Malic enzyme synthesis

B. Dozin, M.A. Magnuson, and V.M. Nikodem, Thyroid hormone regulation of malic enzyme synthesis, J. Biol. Chem. 261 10290 (1986). 

Cholesterol is required for membrane synthesis, steroid synthesis, and in the liver, bile acid synthesis. Most cells derive their cholesterol from LDL or HDL, but some cholesterol may be synthesized de novo. Most de novo synthesis occurs in the liver, vfhere cholesterol is synthesized from acetyl CoA in the cytoplasm. The citrate shutde carries mitochondrial acetyl CoA into the cytoplasm, and NADPH is provided by the HMP shunt and malic enzyme. Important points are noted in Figure 1-15-9,... 

The first step is carboxylation of acetyl CoA to malonyl CoA. This reaction is catalyzed by acetyl-CoA carboxylase [5], which is the key enzyme in fatty acid biosynthesis. Synthesis into fatty acids is carried out by fatty acid synthase [6]. This multifunctional enzyme (see p. 168) starts with one molecule of ace-tyl-CoA and elongates it by adding malonyl groups in seven reaction cycles until palmi-tate is reached. One CO2 molecule is released in each reaction cycle. The fatty acid therefore grows by two carbon units each time. NADPH+H is used as the reducing agent and is derived either from the pentose phosphate pathway (see p. 152) or from isocitrate dehydrogenase and malic enzyme reactions. 

Fig. 5.8. Respiratory pathways in Echinococcus spp., sites of ATP synthesis (ox) oxidative, and (red) reductive processes PK, pyruvate kinase OAA, oxaloacetate ME(c), ME(m), malic enzyme (cytosolic) or (mitochondrial) FR, fumarate reductase PDH, pyruvate dehydrogenase complex. (After McManus Bryant, 1986.)...

The synthesis of several lipogenic enzymes is stimulated in the liver by thyroid hormones. For instance the concentration of acetyl Co A carboxylase, fatty acid synthetase and malic enzyme is increased in vivo after T3 injection [66-69],... 

Malic enzyme catalyses the NADP-dependent oxidative decarboxylation of malate to pyruvate and C02 with the production of NADP which is utilized for the synthesis of long chain saturated fatty acids from malonyl CoA. 

A diet high in carbohydrates also stimulates malic enzyme activity and synthesis in rodent and avian liver while starvation or low carbohydrates has the opposite effect [67,74]. Recent data demonstrated [75] that a high-carbohydrate diet increases the cytoplasmic malic enzyme mRNA at a post-transcriptional level probably by retarding its degradation. Such a control is liver specific since no response was observed in brain, heart, kidney and other non-hepatic tissues. The amplitude of the response to the high carbohydrate diet is increased several fold by T3. 

Answer Oxaloacetate might be withdrawn for aspartate synthesis or for gluconeogenesis. Oxaloacetate is replenished by the anaplerotic reactions catalyzed by PEP carboxykinase, PEP carboxylase, malic enzyme, or pyruvate carboxylase (see Fig. 16-15, p. 632). 

A total of 14 NADPH molecules are utilized to make each palmitate molecule. It comes from three sources the malic enzyme (see earlier) provides one NADPH molecule for every acetyl-CoA molecule generated from citrate. For palmitate, this accounts for eight NADPH molecules. The rest must be derived largely from the hexose monophosphate shunt (see Chapter 18). A minor source of NADPH is cytosolic isocitrate dehydrogenase (see Chapter 18). The synthesis of one palmitate molecule thus requires an equivalent of 7 + (3)14 = 49 ATP molecules. 

In animals and fungi there is a similar dichotomy. NADPH can be generated by cytosolic malic enzyme which catalyses the reaction malate + NADP+ — pyruvate + COg + NADPH. Cytosolic malate derives from the following successive reactions the pyruvate/ citrate shuttle on the mitochondrial inner membrane takes pyruvate to the mitochondrion in exchange for citrate cytosolic ATP citrate lyase catalyses ATP + citrate + CoA-SH —> acetylCoA (CH3CO-S-C0A) + oxaloacetate and cytosolic malate dehydrogenase, which catalyses NADH + oxaloacetate NAD+ + malate. This scheme provides both acetylCoA and NADPH for subsequent long chain fatty acid synthesis (see section on Fatty acid synthesis ). 

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... 

F[GURE 5,10 MaJic enzyme/cittate tyasc pathway Citrate lyase is used for producing acetyl-Co A in the cytoplasm, the site of fatty acid synthase. Acetyl-CoA is used by fatty acid synthase for the synthesis of fatly acids. Malic enzyme catalyzes the reduction of NADr , which is required as a cofaclor by fatty acid synthase. 

NADPH, which provides the reducing equivalents for fatty acid synthesis, is produced by the inducible malic enzyme and by the inducible enzymes of the pentose phosphate pathway, glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. 

B. The synthesis of fatty acids from glucose occurs in the cytosol, except for the mitochondrial reactions in which pyruvate is converted to citrate. Biotin is required for the conversion of pyruvate to oxaloacetate, which combines with acetyl CoA to form citrate. Biotin is also required by acetyl CoA carboxylase. Pantothenic acid is covalently bound to the fatty acid synthase complex as part of a phosphopantetheinyl residue. The growing fatty acid chain is attached to this residue during the sequence of reactions that produce palmitic acid. NADPH, produced by the malic enzyme as well as by the pentose phosphate pathway, provides reducing equivalents. Citrate, not isocitrate, is a key regulatory compound. 

When citrate, a citric acid cycle intermediate, moves from the mitochondrial matrix into the cytoplasm, it is cleaved to form acetyl-CoA and oxaloacetate by citrate lyase. The citrate lyase reaction is driven by ATP hydrolysis. Most of the oxaloacetate is reduced to malate by malate dehydrogenase. Malate may then be oxidized to pyruvate and CO, by malic enzyme. The NADPH produced in this reaction is used in cytoplasmic biosynthetic processes, such as fatty acid synthesis. Pyruvate enters the mitochondria, where it may be converted to oxaloacetate or acetyl-CoA. Malate may also reenter the mitochondria, where it is reoxidized to form oxaloacetate. 

---