A switch from catabolic to anabolic metabolism: the synthesis of fat in the body Explain why the process of fat synthesis leads to the formation of fatty acids with even numbers of carbon atoms Explain why animals cannot carry out net glucose synthesis from fatty acids Distinquish between the carrier for fat oxidation (CoA) and that for fat synthesis (ACP) Outline the steps in the elongation of a fatty acid by two carbon atoms including the role of NADPH Explain how acetyl groups are transferred from the mitochondria to the cytosol including the role of citrate cleavage enzyme, malic enzyme and pyruvate carboxylase Refer to chapters 12 & 22, Stryer, 5e Lecture 26, Michael Schweizer
Acetyl-CoA Carboxylase The overall reaction catalyzed by Acetyl-CoA Carboxylase is spontaneous. Acetyl-CoA Carboxylase, which converts acetyl-CoA to malonyl-CoA, is the committed step of the fatty acid synthesis pathway. The mammalian enzyme is regulated. There is regulation by phosphorylation, and allosteric regulation by local metabolites. A cAMP cascade, activated by hormones glucagon & epinephrine when blood glucose is low, results in phosphorylation of Acetyl-CoA Carboxylase by cAMP-Dependent Protein Kinase.
The input to fatty acid synthesis is acetyl-CoA, which is carboxylated to malonyl-CoA. The ATP-dependent carboxylation provides energy input. TheCO2 is later lost during condensation with the growing fatty acid. The spontaneous decarboxylation drives the condensation reaction.
Acetyl-CoA Carboxylase catalyzes a 2-step reaction by which acetyl-CoA is carboxylated to form malonyl-CoA. The prosthetic group is biotin. There is a separate active site for each reaction. Overall: HCO3- +ATP+ acetyl-CoAADP+ Pi + malonyl-CoA
Fatty Acid Synthase Fatty acid synthesis from acetyl-CoA & malonyl-CoA occurs by a series of reactions, catalyzed in bacteria by seven different enzymes. Mammalian Fatty Acid Synthase is a dimer consisting of 2 copies of a large protein that includes domains containing each of the several catalytic sites. Its evolution has apparently involved gene fusion. NADPH serves as electron donor in two reactions involving substrate reduction. The NADPH is produced mainly by the Pentose Phosphate Pathway.
Fatty Acid Synthase Palmitate, a 16-C saturated fatty acid, is the product. Summary (ignoring H+ & water): acetyl-CoA + 7malonyl-CoA + 14NADPH palmitate + 7CO2 + 14NADP+ + 8CoA Accounting for ATP-dependent synthesis of malonate: 8acetyl-CoA + 14NADPH + 7ATP palmitate + 14NADP++ 8 CoA+7ADP+7Pi Fatty acid synthesis occurs in the cytosol. Acetyl-CoA generated in mitochondria is transported to the cytosol via a shuttle mechanism involving citrate.
Chemical similarities between oxidation and synthesis of a fatty acid
Fatty Acid Synthase prosthetic groups include: • a cysteine thiol • a thiol of phosphopantetheine, equivalent to part of coenzyme A.
Phosphopantetheine is linked via phosphate ester to a serine OH of the Acyl Carrier Protein domain. The long flexible arm of phosphopantetheine allows its thiol to move from one active site to another within the complex.
The thiolsof cysteineandphosphopantetheine, form thioesters with carboxyl groups of acetate, malonate, &/or the growing fatty acid.
Fatty Acid Synthase: The first three steps of the pathway are catalyzed by the catalytic domains listed. The cysteine of one protein subunit and phosphopantetheine of the acyl carrier protein (ACP) domain of the other subunit are shown.
In steps 4-6, the b-ketone is reduced to an alcohol, by e- transfer from NADPH; dehydration yields a trans double bond; and reduction at the double bond by NADPH yields the saturated chain shown.
Following inter-subunit transfer of the fatty acid from phosphopantetheine to cysteine sulfhydryl, the cycle begins again, with reaction of another malonyl-CoA.
Acetyl CoA as a key intermediate between fat and carbohydrate metabolism
Sources of acetyl groups (acetyl CoA) and reducing equivalents for fatty acid synthesis