8-423-08AASynth - Amino acid Biosynthesis OBJECTIVES-Be...

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Amino acid Biosynthesis OBJECTIVES -Be able to outline the biosynthesis of the amino acids discussed here (do NOT need to know structures, details, just the general strategies and common reactions) -What is an essential amino acid? -Why can't patients with PKU use the artificial sweetener aspartame? -How is the enzyme glutamine synthetase regulated? OUTLINE I. Review All tissues have some capability for synthesis of the non-essential amino acids, and conversion of non-amino acid carbon skeletons into amino acids. However, the liver is the major site of nitrogen metabolism in the human body. In times of dietary surplus of proteins, the potentially toxic nitrogen of amino acids is eliminated via transaminations, deamination, and urea formation; the carbon skeletons are generally conserved as carbohydrate, via gluconeogenesis, or as fatty acid via fatty acid synthesis pathways. In this respect amino acids fall into two categories: glucogenic or ketogenic. Glucogenic amino acids are those that give rise to a net production of pyruvate or CAC intermediates, such as α- ketoglutarate or oxaloacetate, all of which are precursors to glucose via gluconeogenesis. All amino acids except lysine and leucine are at least partly glucogenic. Lysine and leucine are the only amino acids that are solely ketogenic, giving rise only to acetylCoA or acetoacetylCoA, neither of which can bring about net glucose production. During times of starvation the reduced carbon skeletons of amino acids are used for energy production through the usual pathways, with the result that they are oxidized to CO 2 and H 2 O. II. Essential and non-essential amino acids Organisms vary greatly in their ability to synthesize the 20 standard amino acids. Most bacteria and plants can synthesize all; mammals can synthesize only about half. Those that mammals can make are called non-essential amino acids, that is, NOT needed in the diet (but still necessary for life!) The remainder must be obtained from food; they are "essential" in the diet. III. Biosynthesis of amino acids One way to organize the synthesis of the amino acids is to group them according to their metabolic precursors; The take-home message here: the beginning points of amino acids synthesis are mostly compounds you already know about, from glycolysis, the CAC, and the PPP.
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We will not cover all the pathways, just a few to illustrate certain key principles. Note that transamination and one-carbon transfer reactions are common. Glutamate and glutamine are usually the source of nitrogen in amino acid biosynthesis. Serine and Glycine Biosynthesis The main pathway to serine starts with the glycolytic intermediate 3-phosphoglycerate. An NADH-linked dehydrogenase converts 3-phosphoglycerate into a keto acid, 3-phosphohydroxypyruvate, which is then subsequently transaminated using our old friend glutamate, producing 3-phosphoserine, which is converted to serine by phosphoserine phosphatase. Glycine can be made from serine through removal of the
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8-423-08AASynth - Amino acid Biosynthesis OBJECTIVES-Be...

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