8) Phosphoglycerate Mutase: 3-PGA to 2-PGA, begins the third stage of Glycolysis and our ATP energy "profit."
This enzyme requires 2,3-bis PGA (2,3BPG; DPG) as a cofactor to phosphorylate the enzyme and to maintain the E-P intermediate:
2,3-bisPGA + E 2-PGA + E-P
A detailed mechanism for this enzyme is shown below: [overhead]
Note that we need another enzyme to produce the BPG cofactor: Bisphosphoglycerate mutase. This enzyme catalyses the interconversion of 1,3-bis PGA to 2,3-bis PGA, taking a high energy compound to a low energy compound: this enzyme is thus an obvious candidate for control, since if it had much activity it could drain Glycolysis of ATP production! Normally of very lo activity. (But enhanced in RBC's, since they use BPG to control the binding of oxygen by hemoglobin. RBC's also have another enzyme, 2,3-bis PGA Pase to bring BPG back into Glycolysis as 2-PGA, but without making ATP.)
9) Enolase: 2-PGA to PEP
This is an alcohol elimination reaction as you've seen in OChem, with catalysis by Magnesium and using general base catalysis by the enzyme:
Note that a low energy compound (2-PGA, approx 10 kJ) is converted to a high energy compound (PEP, greater than 60 kJ) with very little change in energy overall. Essentially have made the phosphate bond much less stable, while increasing the stability of other bonds in the molecule.
10) Pyruvate Kinase: PEP to Pyruvate
Here we have an attack by ADP:
The resulting enol then spontaneously tautomerizes to pyruvate.
PK is a regulatory enzyme in some tissues. There are three isozymes:
- L-PK (Liver, Kidney, RBC's): greatest regulation. Sigmoidal vs. [PEP] & [K+]; F-1,6-bis P is a (+) effector to both. Hormonal control operates via phosphorylation to inactivate the enzyme.
- M-PK (Muscle, brain): least regulation - product inhibition by Pyruvate and MgATP.
- K-PK (Adipose, kidney, liver): intermediate regulation. Sigmoidal vs. [PEP] & [K+]; F-1,6-bis P is a (+) effector to both.
PK completes the reactions of Glycolysis. However, for Glycolysis to proceed NAD+ needs to be regenerated. For aerobic tissues this is done via the Kreb's TCA Cycle. Later we will look at this process for aerobic cells.
Lactate DH is used to regenerate NAD+ in anaerobic tissue in mammals, and takes Pyruvate to Lactate:
Again the NAD+ abstracts a Hydride ion in the reverse reaction:
while a general base aids the formation of the carbonyl carbon, and a positive charge draws electron charge up to the carboxyl group and aids the removal of the hydride ion.
Lactate DH also has isozymes. It is a tetramer of two types of monomers, H & M. Can thus have 5 possible isomers: H4, H3M, H2M2, HM3, & M4, with one active site per monomer.The kinetic properties of the pure monomer LD isozymes are given in the Table:
| H | M | |
| Pyruvate | 1.4 x 10-4 | 5.2 x 10-4 |
| Lactate | 9 x 10-3 | 2.5 x 10-2 |
| Pyruvate Inhibition? | yes | no |
Pathway Diagrams
