3 Regulatory Enzymes and rate limiting step of Glycolysis

 Glycolysis (Glyco=Glucose; lysis= splitting) is the oxidation of glucose (C 6) to 2 pyruvate (3 C) with the formation of ATP and NADH.

It is also called as the Embden-Meyerhof Pathway
Glycolysis is a universal pathway; present in all organisms:
from yeast to mammals.

It is a universal anaerobic process where oxygen is not required.

Lear more on 10 steps of Glycolysis

In metabolic pathways, enzymes catalyzing essentially irreversible reactions are potential sites of control. In glycolysis, the reactions catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase are virtually irreversible; hence, these are the regulatory enzymes in Glycolysis.

This is our 5 minute video Regulatory Enzymes and rate limiting step of Glycolysis

Regulatory Enzyme 1 : Hexokinase

Step 1: Phosphorylation of glucose to glucose-6 phosphate (Hexokinase)

This reaction requires energy and so it is coupled to the hydrolysis of ATP to ADP and Pi.
• Enzyme: hexokinase. It has a low Km for glucose; hexokinase phosphorylates glucose that enters the cell
• Irreversible step. So the phosphorylated glucose gets trapped inside thecell. Glucose transporters transport only free glucose

Hexokinase Activators:AMP/ADP (indicating low energy or ATP therefore activates hexokinase

Inhibitors: Glucose-6-phosphateInhibitors

If G6P accumulates in the cell, there is feedback inhibition of hexokinase till the G6P is consumed

3 Regulatory Enzymes and rate limiting step of Glycolysis

Regulatory Enzyme 2 and Rate limiting step : Phosphofructokinase (PFK)

Step 3: Phosphorylation of fructose-6- bisphosphate.(PFK)

Phosphorylation of the hydroxyl group on C1  forming fructose-1,6- bisphosphate.
Enzyme: phosphofructokinase. This allosteric enzyme regulates the pace of glycolysis (rate limiting step).

The rate limiting step is the slowest (irreversible) step in a pathway, which determines how fast the whole pathway can be carried out.

ATP is used
Second irreversible reaction of the glycolytic pathway.

PFK Activators: AMP/ADP, Fructose-2,6-bisphosphate

Inhibitors: ATP, Citrate

Citrate inhibits PFK by enhancing the inhibitory effect of ATP.

Fructose 2,6-bisphosphate (PFK-2) activates PFK by increasing its affinity for fructose 6-phosphate and diminishing the inhibitory effect of ATP 

This reaction is unique to Glycolysis therefore rate limiting step

Regulatory Enzyme 3: pyruvate kinase.

Step 10: Enolphosphate is a high energy bond. It is hydrolyzed to form the enolic form of pyruvate with the synthesis of ATP. Irreversible step

Enzyme: pyruvate kinase.

Enol pyruvate quickly changes to a more stable keto pyruvate.

Pyruvte kinase Activators:AMP/ADP , Fructose-1,6-bisphosphate

Inhibitors: ATP, Acetyl CoA, Alanine

If fructose 1,6 bisphosphate is formed, it acts a allosteric feedforward activator and drives the pyruvate kinase reaction forward.

Alanine, an aminoacid derived from pyruvate, is a negative
effector of catabolism.

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