Question

For the following question, please answer in essay format( a page and half long minimum)( if hand written please print legibly and be clear about which part of the question are answering, in complete sentences, with as much detail as possible. Thanks!

Describe the Embden-Meyerhof pathway found in most eukaryotic cells. List all ten enzymes needed for this metabolic pathway, along with their substrates (include the number of carbon atoms in each substrate), products (include the number of carbon atoms in each product), and any high-energy molecules generated or consumed by each enzyme.

Answer 1

Embden Meyerhoff Parnas pathway /EMP pathway / Glycolysis:

Glycolysis is ten enzyme catalysed reaction in which glucose (6C) is converted to Pyruvate (3C) which occurs in all eukaryotic cells. Gustavw Embden, Otto Meyerhof and Jacob Parnas were the pioneer workers of this pathway so it is called EMP pathway.

It occurs in two phases

i) Preparatory Phase - Conversion of glucose (6C) to glyceraldehyde 3 phosphate (3C) which utilises 2 ATP.

ii) Payoff Phase - conversion of glyceraldehyde 3 phosphate (3C) to pyruvate in which 4 ATP molecules and 2 NADH molecules are produced.

Reaction Steps of Glycolysis:

  PREPARATORY PHASE REACTIONS

1) Phosphorylation of Glucose:

Very first reaction of glycolysis is activation of glucose by phosphorylation reaction. In this glucose is conveted to glucose 6 phosphate by the enzyme Glucokinase (in all cells ) / Hexokinase (Liver) using 1 ATP . This is irreversible reaction and rate limiting step of glycolysis. Mg2+ is the cofactor in this reaction.

Glucose (6C) + ATP --------------------------> Glucose 6 phosphate (6C) + ADP

2) Isomerization of Glucose 6 phosphate (6C) to Fructose 6 phosphate (6C):

Glucose 6 phosphate (6C) is converted to Fructose 6 phosphate (6C) by the enzyme Phospho gluco isomerase.

Glucose 6 phosphate (6C) <------------------->  Fructose 6 phosphate (6C)

3) Phosphorylation of Fructose 6 phosphate:

Fructose 6 phosphate (6C) is phosphorylated again at first carbon atom and converted to Fructose 1, 6 diphosphate (6C) by the enzyme Phosphofructokinase. In this reaction 1 ATP is used. this is again irreversible reaction and rate limiting step. Mg2+ is the cofactor in this reaction.

Fructose 6 phosphate (6C) ----------------------> Fructose 1, 6 diphosphate (6C)

4) Cleavage of Fructose 1, 6 diphosphate:

Fructose 1, 6 diphosphate (6C) is split into two 3 carbon molecules Dihydroxy acetone phosphate (3C) + Glyceraldehyde 3 phosphate (3C)  by the enzyme Aldolase. The bond between C3 and C4 is broken.

Fructose 1, 6 diphosphate (6C) <---------------> Dihydroxy acetone phosphate (3C) + Glyceraldehyde 3 phosphate (3C)

5) Isomerization of Dihydroxy acetone phosphate:

Dihydroxy acetone phosphate(3C) is isomerized by triose phosphate isomerase enzyme to Glyceraldehyde 3 phosphate (3C).

Dihydroxy acetone phosphate (3C) <-------------------------------------> Glyceraldehyde 3 phosphate (3C).

From this step as the 6 C glucose is broken down to two molecules of 3C the substrates and products formed are in 2 molecules.

   PAYOFF PHASE REACTIONS

6) Oxidative phosphorylation of Glyceraldehyde 3 phosphate:

Two molecules of Glyceraldehyde 3 phosphate (3C) is converted to two molecules of 1, 3 diphosphoglycerate (3C)/ DPG by the enzyme Glyceraldehyde 3 phosphate dehydrogenase. In this reaction 1 NAD is oxidised to NADH. Phosphorylation at first C takes place.

2 Glyceraldehyde 3 phosphate (3C) + NAD + Pi <-----------------> 2 1, 3 - diphosphoglycerate (3C) + NADH + H+

7) Transfer of Phosphate from 1,3 DPG to ADP:

two molecules of 1, 3 diphosphoglycerate (3C) is converted to 3 phosphoglycerate (3C) by the enzyme phospho glycerate kinase. 2 ATP molecules are  released. Mg2+ is the cofactor in this reaction.

2 1, 3 diphosphoglycerate (3C) + 2 ADP <---------------------> 2 3- Phospho glycerate + 2ATP

8) Isomerization of 3 Phospho glycerate:

The enzyme phospho glycerate mutase transfers the phosphate group from carbon 3 to 2. So 2 molecules of 2 Phospho glycerate is formed. Mg2+ is the cofactor in this reaction.

2 3- Phospho glycerate (3C) <-----------------------> 2   2 - phosphoglycerate (3C)

9) Dhydration of 2 - phosphoglycerate:

The enzyme Enolase by using cofactors Mg2+/Mn2+   converts 2 - phosphoglycerate (3C) to Phospho enol pyruvate (3C)/ PEP.

2 2 - phosphoglycerate (3C) <--------------------------> 2 PEP (3C) + 2H2O

10) Transfer of Phosphate from PEP to ADP:

Second ATP generating step of glycolysis. 2 ATP molecules are  released during the conversion of PEP to Pyruvate (3C). This is again irreversible reaction and rate limiting step. Cofactors used is Mg2+.

PEP (3C) + 2ADP -----------------------> 2 Pyruvate (3C) + 2 ATP

Regulation of Glycolysis:

reactions 1,3,10 are the rate limiting steps of glycolysis. The regulatory enzymes are glucokinse, Phospho fructo kinase and pyruvate kinase. these enzymes are activated by the substrates Glucose, ADP, NAD and inhibited by ATP, Pyruvate, NADH at high concentrations by allosteric modification and feed back inhibition.

Overall Reaction of Glycolysis:

Glucose (6C) + 2ADP + 2Pi + 2 NAD ----------------------> 2 Pyruvate (3C) + 2 ATP + 2 NADH + 2H+ + 2H2O

ENERGETICS OF GLYCOLYSIS:

Reaction 1 and 3 used each one ATP so consumption is 2 ATP molecules.

Production at reaction 7 and 10 is 4 ATPs. So the net gain in ATP is 2 .

In reaction 6, two molecules of NADH + H+  produced will enter ETC and gives 3 ATP each, in total 6 ATP

So the gain of ATP in glycolysis is 8.