Lecture 27 Summary

Our text considers glycolysis as consisting of three stages (see BTS6, Fig.16.2, p.436). Stage 1 sarts with glucose and results in its conversion to fructose 1,6-bisphosphate (F1,6-BP). This stage is energy-requiring, and can be thought of as "priming the pump" - putting in a little bit of energy with the expectation of getting more back in stage 3. Stage 2 is the splitting of F-1,6-BP, yielding two molecules of glyceraldehyde 3-phosphate (GAP). Stage 3 starts with GAP and ends with pyruvate. Two molecules of ATP are produced, along with the reduction of NAD+ to NADH. It is important to realize that for each molecule of glucose entering stage 1, two molecules of GAP enter stage 2. Thus, the net production of glycolysis is 2ATP, 2NADH.

The glycolytic pathway formally begins with glucose, which is phosphorylated and then isomerized to fructose 6-phosphate. There are other common monosaccharides, and a thorough study of glycolysis ought to say something about how other sugars enter the pathway. The text discusses the entry of fructose and galactose (BTS, p.440f) into glycolysis. Since a basic knowlegde of carbohydrate chemistry is a prerequisite for a solid understanding of the reactions and mechanisms of glycolysis, you may find it helpful to review Chapter 11 on Carbohydrates.

Carbohydrates: Simple carbohydrates: monosaccharides. Pentoses and hexoses. Furanose and pyranose rings and their conformations. Complex carbohydrates.

Logic of glycolytic pathway. Review of carbohydrate chemistry (cont.) and aldol addition. Application of carbonyl chemistry to glycolytic reactions converting glucose to glyceraldehyde3-phosphate (GAP).

Glycolysis - Stage 3

After conversion of D-glucose into two three-carbon molecules of glyceraldehyde 3-phosphate (GAP) in stages 1 and 2 of glycolysis. stage 3 starts with GAP and ends with pyruvate. Pyruvate can be considered the end product of glycolysis, and it has a number of possible fates.

Mechanisms of glycolytic enzymes

aldolase - protonated Schiff base intermediate

The mechanism of a Class I aldolase begins with formation of a Schiff base linkage between enzyme (amine group from a lysine side chain) and substrate (aldehyde from open chain form of fructose 1,6-bisphosphate. The protonated Schiff base acts as an electron sink to stabilize the negative charge that develops with carbon-carbon bond cleavage. Comparing the mechanism of aldolase to that of a base-catalyzed aldol cleavage reaction can help us understand its fundamental nature.

TIM - triose phosphate isomerase - enzymatic catalysis: "not different, just better"

The isomerization reaction catalyzed by this enzyme interconverts the ketose dihydroxyacetone phosphate (DHAP) and the aldose glyceraldehyde 3-phosphate (GAP) via a enediol intermediate. We easily envision how such a reaction can be catalyzed by general-acid and general-base catalysis. This is in large measure exactly how TIM works, but the actual mechanism incorporates a surprising twist on this simple scheme.

GAPDH - glyceraldehyde 3-phosphate dehydrogenase - a thioester intermediate

In this mechanism, the energy releasing process of oxidation of the aldehyde group of glyceraldehyde 3-phosphate to a carboxylate group is coupled to the energy-requiring step of forming an acyl phosphate from a carboxylate group. The coupling is achieved via the device of a thioester formed by nucleophilic attack by a specific cysteine residue in GAPDH on the carbonyl carbon of glyceraldehyde 3-phosphate.

Substrate-level phophorylations and formation of pyruvate

The glycolytic payoff of ATP production comes via two substrate-level phosphorylations occurring in stage 3. The following enzymes catalyze these steps:

Phosphoglycerate kinase [EC 2.7.2.3]

Pyruvate kinase [EC 2.7.1.40]

Learning objectives

Page update in progress, 11-01-09

References