Food Chemistry and Fish in Nutrition ( 2+1)

7.1.3.Structure of amino acids

(1)Structure of amino acids

The 20 amino acids that make up food proteins are linked by peptide bonds and have distinct structures and properties. These amino acids consist of an α-carbon atom covalently attached to a hydrogen atom, an amino group, a carboxyl group, and a side- chain R  group. These amino acids differ only in the chemical nature of the side chain R group. 

The solubility, chemical reaction, net charge and hydrogen bond formation of the amno acids are due to the chemical nature of the R group. Based on the degree of interaction of the R groups with water, amino acids are classified into: hydrophilic (Polar) or  hydrophobic(non-polar).

a. R- Group -Hydrophilic(Polar)

Hydrophilic or polar amino acids are soluble in water. They are charged or uncharged.

i. Charged:  Arg, Asp, Glu, His and Lys are charged amino acids. The side chains of Arg and Lys contain quanido and amino groups, respectively and hence are positively charged (basic) at neutral pH. The imidazole group of His is also basic in nature and slightly positive at neutral pH. Asp and Glu acids contain a carboxyl group and carry a net negative charge at neutral pH. The charged amino acids are strongly hydrophilic. The overall charge of a protein depends on the number of positive and negative amino acids present.

ii. Uncharged: The uncharged amino acids have polar groups that form hydrogen bond with water. Ser and Thr are polar due the presence of a hydroxyl group. Tyr contains an ionisable phenolic group that ionizes at alkaline pH and hence considered as polar. But at neutral pH it is a nonpolar aminoi acid. The amide group of Asn and Gln can interact with water and hence polar in nature. Cys is a polar amino acid. Majority of Cys molecules exist as Cystine, a dimer formed by the oxidation of thiol groups to form a disulphide bond. Proline is an imino acid. 

b. Hydrophobic(non-polar): Amino acids with aliphatic and aromatic side chains are hydrophobic and hence are insoluble in water. Ala, Ile, Leu, Met, pro and Val are aliphatic amino acids. Phe, Try and Tyr are aromatic amino acids.