MBIO 2710 / Topic 8a: Carbohydrate Synthesis - Glyoxylate Cycle

> Why can’t the TCA cycle provide organic carbon for biosynthesis?

> How do plants get around it?

> What’s the overall path to get to this, starting from lipids?

> TCA cycle fully oxidizes AcCoA → 2 CO₂ lost → no net organic carbon gained.

> Divert AcCoA into glyoxylate cycle (bypasses CO₂‑releasing steps).

> Overall: lipids → β‑oxidation → acetyl‑CoA → glyoxylate cycle → organic carbon for roots & stems.

In the plant’s growth, when is the glyoxylate shunt generally used, and when does it stop using it? State reasons for both start and stop.

> Used when seedlings w/o stems and roots b/c limited CO₂ exposure.

> Stops when grown and exposed to CO₂ → switches to TCA cycle instead of glyoxylate shunt.

Why do cells polymerize glucose instead of storing it as monomers?

> Free Glc = many small solute particles → draws in H₂O → risk of cell bursting.

> Solution: polymerize into glycogen/starch → one large molecule instead of thousands of small ones.

> Result: each particle has a tiny osmotic effect.

How is glycogen structured?

Main chain: Glc units linked by α‑1,4 glycosidic bonds.

Branches: every ~10 residues, branch formed by α‑1,6 glycosidic bonds.

Why must glycogen phosphorylase (GPase) and glycogen synthase (GSase) not be active at the same time?

> GPase breaks glycogen → G1P

> GSase: consumes G1P to make glycogen

> Net waste of UTP and energy w/ no productive outcome.

Regulation ensures catabolism and anabolism never run together.

How does insulin regulate glycogen synthesis, and where is it produced?

> Pancreas has endocrine & exocrine parts.

> Endocrine has islets of Langerhans (clusters of hormone‑secreting cells) = one of them are β‑cells.

> β‑cells secrete insulin when blood Glc high.

> Insulin tells cells to take up Glc and store it as glycogen/fat → promotes GSase activity.

How does glucagon regulate glycogen breakdown?

> Islets of Langerhans = α‑cells = secrete glucagon when blood Glc low.

> Glucagon tells cells to break down glycogen, fats, and protein, and run gluconeogenesis.

Where is epinephrine secreted from and what does it rely on?

> Secreted from adrenal medulla = part of adrenal glands = on top of kidneys.

> Heavily relies on small glycogen stores for fight-or-flight response (short-term compared to glucagon).

How does epinepherine regulate glycogen breakdown?

> Epi binds to β‑adrenergic receptor → activates adenylate cyclase.

> Adenylate cyclase (AC) uses ATP → cAMP.

> cAMP binds to protein kinase (PK).

> PK phosphorylates & activates subcellular components.

How do AC and PDE control cAMP levels?

> When AC on → cAMP produced faster than PDE can bdown → cAMP levels rise.

> When AC off → PDE still working slowly in the background → cAMP levels drop.