StC L3

What determines the direction of a metabolic pathway?

The direction depends on substrate and product concentrations and the energy changes (ΔG) of the reactions involved.

How is the speed (rate) of a metabolic pathway controlled?

By regulating enzyme activity through mechanisms like allosteric control, covalent modification (e.g., phosphorylation), enzyme concentration, and feedback inhibition.

Why are key metabolic steps often irreversible?

To provide control points that determine pathway direction and prevent futile cycling between synthesis and breakdown.

What role does compartmentalisation play in metabolism?

It separates metabolic processes into specific organelles (e.g., glycolysis in cytosol, TCA cycle in mitochondria), allowing independent control and preventing interference.

What happens when control of metabolic pathways goes awry?

Imbalance in metabolite levels, enzyme dysfunction, or loss of feedback control can lead to disease (e.g., diabetes, metabolic syndrome).

Give an example of failed metabolic regulation.

In diabetes mellitus, insulin signaling fails—cells can’t take up glucose effectively, leading to hyperglycaemia and widespread metabolic disruption.

Where is insulin produced?

In pancreatic β-cells of the islets of Langerhans.

What is preproinsulin?

The initial translated polypeptide that contains a signal sequence directing it to the ER.

What processing steps occur to make active insulin?

• Preproinsulin → signal peptide cleaved → Proinsulin (in ER)

• Disulphide bonds form between A and B chains.

• Proinsulin → cleaved by PC1/3, PC2, and Carboxypeptidase E → Insulin + C-peptide (in Golgi and secretory granules).

How is insulin stored and released?

Stored in secretory vesicles complexed with Zn²⁺ ions; released by regulated exocytosis when blood glucose rises.

Why is insulin processing compartmentalised?

To ensure only mature, active insulin is released; premature release of inactive proinsulin would impair blood glucose regulation.

What is diabetes mellitus?

A group of metabolic disorders characterised by chronic hyperglycaemia due to defects in insulin secretion, insulin action, or both.

How common is diabetes?

It affects over 10% of adults globally (hundreds of millions of people) and is one of the leading causes of death worldwide.

Why is diabetes considered severe?

It leads to serious complications including cardiovascular disease, neuropathy, retinopathy, nephropathy, and impaired wound healing.

Give an example of altered carbohydrate metabolism in another animal species.

Feline diabetes mellitus—cats often develop a type 2–like diabetes due to insulin resistance, obesity, and low physical activity.

Another example?of altered carbohydrate metabolism in another animal species.

Equine metabolic syndrome—horses show insulin resistance and fat accumulation, predisposing them to laminitis (painful hoof inflammation).

Why do these animal conditions matter?

They demonstrate how similar metabolic control mechanisms (insulin signaling and glucose uptake) operate across species and can fail under stress or poor diet.