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Amino acids

Metabolic intermediates

Nucleotides

Energy

FIGURE 17-14 Triacylglycerols as glucose source in seeds. ß Oxidation is one stage in a pathway that converts stored triacylglycerols to glucose in germinating seeds. For more detail, see Figure 16-22.

fatty acids (d-3-hydroxyacyl-CoA epimerase and A3,A2-enoyl-CoA isomerase); the fourth enzyme, thiolase, is a separate, soluble polypeptide.

It is interesting that the enzymes that catalyze essentially the reversal of 3 oxidation in the synthesis of fatty acids are also organized differently in prokaryotes and eukaryotes; in bacteria, the seven enzymes needed for fatty acid synthesis are separate polypeptides, but in mammals, all seven activities are part of a single, huge polypeptide chain (see Fig. 21-7). One advantage to the cell in having several enzymes of the same pathway encoded in a single polypeptide chain is that this solves the problem of regulating the synthesis of enzymes that must interact functionally; regulation of the expression of one gene ensures production of the same number of active sites for all enzymes in the path. When each enzyme activity is on a separate polypeptide, some mechanism is required to coordinate the synthesis of all the gene products. The disadvantage of having several activities on the same polypeptide is that the longer the polypeptide chain, the greater is the probability of a mistake in its synthesis: a single incorrect amino acid in the chain may make all the enzyme activities in that chain useless. Comparison of the gene structures for these proteins in many species may shed light on the reasons for the selection of one or the other strategy in evolution.

The t Oxidation of Fatty Acids Occurs in the Endoplasmic Reticulum

Although mitochondrial 3 oxidation, in which enzymes act at the carboxyl end of a fatty acid, is by far the most

FIGURE 17-15 The enzymes of p oxidation. Shown here are the different subunit structures of the enzymes of 3 oxidation in gram-positive and gram-negative bacteria, mitochondria, and plant peroxisomes and glyoxysomes. Enz-i is acyl-CoA dehydrogenase; Enz2, enoyl-CoA hy-dratase; Enz3, L-3-hydroxyacyl-CoA dehydrogenase; Enz4, thiolase; Enz5, D-3-hydroxyacyl-CoA epimerase, and Enz6, A3,A2-enoyl-CoA iso-merase. (a) The four enzymes of 3 oxidation in gram-positive bacteria are separate, soluble entities, as are those of the short-chain-specific system of mitochondria. (b) In gram-negative bacteria, the four enzyme activities reside in three polypeptides; enzymes 2 and 3 are parts of a single polypeptide chain. (c) The very-long-chain-specific system of mitochondria is also composed of three polypeptides, one of which includes the activities of enzymes 2 and 3; in this case, the system is bound to the inner mitochondrial membrane. (d) In the peroxisomal and glyoxysomal ^-oxidation systems of plants, enzymes 1 and 4 are separate polypeptides, but enzymes 2 and 3, as well as two auxiliary enzymes, are part of a single polypeptide chain, the multifunctional protein, MFP.

FIGURE 17-15 The enzymes of p oxidation. Shown here are the different subunit structures of the enzymes of 3 oxidation in gram-positive and gram-negative bacteria, mitochondria, and plant peroxisomes and glyoxysomes. Enz-i is acyl-CoA dehydrogenase; Enz2, enoyl-CoA hy-dratase; Enz3, L-3-hydroxyacyl-CoA dehydrogenase; Enz4, thiolase; Enz5, D-3-hydroxyacyl-CoA epimerase, and Enz6, A3,A2-enoyl-CoA iso-merase. (a) The four enzymes of 3 oxidation in gram-positive bacteria are separate, soluble entities, as are those of the short-chain-specific system of mitochondria. (b) In gram-negative bacteria, the four enzyme important catabolic fate for fatty acids in animal cells, there is another pathway in some species, including vertebrates, that involves oxidation of the « (omega) carbon—the carbon most distant from the carboxyl group. The enzymes unique to m oxidation are located (in vertebrates) in the endoplasmic reticulum of liver and kidney, and the preferred substrates are fatty acids of 10 or 12 carbon atoms. In mammals « oxidation is normally a minor pathway for fatty acid degradation, but when 3 oxidation is defective (because of mutation or a carnitine deficiency, for example) it becomes more important.

activities reside in three polypeptides; enzymes 2 and 3 are parts of a single polypeptide chain. (c) The very-long-chain-specific system of mitochondria is also composed of three polypeptides, one of which includes the activities of enzymes 2 and 3; in this case, the system is bound to the inner mitochondrial membrane. (d) In the peroxisomal and glyoxysomal ^-oxidation systems of plants, enzymes 1 and 4 are separate polypeptides, but enzymes 2 and 3, as well as two auxiliary enzymes, are part of a single polypeptide chain, the multifunctional protein, MFP.

The first step introduces a hydroxyl group onto the « carbon (Fig. 17-16). The oxygen for this group comes from molecular oxygen (O2) in a complex reaction that involves cytochrome P450 and the electron donor NADPH. Reactions of this type are catalyzed by mixed-function oxidases, described in Box 21-1. Two more enzymes now act on the « carbon: alcohol dehydro-genase oxidizes the hydroxyl group to an aldehyde, and aldehyde dehydrogenase oxidizes the aldehyde group to a carboxylic acid, producing a fatty acid with a car-boxyl group at each end. At this point, either end can be attached to coenzyme A, and the molecule can en-

NADPH, O2 NADP+

mixed-function oxidase

NAD+ NADH

alcohol dehydrogenase

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