Enzymes 3

Question 1

Why must enzymes be regulated?

Answer 1

To increase or decrease their activity in order to maintain metabolic effectiveness

Question 2

Describe enzyme regulation

Answer 2

The slowest ('Rate-limiting') step of a metabolic pathway is the most efficient control point
- longer divergent pathways are controlled at the steps after forks

Regulatory mechanisms include:

• protein-protein interactions
• phosphorylation (and other covalent modifications)
• competitive inhibition and allosteric activation/inhibition
• proteolytic cleavage

Question 3

Describe allosteric regulation of enzymes (give some e.g.)

Answer 3

Many key enzymes in metabolic fuel oxidation pathways are regulated

e. g.
- Phosphofructokinase (glycolysis)
- Isocitrate dehydrogenase (TCA cycle)

Regulation is often by ADP or AMP (allosteric activators)

e. g.
- muscle glycogen phosphorylase degrades glycogen to glucose-1p
- AMP is an allosteric activator (levels increase as ATP is used for muscle contraction)
- Same enzyme also regulated by covalent modification

Question 4

Name some covalent modifications of enzymes

Answer 4

Covalent modification cam also lead to conformational changes
- Phosphorylation

Question 5

Describe phosphorylation of an enzyme,e

Answer 5

Addition of phosphate to

• Serine (S)
• Threonine (t)
• Tyrosine (Y)

(PO4)3- is a bulky and negatively charged group

• it interacts strongly with nearby residues
• It alters ionic interactions and H bonding

It is mediated by protein kinases

• Add phosphates using ATP
• and phosphates (remove by hydrolysis)

e.g. Protein Kinase A
(PKA)

Question 6

Describe the effects of phosphorylation on enzymes

Answer 6

Phosphorylated enzymes may be more/less active than their unphosphorylated forms e.g.

• phosphorylation of glycogen synthase DECREASES it activity
• glycogen phosphorylase activity is INCREASED baby phosphorylation

Question 7

Describe some other less common covalent modifications (and effects on proteins)

Answer 7

Addition/removal of

• acetyl
• ADP-ribose
• lipid moieties

These can affect the ability of the enzyme,e to interact with other proteins/and may change its localisation

Question 8

Name some protein-protein interactions

Answer 8

Interactions between proteins can change their conformation - affecting the active site

• Calcium-Calmodulin family
• Trimeric G proteins

Question 9

Describe proteolytic cleavage

Answer 9

Enzymes can be irreversibly activated (e.g. clotting cascade) or inactivated by proteolytic enzymes
- over a longer timescale, degradation of enzymes by intracellular proteases in lysosomes or proteosomes determines the cell’s enzymatic activity

Question 10

Describe how enzyme activity can be controlled at the point of synthesis

Answer 10

Induction or repression leads to an alteration in the total population of active sites
- generally such control is limited to those enzymes required at specific points in development, or under particular physiological conditions