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FCH 532 Lecture 27. Chapter 26: Essential amino acids Quiz Wed: NIH Shift Quiz Mon: Essential amino acids Exam 3: Next Wed. Table 26-2 Essential and Nonessential Amino Acids in Humans. Page 1030. Glutamate is the precursor for Proline, Ornithine, and Arginine.
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FCH 532 Lecture 27 Chapter 26: Essential amino acids Quiz Wed: NIH Shift Quiz Mon: Essential amino acids Exam 3: Next Wed.
Table 26-2Essential and Nonessential Amino Acids in Humans. Page 1030
Glutamate is the precursor for Proline, Ornithine, and Arginine • E. coli pathway from Gln to ornithine and Arg involves ATP-driven reduction of the glutamate gamma carboxyl group to an aldehyde (N-acetylglutamate-5-semialdehyde). • Spontaneous cyclization is prevented by acetylation of amino group by N-acetylglutamate synthase. • N-acetylglutamate-5-semialdehyde is converted to amine by transamination. • Hydrolysis of protecting group yields ornithine which can be converted to arginine. • In humans it is direct from glutamate-5-semialdehyde to ornithine by ornithine--aminotransferase
Arginine synthesis glutamyl kinase 6. Acetylglutamate kinase N-acetyl--glutamyl phosphate dehydrogense N-acetylornithine--aminotransferase Acetylornithine deacetylase ornithine--aminotransferase Urea cycle to arginine Page 1036
Figure 26-58 The conversion of glycolytic intermediate 3-phosphoglycerate to serine. Conversion of 3-phosphoglycerate’s 2-OH group to a ketone Transamination of 3-phosphohydroxypyruvate to 3-phosphoserine Hydrolysis of phosphoserine to make Ser. Page 1037
Serine is the precursor for Gly • Ser can act in glycine synthesis in two ways: • Direct conversion of serine to glycine by hydroxymethyl transferase in reverse (also yields N5, N10-methylene-THF) • Condensation of the N5, N10-methylene-THF with CO2 and NH4+ by the glycine cleavage system
Cys derived from Ser • In animals, Cys is derived from Ser and homocysteine (breakdown product of Met). • The -SH group is derived from Met, so Cys can be considered essential.
Methionine adenosyltransferase Methyltransferase Adenosylhomocysteinase Methionine synthase (B12) Cystathionine -synthase (PLP) Cystathionine -synthase (PLP) -ketoacid dehydrogenase Propionyl-CoA carboxylase (biotin) Methylmalonyl-CoA racemase Methylmalonyl-CoA mutase Glycine cleavage system or serine hydroxymethyltransferase N5,N10-methylene-tetrahydrofolate reductase (coenzyme B12 and FAD) Page 1002
Cys derived from Ser • In plants and microorganisms, Cys is synthesized from Ser in two step reaction. • Reaction 1: activation of Ser -OH group by converting to O-acetylserine. • Reaction 2: displacement of the acetate by sulfide. • Sulfide is derived fro man 8-electron reduction reaction.
Figure 26-59a Cysteine biosynthesis. (a) The synthesis of cysteine from serine in plants and microorganisms. Page 1038
Figure 26-59b Cysteine biosynthesis. (b) The 8-electron reduction of sulfate to sulfide in E. coli. Sulfate activation by ATP sulfuylase and adeosine-5’-phosphosulfate (APS) kinase Sulfate reduced to sulfite by 3’-phosphoadenosine-5’-phosphosulfate (PAPS) reductase Sulfite to sulfide by sulfite reductase Page 1038
Biosynthesis of essential amino acids • Pathways only present in microorganisms and plants. • Derived from metabolic precursors. • Usually involve more steps than nonessential amino acids.
Biosynthesis of Lys, Met, Thr • First reaction is catalyzed by aspartokinase which converts aspartate to apartyl--phosphate. • Each pathway is independently controlled.
Figure 26-60 The biosynthesis of the “aspartate family” of amino acids: lysine, methionine, and threonine. Page 1039
Figure 26-61 The biosynthesis of the “pyruvate family” of amino acids: isoleucine, leucine, and valine. Page 1040
Figure 26-62 The biosynthesis of chorismate, the aromatic amino acid precursor. Page 1042
Figure 26-63 The biosynthesis of phenylalanine, tryptophan, and tyrosine from chorismate. Page 1043
Figure 26-64 A ribbon diagram of the bifunctional enzyme tryptophan synthase from S. typhimurium Page 1044
