Biochemistry, 4th Edition P84. 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. | How Do Organisms Synthesize Amino Acids 793 Nevertheless four of the five enzymes necessary for isoleucine synthesis are common to the pathway for biosynthesis of valine so discussion of isoleucine synthesis is presented under the biosynthesis of the pyruvate family of amino acids. The Pyruvate Family of Amino Acids Includes Ala Val and Leu The pyruvate family of amino acids includes alanine Ala valine Val and leucine Leu . Transamination of pyruvate with glutamate as amino donor gives alanine. Because these transamination reactions are readily reversible alanine degradation occurs via the reverse route with a-ketoglutarate serving as amino acceptor. Transamination of pyruvate to alanine is a reaction found in virtually all organisms but valine leucine and isoleucine are essential amino acids and as such they are not synthesized in mammals. The pathways of valine and isoleucine synthesis can be considered together because one set of four enzymes is common to the last four steps of both pathways Figure . Both pathways begin with an a-keto acid. Isoleucine can be considered a structural analog of valine that has one extra CH2 unit and its a-keto acid precursor namely a-ketobutyrate is one carbon longer than the valine precursor pyruvate. Interestingly a-ketobutyrate is formed from threonine by threonine deaminase Figure reaction 1 . This PLP-dependent enzyme also known as threonine dehydratase or serine dehydratase is feedback-inhibited by isoleucine the end product. Note that part of the carbon skeleton for Ile comes from Asp by way of Thr. From here on the Val and Ile pathways employ the same set of enzymes. The first reaction involves the generation of hydroxyethyl-thiamine pyrophosphate from pyruvate in a reaction analogous to those catalyzed by transketolase and the pyruvate dehydrogenase complex. The two-carbon hydroxyethyl group is transferred from TPP to the respective keto acid acceptor by acetohydroxy acid synthase acetolactate synthase to
