only amino acid residues andconjugatedwhen other residues besides those of
amino acids occur as integral parts of their structures. For example, haemo-
globin is a conjugated protein because its structure contains a haem residue
(Figure 1.6). These non-amino-acid residues are known as prosthetic groups
when they are involved in the biological activity of the molecule. Conjugated
proteins are classified according to the chemical nature of their non-amino-acid
component. For example, glycoproteins contain a carbohydrate residue,
haemoproteins a haem group and lipoproteins a lipid residue.
1.3.1 Structure
The structures of peptides and proteins are very varied. They basically consist of
chains of amino acid residues (Figures 1.5(b), 1.5(c) and 1.7). These chains may be
branched due to the presence of multi-basic or acidic amino acid residues in the
chain (Figure 1.7(d) ). In addition, bridges (cross links) may be formed between
different sections of the same chain or different chains. Cysteine residues, for
example, are responsible for the SāS bridges between the two peptide chains that
form the structure of insulin (Figure 1.7(e) ). The basic structure of peptides and
proteins is twisted into a conformation (time dependent overall shape) character-
istic of that peptide or protein. These conformations are dependent on both the
nature of their biological environment as well as their chemical structures. The
ability of peptides and proteins to carry out their biological functions is normally
dependent on this conformation. Any changes to any part of the structure of a
NNN NFeNHNCH 2 CHCONHNHCOH 3 CHOOCCOOHCH 3CH 3CH H^2 C
2CH CHH 3 C
Peptide chainsHistidine residue(a) Deoxy-haemoglobin (deoxy-Hb) (b) Oxy-haemoglobin (oxy-Hb)NNN NFeNHNCH 2 CHCONHNHCOH 3 CHOOCCOOHCH 3CH 3CH 2 H 2 CCH CHH 3 COOFigure 1.6 The structure of the haem residue in deoxy- and oxy-haemoglobins. In deoxy-Hb the
bonding of the iron is pyramidal whilst in oxy-Hb it is octahedral
6 BIOLOGICAL MOLECULES