Amino acids and nitrogen compounds

Structure and function of amino acids , . . 244

Glutamate 272

Glutamine 280

Glycine 288

Threonine - 295

Serine 300

Alanine 308

Phenylalanine 314

Tyrosine 321

Tryptophan 328

Methionine 338

Cysteine 348

Lysine. 356

Leucine .. 363

Valine 370

Isoleucine 377

Aspartate 383

Asparagine 389

Argimne 395

Proline 404

Histidine 412

Taurine , 421

Creatine 427

Carnitine 432

Melatonin 439

Choline 447

Muiiiitvxjk, of Nutrient Metabolism ISBN: (M2.417762.X

Copyright ■ 2003 Elsevier Lid All riüh^ of reprnthictinn in any form reserved

Structure and function of amino acids

Abbreviations

CoA coenzyme A

CssC cystine

GABA garn ma-amino butyrate

NMDA N-methyl D-aspartate

Composition and structure

The amino acids relevant for human nutrition comprise a group of dozens of compounds with broad variation of characteristics and functions. By definition they are always made up of an amino group, an acid, and a side chain. The amino group is most often adjacent to the primary carboxyl group (alpha-position), but may be in beta-position instead (e.g. in beta-alanine).

As in carnitine, a tertiary amine may serve the same function. Similarly, the acid group may be represented bv a sulfoxy function, as in taurine.

The side chain may be just a single hydrogen (in glycine), straight (alanine) or branched (valine, leucine, isoleucine) aliphatic chains, contain aromatic rings (phenylalanine. tyrosine, tryptophan), sulfur (methionine, cy steine, taurine|, selenium (seleno-cysteinc). hydroxy! groups (serine, threonine, hydroxy-proline, hydroxy lysine), second (glutamate. aspartate) or third carboxyl groups (gam ma-car boxy I glutamate. gamma-carboxyl aspartate), amido groups (glutamtnc, asparagme). or additional nitrogen-containing groups (lysine, arginme, histidine).

Since the alpha-carbon is chiral for all amino acids except glycine, two stereoisomers conformations are possible. When the carboxylie group in structural diagrams is depicted at the top and the side chain at the bottom, the L form is the one that has the amino group pointing to the left, the I) form has it pointing to the right. 1 luman proteins contain only L-amino amino acids (and glycine). D-amino acids are quantitatively less common in mammals than in fungi and bacteria, but a few (including D-aspartate and D-serine) are synthesized by humans (D'Amello Art at., 1993; Wolosker cr al., 1999; Wolosker et at., 2000).

Some amino acids form only a few specific peptides, such as taurine as part ofglu-taurine, and beta-alanine as part of camosine and anserine. Several other amino acids are not a regular part of human proteins, but serve specific important functions. This is the case with carnitine (fatty acid transport), taurine Iosmolyte, neuronal agent, antioxidant), ornithine (urea cycle, polyamine synthesis), and citrulline (urea cycle).

Endogenous sources

Precursors from intermediary metabolism: Several annuo acids are synthesized through the addition of an amino group to a common alpha-keto acid. Alpha-keloglutarate and oxaloacctate are readily available Krebs cycle intermediates, pyruvate is a glycolysis product. Glyoxylate is a much less abundant intermediary metabolite, which arises from

COOH t

COOH

COOH

L-Alanine

Figur* B.I Different isomeric Forms of alanine

D- Alanine

¡1-Alanine

COOH

4-Hydroxyproline

COOH I

CH-OH I

H2N—CH2 5-Hydroxylysine

COOH

COOH

3-Methylhistidine

COOH

L-Selenocysteine

CH2 I

Taurine

COOH

COOH

COOH 7-Glutamate

COOH 1

OH-CH I

CH2 1

CHa Carnitine

COOH 1

L-Ornithine

Figure H.2 Lu« common forms oF baies and acids in ammo acids the metabolism of hydroxyproline, cthanolamine, giycoiatc, ethylene glycol, atid a few other minor sources. A slightly more complex case is serine, which is used to synthesize 3-phosphog I ycera to in three steps. The main source of the amino groups is other amino acids. The exchange between glutamate and aspartate (aspartate aminotransferase. 2.6.1.1) and glutamate and alanine (alanine aminotransferase; EC2.6.1.2) is particularly important, as underscored by the abundance of the respective aminotransferases in muscle. liver, ami other tissues. The branched-chain amino acids leucine, valine, and

COOH

Glycine IGly. G)

COOH I

cooh I

h3c ch3

cooh I

hjn-ch I

COOH I

L-Isoleucine (lie. 1)

- Branched-chain ammo acids -

COOH I

L-Methionine (Met. M)

cooh I

COOH I

COOH I

L-Threonine (Thr.T)

COOH

Super Serenity Sleepers

Super Serenity Sleepers

Do You Have Problem Getting A Good Night Sleep? Learn To Sleep Like A Cat At Night And Run Like A Lion When You Wake Up.

Get My Free Ebook


Post a comment