain linkage: α-1,4 between C1 of one glucose and C4 of the next. Identify the branch linkage: α-1,6 at branch points connecting C1 of the branching glucose to C6 of the acceptor residue. Exclude β linkages, which are characteristic of cellulose (β-1,4) and not present in glycogen. Conclude that glycogen contains both α-1,4 (chain) and α-1,6 (branch) linkages.

Verification / Alternative check:
Textbook diagrams and enzymology support: glycogen synthase elongates α-1,4; branching enzyme (amylo-α(1→4)→α(1→6)-transglucosidase) creates α-1,6 branches roughly every 8–12 residues.

Why Other Options Are Wrong:
Only α-1,4 (option a) lacks branching; only α-1,6 (option b) cannot form chains; mixed α/β (option d) or β linkages (option e) are not found in glycogen.

Common Pitfalls:
Confusing glycogen with cellulose (β-1,4) or amylopectin (similar linkages but different branching frequency).

Final Answer:
α-1,4 and α-1,6 linkages.