Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 438

Introductory Biochemistry

Spring 2007

Lecture Notes: 28 March

© R. Paselk 2006
 
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GLYCOLYSIS 4

7) Phosphoglycerate Kinase: 1,3-bis PGA to 3-PGA

After a series of unfavorable or marginal reactions now we get a highly favorable reaction again - pulling the pathway forward.

 

These two reactions couple an oxidation (favorable) to a phosphorylation (unfavorable) to give a substrate level oxidative phosphorylation with the capture of the oxidative energy as ATP.

Note that the energy investment in Stage 1 has now been "paid back" and Glycolysis is now energy neutral. This brings us to the third stage of Glycolysis and our ATP energy "profit."

 

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:

3-PGA + E-P 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.

FYI - PK Isozymes

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.

FYI - LD Isozymes

Lactate DH is a tetramer of two types of monomers, H & M. The kinetic properties of these pure monomer LD isozymes are given in the Table:

Michaelis Constant (KM)
  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
The properties of the H(eart) monomer, which predominates in aerobic tissues can be rationalized as better adapted to the aerobic environment. (Heart uses lactate from the serum as a fuel, but doesn't want to lose pyruvate produced in glycolysis to lactate production.

Gluconeogenesis

In order to provide glucose for vital functions such as the metabolism of RBC's and the CNS during periods of fasting (greater than about 8 hrs after food absorption in humans), the body needs a way to synthesis glucose from precursors such as pyruvate and amino acids. This process is referred to as gluconeogenesis. It occurs in the liver and in kidney. Most of Glycolysis can be used in this process since most glycolytic enzymes are operating at equilibrium. However three irreversible enzymes must be bypassed in gluconeogenesis vs. glycolysis: Hexokinase, Phosphofructokinase, and Pyruvate kinase. Phosphofructokinase, and/or hexokinase must also be bypassed in converting other hexoses to glucose.


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Last modified 28 March 2007