Formation of Malonyl CoA

Formation of Malonyl-CoA Activates Acetate Units for Fatty Acid Synthesis... 

The acetate units are activated by formation of malonyl-CoA (at the expense of ATP). 

Acetate Units Are Committed to Fatty Acid Synthesis by Formation of Malonyl-CoA... 

Following the formation of malonyl CoA, another nucleophilic acyl substitution reaction occurs in step 4 to form the more reactive malonyl ACP, thereby binding the malonyl group to an ACP arm of the multienzyme synthase. At this point, both acetyl and malonyl groups are bound to the enzyme, and the stage is set for their condensation. 

Triglyceride and fatty acid synthesis are promoted by insulin stimulation of liver and adipose tissues by causing the phosphorylation of the first and controlling enzyme in the pathway acetyl-CoA carboxylase (see Section 6.3.2). This enzyme catalyses the formation of malonyl-CoA and requires both allosteric activation by citrate and covalent modification for full activity. 

Formation of malonyl CoA is the rate-limiting and principal regulatory step of fatty acid synthesis. 

The formation of malonyl-CoA from acetyl-CoA is an irreversible process, catalyzed by acetyl-CoA carboxylase. The bacterial enzyme has three separate polypeptide subunits (Fig. 21-1) in animal cells, all three... 

Biotin enzymes are believed to function primarily in reversible carboxvlahon-decarboxylation reactions. For example, a biotin enzyme mediates the carboxylation of propionic acid to methylmalonic add, which is subsequently converted to succinic acid, a dtric acid cycle intermediate. A vitamin Bl2 coenzyme and coenzyme A are also essential to this overall reaction, again pointing out the interdependence of the B vitamin coenzymes. Another biotin enzyme-mediated reaction is the formation of malonyl-CoA by carboxylation of acetyl-CoA ( active acetate ). Malonyl-CoA is believed lo be a key intermediate in fatly add synthesis. 

Similarly, factors that stimulate acetyl-CoA carboxylase, the first enzyme in the pathway for fatty acid synthesis, also discourage fatty acid catabolism. This dual effect occurs because the first enzyme in the pathway leads to the formation of malonyl-CoA, which is a potent inhibitor of carnitine acyltransferase I. This inhibition prevents the transport of fatty acids into the mitochondrion, thereby, preventing fatty acid breakdown. 

In the formation of malonyl-CoA via acetyl-CoA carboxylase, biotin which is tightly bound to the enzyme as a prosthetic group acts as a carrier of a carboxyl group that is transferred to acetyl-CoA. 

The formation of malonyl-CoA signals the beginning of the synthesis of palmitic acid (C16 ()). This occurs on a multifunctional enzyme complex, the fatty acid synthase. In mammalian liver, the enzyme complex consists of two identical polypeptides, each with specific binding sites for malonyl and alkanoyl groups, and eight different enzyme activities. 

Stabilization of the 2-carbanion by enolization is thought to be a critical step in the reactions catalyzed by citrate synthase and acetyl-CoA carboxylase. >ftth the former enz5rme, the carbanion attacks an incoming molecule of OAA. As shown in Figure 4.78, condensation of acetyl-CoA with OAA forms citryl-CoA. Hydrolysis of citryl-CoA then produces citric acid and acetyl-CoA. In the case of acetyl-CoA carboxylase, the carbanion attacks an incoming molecule of CO2, resulting in formation of malonyl-CoA (Figure 4.79). 

FIGURE 21.13 The formation of malonyl-CoA, catalyzed by acetyl-CoA carboxylase. 

Refer to Fig. 11-24 None. However, recall that one molecule of ATP is hydrolyzed in the formation of malonyl-CoA, one of the substrates of the reaction pathway. For this reason malonyl-CoA can be viewed as an activated acetyl-CoA. Hence 7 molecules of ATP are used in the formation of 1 molecule of palmitoyl-CoA. 

To begin the next series of reactions, a malonyl unit is transferred from malonyl-CoA to the phosphopantotheine site of ACP. In a manner similar to the initial condensation, the homolog containing two additional carbons (in this case, butyryl-ACP) reacts with the malonyl unit, displaces CO2, and is reduced in the next three steps of the reaction sequence (Fig. 2.5). Energy is required to provide the NADPH reduc-tant and (as ATP) to regenerate the thioester linkage of ace-tyl-CoA, and for formation of malonyl-CoA (Lehninger, 1982). The cycle continues until palmitoyl-ACP is produced. 

Acetyl-CoA carboxylase catalyzes the ATP-dependent carboxylation of acetyl-CoA in the formation of malonyl-CoA [reaction (1)]. Malonyl-CoA is then condensed to acetyl-CoA in the process of long-chain fatty acid synthesis as shown in reaction (2). 

In conclusion, a number of carboxylases have now been described. One type is prokaryotic in that three separate activities can be discerned, and this type appears to be associated with chloroplasts the other type is eukaryotic in nature in that the enzyme appears to be a large complex protein(s). Based on recent evidence discussed in Sections I,C and and III, we can predict at least two types of carboxylases in the plant cell the first would be assigned to proplastids or chloroplasts for the formation of malonyl-CoA to be used as malonyl-ACP for de novo biosynthesis and elongation to stearoyl-ACP, and the second would be assigned to a cytosolic compartment for use in acyl-CoA elongation reactions and/or aromatic ring biosyntheses. 

Scheme 11.36. Potential pathways in concert with evidence for the formation of malonyl-CoA. The cofactor biotin helps produce the enolate anion of acetyl-CoA and to deliver carbon dioxide to that enolate anion to produce malonyl-CoA.

Acetyl-CoA carboxylase (ACCase) catalyzes the first committed step in the fatty acid synthesis, namely, the formation of malonyl-CoA. Two forms of the enzyme, the prokaryotic and the eukaryotic forms, have been found in plants (1-4). The prokaryotic ACCase is composed of several subunits, one of which is encoded in plastid genome and named accD, and exists in plastids. The eukaryotic ACCase is composed of a single multi-functional polypeptide and exists in cytosol. However, Gramineae do not have the prokaryotic ACCase in plastids, but have the eukaryotic ACCase because of the loss of accU gene in plastids. The eukaryotic ACCase substitutes for the prokaryotic ACCase in Gramineae. It is unknown whether plants in other families loss the prokaryotic ACCase. To answer this question, we examined the prokaryotic ACCase in 28 plant families (5). 

Fatty acid biosynthesis utilizes acetyl CoA. Radioactive acetate is the common experimental substitute, but in the developing seed sucrose from the mother plant is the initial source of substrate. Biosynthesis is a multi-step process (Fig. 3.14) which firstly involves the formation of malonyl CoA by carboxylation of acetyl CoA with carbon dioxide. This malonyl CoA is then accepted by acyl carrier protein (ACP) which is part of a multienzyme complex called the ACP fatty acid synthetase complex. The malonyl CoA is then condensed with... 

The most highly oxidized form of a single C atom is carbon dioxide and in certain reactions involving its assimilation, for example the formation of malonyl-CoA (page 2SS) and oxaloacetate (page 248), biotin acts as a carrier. Biotin deficiency has been found to occur in people who eat raw eggs in quantity. It results from the presence in egg white of avidin, a small protein that binds biotin. This property is lost on heating. 

---