Malic enzyme/citrate lyase pathway

The KQ is influenced by the net rate of fatty acid biosynthesis. Fatty acid synthesis involves the conversion of carbohydrate, via the acetyJ-CoA intermediate, to long-chain fatty acids. The synthesis of fatty acids requires reduced NADP as a co/ac-tor. It involves the consumption of two molecules of NADPH + H+ for each 2-carbon unit incorporated into the fatty acid. The NADPH + H is supplied by two separate pathways the pentose phosphate pathway (PPP) and the malic enzyme/citrate Lyase pathway. 

The malic enzyme/citrate lyase pathway is shown in Figure 5.10. The 2-carbon units acetyl groups) for fatty acid synthesis are supplied by the activity of citrate lyase, which may be considered an enzyme of fatty acid biosynthesis. The reduced NADP is Supplied at the point of malic enzyme. Figure 5.10 reveals no net production or utilization of NAD in the cytoplasm. The NADPH + H generated in the cytoplasm is used for fatly acid synthesis, which regenerates NADP. One molecule of CO is produced in the cytoplasm. The diagram reveals no net production or utilization of CO in the mitochondrion. One molecule of NAD is... 

FIGURE 5.10 Malic enzyme/citrate lyase pathway. Citrate lyase is used for producing acetyl-CoA in the cytoplasm, the site of fatty acid synthase. Acetyl-CoA is used by fatty acid synthase for the S3mthesis of fatty acids. Malic enzyme catalyzes the reduction of NADP , which is required as a cofactor by fatty acid synthase. 

Both dehydrogenases of the pentose phosphate pathway can be classified as adaptive enzymes, since they increase in activity in the well-fed animal and when insulin is given to a diabetic animal. Activity is low in diabetes or starvation. Malic enzyme and ATP-citrate lyase behave similarly, indicating that these two enzymes are involved in lipogenesis rather than gluconeogenesis (Chapter 21). 

Figure 2.4. The provision of acetyl-CoA and NADPH for lipogenesis. PPP, pentose phosphate pathway T, tricarboxylate transporter K, a-ketoglutarate transporter. In ruminants, pyruvate dehydrogenase, ATP-citrate lyase and malic enzyme activities are low and perhaps non-functional. (From Murray et al., 1988. Harper s Biochemistry, 21st edn, p. 207, Appleton and Lange, Norwalk, CT reproduced with permission of The McGraw-Hill Companies).

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