Protein control 5: (leyland) regulation of protein function

Question 1

Why are proteins regulated?

Answer 1

Maintenance of cell homeostasis

Responsiveness to the environment

Efficiency

Question 2

Major ways enzyme activity can be regulated?

Answer 2

Changes in subtrate conc.

Binding of small effector molecules

Phosphorylation

Binding of regulatory proteins

Proteolytic activation

Controlling amount of enzyme present

Question 3

Order fastest to slowest regulatory methods:

Binding of small effector molecules
Covalent modification
Change in substrate concentration
Controlling the amount of enzyme present

Answer 3

Change in substrate concentration
Binding of small effector molecules
Covalent modification
Controlling the amount of enzyme present

Question 4

What are isozymes? How are they formed? Properties?

Answer 4

Enzymes that catalyse the same reaction but have a different amino acid sequence

• can be encoded by different genes, or derived by splicing from one gene
• have different kinetic/regulatory properties

Question 5

Roles of isozymes?

Answer 5

Different metabolic uses in different organs

Different locations and metabolic roles in the same cell

Different roles at different stages in development

Different responses to allosteric regulators

Question 6

Examples of lactate dehydrogenase isozymes? Differences in Properties?

Answer 6

Lactate dehydrogenase converts lactate to pyruvate + vice versa - 5 different isozymes
- each contain four polypeptides composed of two different types of protein

Main forms H4 -> found in heart
M4 -> found in muscle

(in between) e.g. HM3, H2M2 all found in different amounts in different tissues

Different isozymes have different km
H4 -> low Km for lactate (favours lactate reduction)
M4 -> low Km for pyruvate (favours pyruvate reduction)

Question 7

Define allosteric regulation

Answer 7

Regulation mediated by interactions of a modulator at a regulatory site away from the active/binding site

Question 8

Whats unusual about the behaviour of allosterically regulated enzyme kinetics

Answer 8

They do not obey Michaelis-Menten kinetics
-> sigmoidal instead of exponential graph shape

-> suggests affinity for substrate is changing over time