2nd Phase of Glycolysi1

2nd Phase of Glycolysi1 - 2nd Phase of Glycolysis Bryant...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
2 nd Phase of Glycolysis Bryant Miles In the preparatory phase of glycolysis two molecules of ATP have been invested. The hexose chain has been cleaved into two triose phosphates. The second phase contains the last five reactions of glycolysis and is called the payoff phase. It is called the payoff phase because in these five reactions two high energy phosphate bonds are produced. These 2 high energy phosphoryl groups are transferred to ADP to generate 2 molecules of ATP. It is important to remember that two molecules of glyceraldehyde 3- phosphate is generated per 1 molecule of glucose. The conversion of two molecules of glyceraldehyde 3- phosphate into two molecules of pyruvate is accompanied with the generation of 4 ATP molecules and 2 molecules of NADH. The net reaction for glycolysis is the production of 2 ATP. Reaction 6 Oxidation and phosphorylation of glyceraldehyde 3-phosphate The first step of the second phase of glycolysis is the oxidation and phosphorylation of glyceraldehyde 3-phosphate to form 1, 3-bisphosphoglycerate. The enzyme is glyceraldehyde 3-phosphate dehydrogenase . Δ G o ’ = 6.3 kJ/mol In erythrocytes Δ G = -1.29 kJ/mol Another reaction near equilibrium. The oxidation of an aldehyde to the carboxylic acid is an exergonic process. This enzyme uses the free energy of aldehyde oxidation to synthesize a high energy acyl phosphate, 1, 3-bisphosphoglycerate. Fun facts about glyceraldehyde 3-phosphate dehydrogenase. 1. Iodoacetate, a reagent that alkylates cysteine residues, inactivates GAPDH. Iodoacetate reacts stiochiometrically with GAPDH. The confirmed presence of a carboxymethylcysteine residue demonstrates an active site cysteine residue that is essential for enzymatic activity. 2. GAPDH quantitatively transfer 3 H from the C1 carbon of glyceraldehyde 3-phosphate to NAD + , establishing a direct hydride transfer. 3. GAPDH catalyzes 32 P exchange between inorganic phosphate and acetyl phosphate. Such isotope exchange is indicative of an acyl-enzyme intermediate. O CH HC OH H 2 CO P O - O - O NAD + + Pi NADH + H + O C HC OH H 2 P O - O - O O P O - O - O ENZ C H 2 SH + I CH 2 CO 2 - HI ENZ C H 2 S CH 2 CO 2 - O C H H H H 2 CP O - O - O O 3 + NAD + + Pi GAPDH GAPDH O C OPO 3 H H H 2 O - O - O O 2- + NAD 3 H P O - O - O HO + H 3 C C O P O - O - O OP O - O - O HO H 3 C C O P O - O - O O 32 32 + GAPDH
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Mechanism of GAPDH First the substrate G-3P binds to the enzyme. The essential cysteine residue acts as a nucleophilic and attacks the aldehyde carbonyl to form a thiohemiacetal intermediate. Next the thiohemiacetal transfer a hydride (2e- + H+) to the active site bound NAD + to form NADH and a thioester intermediate. This NADH molecule dissociates from the active site and another NAD + molecule is bound. The enzyme binds a molecule of inorganic phosphate which is the nucleophile that attacks the thioester To regenerate the active enzyme’s sulfhydryl and form 1, 3- bisphosphoglycerate.
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 8

2nd Phase of Glycolysi1 - 2nd Phase of Glycolysis Bryant...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online