2- Phosphoglycerate in glycolysis

Phosphoglycerate mutase, which interconverts 2-phosphoglycerate and 3-phosphoglycerate in glycolysis (Fig. 10-3, step c), functions by a similar mecha-... 

Most of the enzymes mediating the reactions of the Calvin cycle also participate in either glycolysis (Chapter 19) or the pentose phosphate pathway (Chapter 23). The aim of the Calvin scheme is to account for hexose formation from 3-phosphoglycerate. In the course of this metabolic sequence, the NADPH and ATP produced in the light reactions are consumed, as indicated earlier in Equation (22.3). 

Mn-SOD is an important antioxidant enzyme for the cell due to its role in detoxifying the free radical species superoxide (Oj), so HNE modification of this protein makes the cell more vulnerable to free radical attack. Alpha enolase facilitates the penultimate step of glycolysis by catalyzing the conversion of 2-phosphoglycerate into phosphoenolpyruvate. With HNE modification of alpha enolase, the cell is at risk of inadequate ATP stores due to inhibited production of pyruvate for fueling the citric acid cycle. Similarly, HNE modification of ATPase can lead to inhibited ATP formation due to the direct role of this enzyme in ATP synthesis. Triose phosphate isomerase catalyzes the reversible conversion of dihydroxyacetone phosphate to glyceraldehyde-3-phosphate in glycolysis and MDH catalyzes the oxidation of malate to oxaloacetate, so HNE modification of these proteins can also lead to lower ATP production. 

In addition to the cofactors ATP, NADH, and NADPH, hundreds of metabolic intermediates also must be present at appropriate concentrations in the cell. For example, the glycolytic intermediates dihydroxyacetone phosphate and 3-phosphoglycerate are precursors of tri-acylglycerols and serine, respectively. When these products are needed, the rate of glycolysis must be adjusted to provide them without reducing the glycolytic production of ATP. 

Outline the reactions by which glyceraldehyde 3-phosphate is converted to 3-phosphoglycerate with coupled synthesis of ATP in the glycolysis pathway. Show important mechanistic details. 

PEP is converted to fructose 1,6-bisphosphate in a series of steps that are a direct reversal of those in glycolysis (see Topic J3), using the enzymes enolase, phosphoglycerate mutase, phosphoglycerate kinase, glyceraldehyde 3-phosphate dehydrogenase, triose phosphate isomerase and aldolase (see Fig 1). This sequence of reactions uses one ATP and one NADH for each PEP molecule metabolized. 

In glycolysis, ADP is phosphorylated to ATP during the oxidation of glyceraldehyde 3-phosphate to 3-phosphoglycerate. The phosphorylated intermediate that receives the energy of the oxidation is 1,3-bisphosphoglycerate. 

During glycolysis, glyceraldehyde 3-phosphate is converted to 1,3-bisphosphoglycerate and the equilibrium of the adenylate kinase reaction lies in favor of 3-phosphoglycerate, so the metabolites are drawn through the pathway of reactions. 

After this, phosphoglycero mutase converts 3-phosphoglycerate into 2-phospho-glycerate, which is then dehydrated in phosphoenol pyruvate by the enzyme eno-lase. Phosphoenol pyruvate contains an energy-rich bond that is used by the enzyme pyruvate kinase to phosphorylate ADP into ATP. This reaction generates pyruvate, which is the final product of glycolysis. 

Serine is synthesized from 3-phosphoglycerate, an intermediate in glycolysis. The first step is an oxidation to 3-phosphohydroxypyruvate. This a-ketoacid is transaminated to 3-phosphoserine, which is then hydrolyzed to serine. 

Glycolysis proceeds in the presence of arsenate, but the ATP normally formed in the conversion of 1,3-bisphosphoglycerate into 3-phosphoglycerate is lost. Thus, arsenate uncouples oxidation and phosphorylation by forming a highly labile acyl arsenate. 

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