* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download l8.l The omino ocids
Survey
Document related concepts
Ribosomally synthesized and post-translationally modified peptides wikipedia , lookup
Butyric acid wikipedia , lookup
Metalloprotein wikipedia , lookup
Catalytic triad wikipedia , lookup
Citric acid cycle wikipedia , lookup
Point mutation wikipedia , lookup
Nucleic acid analogue wikipedia , lookup
Fatty acid synthesis wikipedia , lookup
Fatty acid metabolism wikipedia , lookup
Proteolysis wikipedia , lookup
Peptide synthesis wikipedia , lookup
Protein structure prediction wikipedia , lookup
Genetic code wikipedia , lookup
Biochemistry wikipedia , lookup
Transcript
l8.t The Amino Acids 555 l8.l Theomino ocids AIM: To clossifythe 20 commonomino acids occordingto their side-choin structures. Focus There are 20 common natural amino acids. Anarinoacid,is any carboxylicacid (RCO2H)that contains an amino group (-NH, in the same molecule.Tomost chemists and biochemists, however, the term is usually reservedfor the 20 amino acids that are found and used in living organisms.All 2Oamino acids common in nature have a skeleton consisting of a carboxylic acid group and an amino group covalently bonded to a central atom. The central atom of amino acids is called the alpha carbon. The remaining two groups on the alpha carbon are hydrogen and an R group-the amino acid side chain. Alpha carbon-----._ Side chain \ +*\l HrN-C-COOH 1;1p;nal ./H\ \Alphacarboxylic acidgroup The chemical nature of the side chains accounts for differences in amino acid properties. The 20 common naturally occurring alpha amino acids are divided into seven categories according to their side-chain structures. Glycine glycos (Greek): sweet (The amino acid glycine has a sweet taste.) Aliphatic side chains This group consists of amino acids having hydrocarbon (aliphatic) side chains. Glycine, the simplest amino acid, has a hydrogen rather than an aliphatic side chain, but it is still placed in this category.Alanine, with a methyl side chain, is the smallest true member of this group. other aliphatic R groups are those of the amino acids valine (isopropyl), leucine (isobutyl), and isoleucine (sec-butyl)' Leucine and isoleucine have the same molecular formulas and are stluctural isomers. Proline is the only amino acid with its alpha amino group incorporated into a ring. The amino group of proline is a secondary amine. H.C\ H I H2N-C-CO2H lll HHH Glycine H:C., CH, I H2N-C-CO2H /CH. CH I HZN-C-CO2H Valine Alanine CH. ,CH: CH I CHr I H,N-C-CO,H "tl- I CH, I H-C-CH3 I H,N-C-COUH l | ,co,u H,C C. \_-/ \H t\ I H HH Leucine H,C-CH, - Isoleucine Proline 5!t4 CHAPTERIS Arnino Acids, Peptides;and proteins lrydrorylic side chains The amino acids in this group are serine and threonine. Both have aliphatic side chains containing hydroxy funciional groups. OH OH I I CH, H-C-CH3 I I{rN-C-COrH H2N-C-CO2H rt H Serine Eight of the amino acids, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, andvaline, are essential amino acids. These amino acids cannot be synthesized by the bodybut are essential to its proper functioning. They must be supplied in our diet. Threonine Aromatic side chains There are three amino acids that have aromatic rings in their side chains. Phenylalanine is an amino acid in which one of $e methyl hydrogens of alanine is replaced by a phenyl group. In tyrosine" the phenyl ring of phenylalanine is substitute dintt'e. paraposition with a phenolic hydroxyl group. The aromatic ring system of tryptophan is derived from indole (seeSec.l5.l). OH I <-\ ttl H Y CH" Cy-c-cH,-l-co,n NH, CH, I- t- I{2N-C-CO2H I H V*-tt H2N-C-CO2H I H Phenylalanine H T\nosine Tryptoph-an Acidic side chains Aspartic and glutamic acids have side chains that are terminated by carbo;rylic acid groups. At the usual biorogical pH, slightly abovepH 7, these carbo4ylic acid groups are ionized. For this reison, aspartic acid and glutamic acid are often referred to'as-their carboxylate i-ons, aspartate and glutamate. i ?o," CH, l-r_, '! CH, H2N-C-CO2H t- HrN-f-COrH HH Aspartic acid Glutamic acid ia Amide side chains Asparagine and glutamine are the amides of aspartic and glutamic acids, respectively.Their side chains are €lectricallv neutral at pH 7.0. 1 I l8.l The Amino Acids O O. \a, I ,NH, CH, I- H2N-C-COzH I H Asparagine \a, I 555 ,NH, CH, I f", HZN-C-CO2H I H Glutamine Basic side chains This group consists of ttrree amino acids containing weakly basic nitrogens. The nitrogens on the side chains of lysine and arginine are such sufficiently strong bases that they remove a proton from water at neutral pH. At pH 7.0, therefore, the side chains of arginine and Iysine are positively charged. The nitrogens of the side chains of histidine are more weakly basic than those of lysine and arginine. In solution at pH 7.0, the side chains of only about half of the histidine molecules are positively charged at any moment. NH, I NH, I C:NH I I CH, NH HC-NH CH, CH, NCH CH, CH, t- I I I CH, I H,N-C-CO,H I H Lysine I I CH, I H,N-C-CO,H I Il \/ C I CH, I H,N-C-CO,H I H H Arginine Histidine Sulfur-containing side chains Methionine and cysteine are two common sulfur-containing amino acids. Cysteine is often found linked to another cysteine 4rough formation of a disulfide bond (-S-S-) to form the amino acid cystine.Cystine is not considered one of the 20 common amino acids, since it is formed from two molecules of cysteine. H I CH. I b I H2N-C-CO2H I CHr I s I I SH CH, S CH, CH, CH, I I H2N-C-CO2H I H Cysteine I I HzN-C-CO2H I H Methionine I H2N-C-CO2H I H Cystine 556 CHAPTER 18 Amino Acids,Peptides,and Proteins PRACTICEEXERCISE I8.I Categorizethe following amino acids according to their.side-chain groups: (a) leucine (b) lysine (c) serine (d) tyrosine 18.2Stereoisomers of amino ocids AIMS: To stote the handedness of omino ocidsfound in noture. To drow Fischerproiection formulosfor olpho omino ocids. Focus All the amino acids but glycine have handedness. A glance at the structures of common amino acids will show that, except for glycine, there are four different groups attached to the alpha carbons. This is a criterion for handednessin organic molecules (seeSec.16.1).Amino acids fit this rule and may exist as D or r forms that are nonsuperimposable mirror images of each other. The handedness of amino acids is related to that of o- and r-glyceraldehyde as follows: CHO CHO I H-toH c02H HOtH lr RR o-Glyceraldehyde r-Glyceraldehyde CO,H CO,H l-tHtNH, HrNtH tl RR Although the amino acids found in nature are mostly the r forms, the n forms of a few amino acids are found in the cell walls of certain bacteria. 'lxRmplE'lg.l'''j r ' r r rI r l R o-Amino acid r-Amino acid Configuration of r-amino acids Nature shows a marked preference for the r forms of amino acids.With few exceptions, the amino acids found in lMng systems belong to the r family. o r a w i n g ' r i s c h a i ' t r o i afco r m ' u t a s,, : '' i-:.':::"',, ',',i' ' .' Draw the Fischer projection formula of r-alanine sotuTtoN Place the carboxylic acid group at the top and the methyl group (the R group) at the bottom. For the l isomer, the amino group is on the left. COOH g,N-_]-H -t CH.