Question

[Biochemistry]

Compare phosphorylation and proteolysis as mechanism for activating an enzyme.

Explain why a lipid hormone have intracellular receptors.

Answer 1

Enzyme activation by phosphorylation:

• The conformational change to an enzyme caused by the addition of one or more phosphate groups is known as enzyme activation by phosphorylation.
• It is a reversible reaction.
• Many metabolic enzyme present as pro enzyme, get converted into active form by this method.

Enzymes that get activated by phosphorylation are:

Enzymes of Carbohydrate Metabolism: Glycogen phosphorylase, phosphorylase kinase, gylcogen synthase, pyruvate kinase.

Enzymes of Lipid Metabolism: acetyl-CoA carboxylase, triacylglycerol lipase.

Enzyme activation by proteolysis:

• Many enzymes are activated by cleavage of one or a few specific peptide bonds, this type of enzyme activation called activation by proteolysis.
• Energy source, ATP, is not needed for activation.
• It is irreversible process.
• Specific proteolysis is a common means of activating enzymes.
• Blood clotting is its example, regulated by a cascade of proteolytic activations that lead to rapid response to injury.

​​- Lipid hormone have intracellular receptor because lipid hormone are able to diffuse from the cell membrane to inside of the cell. There they bind to the receptor and exert it's effect. Mostly , receptor binding stimulates transcription. The hormone-receptor complex thus functions as a transcription factor.