【多选题】How are long-chain saturated fatty acids synthesized from palmitate?
A.
Palmitate, the principal product of the fatty acid synthase system in animal cells, is the precursor of other long-chain fatty acids. It may be lengthened to form stearate (18:0) or even longer saturated fatty acids by further additions of acetyl groups, through the action of fatty acid elongation systems present in the smooth endoplasmic reticulum and in mitochondria.
B.
The more active elongation system of the ER extends the 16-carbon chain of palmitoyl-CoA by two carbons, forming stearoyl-Co Although different enzyme systems are used, and coenzyme A rather than ACP is the acyl carrier in the reaction, the mechanism of elongation in the ER is otherwise identical to that in palmitate synthesis: donation of two carbons by malonyl-CoA, followed by reduction, dehydration, and reduction to the saturated 18-carbon product, stearoyl-Co
C.
Two key products of elongation pathways are linoleate, an ω-6 fatty acid, and α-linolenate, an ω-3 fatty acid. These are precursors for two extensive families of derivative unsaturated fatty acids, the ω-6 and ω-3 families. Humans cannot synthesize linoleate and α-linolenate and must obtain them in the diet.
D.
The ratio of ω-6 to ω-3 fatty acids in the diet, if too high, can lead to cardiovascular disease. The importance of this ratio may reflect the multitude of signaling molecules in the ω-6 and ω-3 families, with their equally complex physiological effects.
【判断题】When bacteria growing at 20°C are warmed to 30°C, are they more likely to synthesize membrane lipids with saturated fatty acids and long-chain fatty acid. ( )
【多选题】How are odd-number fatty acids oxidized in cells?
A.
Long-chain odd-number fatty acids are oxidized in the same pathway as the even-number acids, beginning at the carboxyl end of the chain. However, the substrate for the last pass through the β-oxidation sequence is a fatty acyl-CoA with a five-carbon fatty acid. When this is oxidized and cleaved, the products are acetyl-CoA and propionyl-Co The acetyl-CoA can be oxidized in the citric acid cycle, of course, but propionyl-CoA enters a different pathway, having three enzymes.
B.
Propionyl-CoA is first carboxylated to form the D stereoisomer of methylmalonyl-CoA by propionyl-CoA carboxylase, which contains the cofactor biotin. In this enzymatic reaction, as in the pyruvate carboxylase reaction, CO 2 (or its hydrated ion) is activated by attachment to biotin before its transfer to the substrate, in this case the propionate moiety.
C.
The D-methylmalonyl-CoA thus formed is enzymatically epimerized to its L stereoisomer by methylmalonyl-CoA epimerase.
D.
The L-methylmalonyl-CoA then undergoes an intramolecular rearrangement to form succinyl-CoA, which can enter the citric acid cycle. This rearrangement is catalyzed by methyl-malonyl-CoA mutase, which requires as its coenzyme 5′-deoxyadenosylcobalamin, or coenzyme B 12 , which is derived from vitamin B 12 (cobalamin).