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Biochemistry, 4th Edition P83

Biochemistry, 4th Edition P83. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | 25.4 How Do Organisms Synthesize Amino Acids 783 The a-Ketoglutarate Family of Amino Acids Includes Glu Gln Pro Arg and Lys Amino acids derived from a-ketoglutarate include glutamate Glu glutamine Gln proline Pro arginine Arg and in fungi and protoctists such as Euglena lysine Lys . The routes for Glu and Gln synthesis were described when we considered pathways of ammonium assimilation. Proline is derived from glutamate via a series of four reactions involving activation then reduction of the y-carboxyl group to an aldehyde glutamate-5-semialdehyde which spontaneously cyclizes to yield the internal Schiff base A1-pyrroline-5-carboxylate Figure 25.20 . NADPH-dependent reduction of the pyrroline double bond gives proline. Arginine biosynthesis involves enzymatic steps that are also part of the urea cycle a metabolic pathway that allows certain animals including humans to excrete any excess nitrogen arising from overconsumption of protein. Net synthesis of arginine depends on the formation of ornithine. Interestingly ornithine is derived from glutamate via a reaction pathway reminiscent of the proline biosynthetic pathway Figure 25.21 . Glutamate is first N-acetylated in an acetyl-CoA-dependent reaction to yield N-acetylglutamate Figure 25.21 . An ATP-dependent phosphorylation of N-acetylglutamate to give N-acetylglutamate-5-phosphate primes this substrate for a reduced pyridine nucleotide-dependent reduction to the semialdehyde. N-acetylglutamate-5-semialdehyde then is aminated by a glutamatedependent aminotransferase giving N-acetylornithine which is deacylated to ornithine. In mammals ornithine is made from glutamate via a pathway that does not involve an N-acetyl block. Ornithine has three metabolic roles 1 to serve as a precursor to arginine 2 to function as an intermediate in the urea cycle and 3 to act as an intermediate in Arg degradation. In any case the 3-NH3 of ornithine is carbamoylated in a reaction catalyzed by ornithine transcarbamoylase. The carbamoyl