. Biochemistry of plants and animals, an introduction. Biochemistry. 450 ANIMAL BIOCHEMISTRY oxaloacetate citrate fumarate. isocitrate FIGURE 18-1. The glyoxylatc by-pass mechanism ot tlic tiicarboxylic acid cycle. A more complete presentation of the cycle itself is given in Figure 7-2, page 171. ply intermediates for biosynthetic purposes. li a-ketoglutarate, for ex- ample, is withdrawn in making glutamic acid, the original supply of oxaloacetate is not replaced and the cycle should stop. This problem of using the Krebs cycle for both complete oxidation and the supply of intermediates has rec
. Biochemistry of plants and animals, an introduction. Biochemistry. 450 ANIMAL BIOCHEMISTRY oxaloacetate citrate fumarate. isocitrate FIGURE 18-1. The glyoxylatc by-pass mechanism ot tlic tiicarboxylic acid cycle. A more complete presentation of the cycle itself is given in Figure 7-2, page 171. ply intermediates for biosynthetic purposes. li a-ketoglutarate, for ex- ample, is withdrawn in making glutamic acid, the original supply of oxaloacetate is not replaced and the cycle should stop. This problem of using the Krebs cycle for both complete oxidation and the supply of intermediates has recently been resolved as follows. When any of the compounds of the cycle are withdrawn, isocitrate is diverted through the glyoxalate by-pass shown in Figure 18-1. The two key enzymes yield succinate and glyoxylate first, and the latter then is converted to malate by addition of carbon from acetyl CoA. Oxaloacetate is regenerated by oxidation. The net effect, the forma- tion of a new molecule of a four-carbon acid by utilizing two mole- cules of acetyl CoA, becomes possible because the steps eliminating carbon dioxide are by-passed. This new molecule may be used in other reaction systems, or it may be enlarged by reaction with still an- other acetyl CoA to form citrate, which may proceed in turn to a-keto- glutarate by decarboxylation. In this way the tricarboxylic acid cycle continuously supplies a-ketoglutarate for glutamic acid, fumarate and oxaloacetate for aspartic Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Mallette, M. Frank (Manney Frank), 1917-. New York, Wiley
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