Bio Chemistry

BIOCHEMISTRY 3(2+1)

Lesson 9 : Proteins – Classification, Functions, Structure - Amino Acids

Protein Structure

1. Primary structure - the amino acid sequence of the peptide chains.

2. Secondary structure - highly regular sub-structures (alpha helix and strands of beta sheet) which are locally defined, meaning that there can be many different secondary motifs present in one single protein molecule.

3. Tertiary structure - three-dimensional structure of a single protein molecule; a spatial arrangement of the secondary structures. It also describes the completely folded and compacted polypeptide chain.

4. Quaternary structure - complex of several protein molecules or polypeptide chains, usually called protein subunits in this context, which function as part of the larger assembly or protein complex.

5. The primary structure of a protein is the linear sequence of amino acids that are covalently bonded to form a polypeptide chain.

6. The primary structure is held together by covalent or peptide bonds, which are made during the process of protein biosynthesis or translation. These peptide bonds provide rigidity to the protein. The two ends of the amino acid chain are referred to as the C-terminal end or carboxyl terminus (C-terminus) and the N-terminal end or amino terminus (N-terminus) based on the nature of the free group on each extremity.
   

7. The sequence of the different amino acids is called the primary structure of the peptide or protein. Counting of residues always starts at the N-terminal end (NH2-group), which is the end where the amino group is not involved in a peptide bond. The primary structure of a protein is determined by the gene corresponding to the protein

Peptide Bonds

The peptide bond is the primary linkage of all protein structures and is the chemical bond between the carboxyl groups and amino groups that unites a peptide. A peptide bond (amide bond) is a chemical bond formed between two molecules when the carboxyl group of one molecule reacts with the amine group of the other molecule, thereby releasing a molecule of water (H2O).

The polypeptide chain