Protein structure

Protein molecules have defined structures that specify their function and allow them to make highly specific contributions to cellular processes. Proteins are composed of a linear sequence of amino acids (residues) linked by peptide bonds. Each protein is encoded by a specific gene. The genetic code is transferred from the nucleus to the cytoplasm by mRNA, which is translated on ribosomes into protein sequences. Each codon directs the incorporation of one of 20 possible L-amino acids into the growing polypeptide chain (Fig. 3.6.1). Amino acids comprise a central alpha carbon atom linked to an amino group, a carboxyl group, a hydrogen atom and, importantly, a variable R group, which defines the character of the amino acid. A peptide bond (N–C) is formed when an amino and carboxyl group of two amino acids react. The bond is planar owing to its partial double-bond character. This restriction in motion around the axis of the linear sequence of residues, together with steric hindrance between the R groups of adjacent residues, limits the possible structures available to the protein. A specific amino acid sequence, therefore, defines a unique three-dimensional structure and proteins are said to be either fibrous (elongated structure, e.g. myosin) or globular (highly folded structure, e.g. haemoglobin).

Fig. 3.6.1 General formula for an L-amino acid at neutral pH and formation of a peptide bond.

Protein structure can be described at four levels.

Stay updated, free articles. Join our Telegram channel

Join