Question

What four enzyme activities unique to gluconeogenesis are needed to bypass the three irreversible steps of glycolysis. Draw the reactions catalyzed by these four enzymes.

Answer 1

Glycolysis isthe process of breaking down of six-carbon glucose molecule into two molecules of three carbon pyruvate. the wholw process can be divided into two phases- 1. Preparatory Phase and 2.Payoff Phase.Both phases are constituted of 5 steps each (Total 10 steps).

There are two irreversible steps in the preparatory phase and one step in payoff phase.

1. phosphorylation of glucose at -OH group at C6; thus Glucose-6-phosphate is formed and one ATP is converted into ADP; Enzyme-Hexokinase

2. Fructose-6-phosphate is phosphorylated at C1 position;thuse Fructose-1-6-bisphosphate is formed and one ATP is converted to ADP; Enzyme-Phosphofructokinase-1

These two steps belong to preparatory phase.

3.two molecules of phosphoenol pyruvate are converted in two molecules of Pyruvate; two ADPs are phosphorylated to form two ATPs. This step belongs to payoff phase.

On the other hand gluconeogenesis is the process of synthesizing glucose from non-carbohydrate precurssor such as pyruvate, lactate etc.

The reactions of gluconeogenesis proceed in opposite fashion of glycolysis reactions. However to accomplish that gluconeogenesis reactons need to bypass those three written above irreversible steps of glycolysis. four unique enzymes are involved in the process of bypassing those steps. Those are stated below along with the reactions they catalyze.

1. Pyruvate(3C) (starting molecule in gluconeogenesis) is converted into Oxaloacetate(4C) by an enzyme named Pyruvate Carboxylase; ATPs are broken down into ADPs in the process.

2. Oxaloacetate is converted to Phosphoenol Pyruvate by PEP carboxylase; GTPs are converted into GDPs in this process.

3.Fructose-1-6 bisphosphate is converted to Fructose 6-Phosphate by an enzyme called Fructose-1-6-bisphosphatase; in this reaction one P_i is released.

4.Glucose -6-Phosphate is converted into Glucose( final product of the process) by an enzyme called Glucose-6-Phosphatase;  in this reaction one P_i is also released.

So four unique enzymes in Gluconeogenesis that are essential to bypass three irreversible steps of glycolysis are- Pyruvate kinase, PEP carboxylase, Fructose-1-6 bisphosphatase and Glucose-6-phosphatase.