Proteins are linear macromolecules (polypeptides)consisting of amino acids joined by peptide bonds, arranged in a complex three-dimensional structure that is specific for each protein.
Proteins are involved in all chemical processes in living organisms. As enzymes, they drive chemical reactions that in living cells would not occur spontaneously. They serve to transport small molecules, ions, or metals and have important functions in cell division during growth and in cell and tissue differentiation. Proteins control the coordination of movements by regulating muscle cells and the production and transmission of impulses within and between nerve cells; they control blood homeostasis (blood clotting)and immune defense. They have mechanical functions in skin, bone, blood vessels, and other areas.
A. Peptide bonds
Amino acids are easily joined together owing to their dipolar ionization (zwitterions).The carboxyl group of one amino acid bonds to the amino group of the next (a peptide bond, sometimes also referred to as an amide bond).When many amino acids are bound together by peptide bonds, they form a polypeptide chain. Each polypeptide chain has a defined direction, determined by the amino group (—NH2)a to neend and the carboxyl group (—COOH)at the other. By convention, the amino group represents the beginning, and the carboxyl group the end of a peptide chain.
B. Primary structure of a protein
Insulin is an example of a relatively simple protein consisting of two polypeptide chains, an A chain of 21 amino acids and a B chain of 30 amino acids. The determination of its complete amino acid sequence by Frederick Sanger in 1955 was a landmark accomplishment. It showed for the first time that a protein, in genetic terms a gene product,has a precisely defined amino acid sequence. Insulin is synthesized from two precursor molecules, preproinsulin and proinsulin. Preproinsulin consists of 110 amino acids, including 24 amino acids of a leader sequence at the amino end. The leader sequence directs the molecule to the correct site within the cell, where it is removed to yield proinsulin with 86 amino acids. From this, a connecting (C)peptide is removed (amino acids number 31 –65).This yields the two chains, B (amino acids no.1 –30)and A (amino acids 1 –21).The A and B chains are connected by two disulfide bridges, which respectively join the cysteines in positions 7 and 19 of the B chain with positions 6 and 20 of the A chain. The A chain contains a disulfide bridge between positions 6 and 11.The linear sequence of the amino acids is called the primary structure. It yields important information about the function and evolutionary origin of a protein. The positions of the disulfide bridges reflect the spatial arrangements of the amino acids, called the secondary structure.
C. Secondary structural units
The secondary structure of a protein refers to regions with a defined spatial arrangement.
Two basic units of global proteins are α helix formation (α helix)and a flat sheet (-pleated sheet).Insulin is made up of 57%α –helical areas,6%-pleated sheets,10%-turns,and 27%other areas without a defined secondary structure.(Figure adapted from Stryer,1995.)
D. Tertiary structure
The tertiary structure of a protein is the complete three-dimensional structure that is required for its biochemical and biological function. All functional proteins assume a well defined three-dimensional structure. This structure is based on the primary and secondary structures. The tertiary structure may result in a specific spatial relationship of amino acid residues that are far apart in the linear sequence. The quaternary structure involves further folding of the protein, resulting in a specific three-dimensional spatial arrangement of different subunits that affects their interactions. The correct quaternary structure ensures proper function.
Numerous genetic diseases involve a defective or absent protein.