In this article we will discuss about the two main types of amino acids, i.e, alphatic amino acids and cyclic amino acids.
1. Aliphatic Amino Acids:
A. Hydrocarbon Chain Amino Acids:
1) Glycine (Gly) or glycocoll, or α-aminoacetic acid is the simplest of the amino acids because R = H:
Glycine is very widely distributed in proteins; it is particularly abundant in gelatine and fibroin (silkworm protein) where it represents respectively 25% and 40% of the total amino acids.
Glycine is present in bile acids (see fig. 5-7).
On the other hand, glycine contributes to the detoxication processes in the liver; benzoic acid is thus eliminated in the form of a derivative where it is conjugated with glycine, giving N-benzoyl-glycine or hippuric acid, C6H5 – CO – NH – CH2 – COOH.
The monomethyl derivative of glycine is called sarcosine. Betaine is the trimethyl derivative: (CH3)3 ≡ N—CH2—COO–, which is an oxidation product of the corresponding alcohol, choline (see fig. 7-10).
2) Alanine (Ala), or α-aminopropionic acid is the higher homologue of glycine (it is also a widely distributed amino acid).
The structure of other amino acids is derived from that of alanine by substitution of various groups to one H of the methyl radical. Let us note that one sometimes finds β-alanine: H2N —CH2 —CH2—COOH (particularly in coenzyme A, see fig. 2-18).
3) Valine (Val), or α-aminoisovaleric acid.
4) Leucine (Leu), or α-aminoisocaproic acid.
5) Isoleucine (Ile), or α-amino-β-methylvaleric acid.
These three amino acids have a non-polar hydrocarbon chain which man and most animals cannot synthesize; they must be supplied through diet and are therefore called indispensable or essential amino acids.
B. Hydroxy Amino Acids:
1. Serine (Ser), or α-amino-β-hydroxypropionic acid, is found in most proteins.
The hydroxyl group can be esterified by phosphoric acid; this gives phosphoserine, which is characteristic of phosphoproteins (like vitellin of egg yolk):
Certain antibiotics are derivatives of serine (azaserine, cycloserine, chloramphenicol).
2. Threonine (Thr), or α-amino-β-hydroxybutyric acid. This is an amino acid indispensable to man.
C. Sulphur-Containing Amino Acids:
1. Cysteine (Cys), or α-amino-β-thiopropionic acid, differs from serine only by an SH in place of an OH.
This thiol group has an important role in certain enzymes which lose their catalytic activity when the SH is blocked, or even when it is simply oxidised according to the following reaction, with formation of a disulphide bridge:
A more extensive oxidation of cysteine gives cysteic acid. The latter, by decarboxylation gives taurine, which — like glycine — can be conjugated with cholic acid to produce the bile acids (see fig. 5-7).
The decarboxylation of cysteine gives cysteamine or thioethanolamine, which forms the reactive part of the molecule of coenzyme A (see fig. 2-18): HS-CH2-CH2-NH2.
2. Methionine (Met), or α-amino-γ-methyl-mercapto-butyric acid:
D. Dicarboxylic Amino Acids and their Amides:
As indicated by their name, the dicarboxylic amino acids have two acidic groups.
1. Aspartic acid (Asp), or aminosuccinic acid and its β-amide asparagine (Asn).
2. Glutamic acid (Glu) or α-aminoglutaric acid, and its γ-amide, glutamine (Gln).
Aspartic and glutamic acids are very widely distributed in proteins. Besides, they play an important role in transamination processes as will be seen in connection with the metabolism of amino acids.
Both amides are also found in proteins. Furthermore, they play an important role in the transport and storage of amino nitrogen in plants (asparagine) and animals (glutamine).
E. Basic Amino Acids:
We find in proteins, three basic amino acids having an additional cationic group besides the α-amino group.
1. Lysine (Lys), or α-ε-diamino-caproic acid; it has a second NH2 in ε. This amino acid is indispensable to man.
In certain proteins (collagen, gelatin) one finds δ-hydroxylysine, the δ- hydroxylated derivative of lysine.
2. Arginine (Arg), or α-amino-δ-guanidino-valeric acid is – like lysine — an amino acid particularly abundant in the basic proteins of cell nuclei: histones and protamines (certain protamines contain 80 to 90% arginine).
Arginine plays a very important role in the ureogenesis cycle (see fig. 7-30).
Besides, arginine-phosphate plays an important role in the storage of energy in Invertebrates.
3. Histidine (His), or α-amino-β-imidazole propionic acid, or imidazolyl- alanine, is not an aliphatic amino acid because it contains an imidazole ring; we shall however study it here together with the other two basic amino acids. Certain authors prefer to classify histidine with tryptophan and proline in the group of heterocyclic amino acids.
The weakly basic character of histidine is due to the imidazole ring. It is a widely distributed amino acid, particularly in globines.
2. Cyclic Amino Acids:
A. Aromatic Amino Acids:
1. Phenylalanine (Phe), or α-amino-β-phenyl propionic acid, is indispensable to man.
2. Tyrosine (Tyr), or α-amino-β-parahydroxyphenyl propionic acid, differs from the above amino acid by a phenolic group which gives it a weakly acid character.
Tyrosine — together with tryptophan — is mainly responsible for the absorption in U.V. (between 260 and 290 nm) observed in proteins. Tyrosine is also responsible for the xanthoprotein reaction, i.e. yellow coloration observed on addition of HNO3 and due to the formation of nitrophenol type compounds. The metabolism of tyrosine leads to several important compounds like hormones (adrenaline, thyroxine) and pigments (melanines).
3. Tryptophan (Trp), or α-amino-β-indolepropionic acid, or β-indolyl- alanine. This is an amino acid indispensable to man. Tryptophan has an indole ring (resulting from the juxtaposition of a benzene ring and a pyrrole ring). It owes its name to the fact that it was discovered after hydrolysis by trypsine, as it is destroyed by the acidic hydrolysis generally used to hydrolyse proteins into amino acids.
B. Heterocyclic Amino Acids:
As we included histidine among basic amino acids and tryptophan among aromatic amino acids, only proline and hydroxyproline are left in this group.
— Proline (Pro), or pyrrolidine-2-carboxylic acid and its 4-hydroxylated derivative, hydroxyproline, contain a heterocycle having 4 carbon atoms and one nitrogen atom.
These two amino acids have a secondary amino group, not a primary one; they are substituted α-amino acids.
Proline is very widely distributed; hydroxyproline less so, but it is found in appreciable quantities in collagen (13%).
Besides these amino acids constituting the proteins, one may find some less common amino acids, sometimes observed only in a small number of species, either in free slate, or in peptides.
For example in the toxic polypeptides of Amanita phalloides one finds hydroxyleucine and allothreonine; and in various cyclic peptides with antibiotic activity (tyrocidines, gramicidines, bacitracines) isolated from certain bacteria, one finds ornithine (see fig. 1-16); we will refer again to this amino acid because ornithine and citrulline are two amino acids which play an important role in ureogenesis.