Download MBBS Biochemistry PPT 12 Enzymes Regulation Lecture Notes

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Enzyme Regulation, 2018

Basic Principles of Metabolic Regulation

Passive: Substrate Availability and

Compartmentation

Active: Regulation of Rate Limiting Enzyme
Substrate availability and Compartmentation

? Passive mechanism
? Limited capacity
? Anabolic and Catabolic pathways separated
E.g. Fatty acid synthesis & Fatty acid oxidation
? Specialized subcellular compartments (Hydrolases in Lysosomes)

Controlling Rate Limiting Enzyme

Regulation of

? Regulation of Enzyme

Enzyme quantity

synthesis

Regulation of

? Al osteric Regulation

Enzyme catalytic

activity

? Covalent Modification
Regulation of enzyme synthesis

? Gene Transcription induction and Repression
? HMG-CoA reductase by cholesterol
? PEP carboxykinase by insulin and glucagon
? Cytochrome P450 by various drugs
? Slow Process
? Long Term Effect

Regulation of enzyme catalytic activity

Allosteric Regulation

? Within seconds
? Allosteric Enzymes: Catalysis at active site modulated

by presence of effector at allosteric site

? Positive or negative effectors
? May affect affinity (K series) or
? Catalytic activity (V series)
Homotropic or Heterotropic effectors

? Homotropic:

? Heterotropic

? Substrate itself an effector

? Effector different from

? Mostly, Positive effector

substrate

? Exhibit cooperativity

? Feedback inhibition

? Hyperbolic curve
? Hills equation define

characteristics

Examples of allosteric regulation

? Most of the rate limiting steps in metabolic pathways
? Feedback inhibitions
? Phosphofructokinase
? Aspartate transcarbamoylase
? Which of the following describes a characteristic of

most allosteric enzymes?

? (A) They are composed of single subunits.
? (B) They show cooperativity in substrate binding.
? (C) They have allosteric activators that bind in the catalytic site.
? (D) They have irreversible allosteric inhibitors that bind at allosteric

sites.

Covalent modification

Partial Proteolysis

Phosphorylation
Partial proteolysis

? Proteases synthesized as inactive precursor:

Proproteins/Proenzymes/Zymogens

? Eg. Pepsin, Tr ypsin, Chymotrypsin, Clotting factors

? Irreversible modification
? Selective Proteolysis leads to conformation change and

configures active site

Phosphorylation/Dephosphorylation

? Catalysed by Protein kinases and Phosphoprotein phosphatases
? Act on serine, threonine and tyrosine residues
? May increase or decrease activity
Example

? High Insulin/Glucagon Ratio decreases c AMP and Protein kinase

A causing dephosphorylation of PFK-2 (Active)

? Active PFK2 increases Fructose 2,6-bisphosphate that increases

PFK-1 activity causing increased glycolysis

? Reverse happens under the effect of glucagon that increases

phosphorylation by in increasing c AMP

Phosphorylation/Dephosphorylation

? Most common mechanism employed for regulation
? Ease of interconversion
? Chemical nature of phosphoryl group helps in conformational changes of

enzymes

? H bond formation by O
? Negative charge

? Insulin/ Glucagon hormones regulates enzymes
Summary

Enzyme

Regulation

Active:

Passive:

Regulation

substrate

of Enzyme

availability/Co

activity

mpartmentatio

n

Enzyme

Allosteric

Covalent

Synthesis

Regulation

Modification

References

? Victor W. Rodwell, David A. Bender, Kathleen M. Botham, Peter J. Kennelly,

P. Anthony Weil. Harper's Illustrated Biochemistry, 30th Edition

? Denise R. Ferrier; Lippincott Illustrated Reviews Biochemistr y, 7th Edition

Thank You!

This post was last modified on 05 April 2022