1. The primary structure of proteins is the amino acid sequence of which they are composed (Devlin). Amino acids are strung together on the ribosomes in the cytoplasm of the cell according to the blueprint provided by mRNA which is a copy of a gene read off a stretch of DNA on a chromosome in the nucleus. Protein synthesis occurs in a stepwise fashion, with ribosomal peptidyltransferase transferring the growing peptide chain from its tRNA carrier to the alpha-amino group of the amino acid residue of the aminoacyl-tRNA specified by the next codon, or in the case of the first step, to the initiating methionine residue. Nonribosomal elongation factors aid in the process. A chain-terminating UAG, UAA, or UGA codon promotes binding of a release factor once the protein chain is complete, and it is released from the ribosome.
The secondary structure of a protein refers to the local three-dimensional folding of the polypeptide chain of the protein (Devlin). The polypeptide chain has covalently linked atoms of the peptide bonds and alpha-carbon linkages that sequentially link the amino acid residue of the protein. Side chains of amino acid are not considered at the level of the secondary structure of the protein. The tertiary structure of a protein refers to the three-dimensional structure of the polypeptide, and it includes the conformational relationships in space of the side chains and the geometric relationship between distant regions of the polypeptide chain. The quaternary structure of a protein refers to the structure and interactions of the noncovalent association of discrete polypeptide subunits into a multisubunit protein: not all proteins display a quaternary structure.
Proteins usually have unique secondary, tertiary and quaternary structures as determined by their particular amino acid sequence, which is termed their native conformation (Devlin). The folding of a protein into its primary structure is usua...