Amino Acids

Structure and Properties

ByDr. Walaa bayoumie el gazzar

Neutral Aromatic amino acids

• These are amino acids that contain an

aromatic ring (benzene ring). They include

Phenylalanine and Tyrosine. Tryptophane may

also be included in this group.also be included in this group.

Neutral Heterocyclic amino acids

• These are amino acids that contain a

heterocyclic ring ( a ring containing at least

one atom other than carbon). They include

Tryptophan , proline.Tryptophan , proline.

� Proline is an imino acid ( contain imino group

(-NH) rather than amino group)

�The heterocyclic amino acids also include

histidine, but histidine is a basic amino acid.

Trytophan

Proline

�Acidic amino acids and their amides:

• The acidic amino acids are monoamino-dicarboxylicacids. They include Aspartic acid, Glutamic acid.

• Asparagine and glutamine, the amides of aspartic andglutamic acids, respectively, are neutral.

�Basic amino acids

• Histidine, Arginine and Lysine are the only members ofthis group required for protein synthesis.

• Ornithine is not found in proteins but is important inmetabolism.

Nutritional Classification of Amino acids

• Twenty amino acids are needed for proteinsynthesis.

• Nine of these amino acids can not be synthesizedin the body:(phenylalanine, valine, threonine, tryptophan,methionine, leucine, isoleucine, lysine,and histidine). They should be supplied in the diet,methionine, leucine, isoleucine, lysine,and histidine). They should be supplied in the diet,and hence the name ESSENTIAL (Indispensable)AMINO ACIDS.

• Arginine is only essential for growing infants, butnot for adults hence the name semiessential. (10essential amino acids for infants).

• NONESSENTIAL (Dispensable )AMINO ACIDS:

• Nonessential means that our bodies producean amino acid, even if we do not get it fromthe food we eat. Nonessential amino acidsinclude: alanine, asparagine, aspartic acid,cysteine, glutamic acid, glutamine, glycine,cysteine, glutamic acid, glutamine, glycine,proline, serine, and tyrosine.

• Proteins that are rich in essential amino acidsare known as proteins of high biologicalvalue.

PROPERTIES OF AMINO ACIDS

�Solubility: all amino acids are soluble in water.

However, cystine is poorly soluble; that is why

excretion of large amounts of cystine in urine

(cystinuria) leads to stone formation.(cystinuria) leads to stone formation.

�Amphoteric properties:• Amino acids contain at least one carboxyl and one

amino group. The carboxyl group is acidic and candissociate into a negatively charged carboxylate ionand a hydrogen ion.

• The amino group is basic; it combines with a hydrogenion to form the positively charged ammonium ion.

• At the physiologic pH the amino acid carries both• At the physiologic pH the amino acid carries bothpositive and negative charges and has the followingstructure:

• If the medium is alkaline, both carboxyl andamino groups are dissociated, and the amino acidis negatively charged (anion).

• If the medium is acidic, both carboxyl and aminogroups are undissociated, and the amino acid ispositively charged (cation).

• At an intermediate reaction between acidity and• At an intermediate reaction between acidity andalkalinity, the amino acid carries equal positiveand negative charges and is called a zwitterion ordipolar ion.

• The pH at which this form exists is calledisoelectric point or isoelectric pH (pI).

�PEPTIDE BOND FORMATION

• Alpha carboxyl group of one amino acid reacts

with alpha amino group of another amino acid

to form a peptide bond or CO-NH bridge

�Reaction with carbon dioxide:

• Carbon dioxide combines with the amino

group of amino acids to form a carbamino

derivative, or carbamic acid, which is a much

weaker acid than carbonic acid. This reaction

is important in the buffering of the CO2 by theis important in the buffering of the CO2 by the

plasma proteins and hemoglobin.

Hb—NH2 + CO2 → Hb—NH—COOH (Carbamino-Hb)

�Decarboxylation

• The amino acids will undergo alpha

decarboxylation to form the corresponding

amine. e.g. Histidine gives histamine. Many

primary amines are of great physiologicprimary amines are of great physiologic

importance

�Polarity of the R groups

• According to the polarity of the R group,amino acids are classified into:

�Amino acids having nonpolar (hydrophobic)side chains: These include Alanine, Valine,side chains: These include Alanine, Valine,Leucine, Isoleucine, Methionine, Proline,Phenylalanine and Tryptophan.

�Amino acids having uncharged or non-ionicpolar side chains: Serine, Threonine, Cysteine,Tyrosine, Glutamine and Asparagine

�Amino acids having charged or ionic polar

side chains (hydrophilic): These are amino

acids in which the R group carries a full charge

due to ionization of the acidic groups (aspartic

and glutamic acids), giving amino acidsand glutamic acids), giving amino acids

negative charges or due to protonation of

basic groups (arginine, lysine and histidine)

giving amino acids popsitive charges.

When amino acids are connected together to form peptides and proteins their

carboxyl and amino groups lose their charges. However, the R groups of some

amino acids contain polar groups, giving water- solubility to peptides and proteins.

Peptides

Numbering of Amino Acids in Proteins

i. In a polypeptide chain, at one end there will be one free alpha amino group. This end is called the amino terminal (N-terminal) end and the amino acid contributing the alpha-amino group is named as the first amino acid.

ii. Usually the N-terminal amino acid is written on the left hand side when the sequence of the protein is denoted. hand side when the sequence of the protein is denoted. Incidentally, the bio-synthesis of the protein also starts from the amino terminal end.

iii. The other end of the polypeptide chain is the carboxyterminal end (C-terminal), where there is a free alpha carboxyl group which is contributed by the last amino acid

Biologically Important Peptides

• When 10 or less number of amino acids arejoined together, it is called an oligopeptide. Someof them are biologically active.

• Thyrotropin releasing hormone (TRH) is atripeptide.tripeptide.

• Vasopressin (ADH) is a nonapeptide; with 9amino acids, secreted by posterior pituitary.

• Glutathione is a tripeptide. It is gamma glutamylcysteinyl glycine. It is involved in erythrocytemembrane integrity.

Glutathione