Malate glycolysis

There are several possible reasons why storage lipid is mobilized and the glyoxylate cycle stimulated following nitrate addition to N-starved cultures. These are listed as follows 1.) to replenish, via gluconeogenesis, carbohydrates exhausted during nitrate assimilation 2.) to replenish, anaplerotically, TCA cycle intermediates drawn off for protein synthesis during rapid nitrate assimilation 3.) to increase, via malate glycolysis, the amount of cytosolic NADH available for NR. These possibilities are detailed in Fig.3. 

Figure 3. Model showing possible fates for lipid and carbohydrate carbon during nitrogen assimilation in P. tricomutum. Ruxes shown do not represent actual stoichiometries. (1) glutamine synthetase (2) nitrite reductase (3) nitrate reductase (4) malate glycolysis (5) cytosolic malate dehydrogenase (6) anaplerotic carbon flux (7) gluconeogenesis (8) glycolysis (9) carnitine acyltransferase (10) isocitrate lyase.

In 1937 Krebs found that citrate could be formed in muscle suspensions if oxaloacetate and either pyruvate or acetate were added. He saw that he now had a cycle, not a simple pathway, and that addition of any of the intermediates could generate all of the others. The existence of a cycle, together with the entry of pyruvate into the cycle in the synthesis of citrate, provided a clear explanation for the accelerating properties of succinate, fumarate, and malate. If all these intermediates led to oxaloacetate, which combined with pyruvate from glycolysis, they could stimulate the oxidation of many substances besides themselves. (Kreb s conceptual leap to a cycle was not his first. Together with medical student Kurt Henseleit, he had already elucidated the details of the urea cycle in 1932.) The complete tricarboxylic acid (Krebs) cycle, as it is now understood, is shown in Figure 20.4. 

Because the 2 NADH formed in glycolysis are transported by the glycerol phosphate shuttle in this case, they each yield only 1.5 ATP, as already described. On the other hand, if these 2 NADH take part in the malate-aspartate shuttle, each yields 2.5 ATP, giving a total (in this case) of 32 ATP formed per glucose oxidized. Most of the ATP—26 out of 30 or 28 out of 32—is produced by oxidative phosphorylation only 4 ATP molecules result from direct synthesis during glycolysis and the TCA cycle. 

Oxidation of 2 molecules each of isocitrate, n-ketoglutarate, and malate yields 6 NADH Oxidation of 2 molecules of succinate yields 2 [FADHg] Oxidative phosphorylation (mitochondria) 2 NADH from glycolysis yield 1.5 ATP each if NADH is oxidized by glycerol-phosphate shuttle 2.5 ATP by malate-aspartate shuttle + 3 + 5... 

FIGURE 25.1 The citrate-malate-pyruvate shuttle provides cytosolic acetate units and reducing equivalents (electrons) for fatty acid synthesis. The shuttle collects carbon substrates, primarily from glycolysis but also from fatty acid oxidation and amino acid catabolism. Most of the reducing equivalents are glycolytic in origin. Pathways that provide carbon for fatty acid synthesis are shown in blue pathways that supply electrons for fatty acid synthesis are shown in red. 

NADPH can be produced in the pentose phosphate pathway as well as by malic enzyme (Figure 25.1). Reducing equivalents (electrons) derived from glycolysis in the form of NADH can be transformed into NADPH by the combined action of malate dehydrogenase and malic enzyme ... 

Six ATPs will be synthesized if the aspartate-malate shuttle is used to transfer NADH generated through glycolysis to NADH in the mitochondrial matrix four molecules of ATP will be made if the glycerol phosphate shuttle delivers the electrons to ubiquinone in the inner mitochondrial membrane. 

The transport system that conveys malate and a-ketoglu-tarate across the inner mitochondrial membrane (see Fig. 19-27) is inhibited by n-butylmalonate. Suppose n-butyl-malonate is added to an aerobic suspension of kidney cells using glucose exclusively as fuel. Predict the effect of this inhibitor on (a) glycolysis, (b) oxygen consumption, (c) lactate formation, and (d) ATP synthesis. 

Catabolic pathways used Glycolysis, Krebs cycle Glycolysis, malate dismutation... 

Fatty acid biosynthesis (and most biosynthetic reactions) requires NADPH to supply the reducing equivalents. Oxaloacetate is used to generate NADPH for biosynthesis in a two-step sequence. The first step is the malate dehydrogenase reaction found in the TCA cycle. This reaction results in the formation of NAD from NADH (the NADH primarily comes from glycolysis). The malate formed is a substrate for the malic enzyme reaction, which makes pyruvate, CO2, and NADPH. Pyruvate is transported into the mitochondria where pyruvate carboxylase uses ATP energy to regenerate oxaloacetate. 

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