Problem 3) Compare and contrast pathways for biosynthesis and beta oxidation of fatty acids starting with excess glucose and fatty acid substrates in the blood, respectively. This problem requires putting all of the parts of Problem 2 of Problem Set Module 6 together. Problem Set Module 6 revealed that the pathways are not simply the reverse of each other. Whereas the initial substrate of the biosynthesis pathway goes into and out of the mitochondria, the beta oxidation pathway is strictly one way into the mitochondria. The Problem Set also revealed that the electron carriers do not overlap in the cyclic parts of the pathways but the cyclic stages are only part of the biosynthesis and beta oxidation pathways given the initial substrates glucose and fatty acids, respectively. What is the coenzyme usage if you consider the entireties of the two pathways? How much of the citric acid cycle is required if citrate is transported to the cytosol? And, why should this matter if you are accounting for coenzyme usage? Finally, the Problem Set reminds us that the nomenclature for the acyl group of acyl-CoA refers to long chain fatty acyl groups, which includes palmitoyl but not acetyl groups. Recall that palmitic acid, which is the most abundant fatty acid found in the diet and body, circulates with the other fatty acids in the blood and passes by simple diffusion into all cells of the body. Palmitoyl-CoA that forms in the activation reaction (see footnote in Table 4 below) has special significance with respect to the enzyme that catalyzes the reaction that forms malonyl-CoA and the rate of the pathway.

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