Proteins
Nearly half of the dry weight of a typical animal cell is protein. Structural components of the cell, antibodies, and many of the hormones are proteins but as much as 90% of cellular proteins are the enzymes upon which fundamental cellular function depends. They may be as many as 1000 different enzymes in a single cell.
The protein molecule is a polymer of amino acids joined in peptide linkages. Although the molecular weight is usually high, there is a vast range in both structure and complexity of protein molecules. Hemoglobin for example, has a molecular weight of about 64,500; myosin, a muscle protein is estimated to have a molecular weight of about 489,000. It is not uncommon for peptide structures of fairly low molecular weight (less than 10,000 and containing less than 100 amino acids) to be designated polypeptides rather than proteins. On the average, about 20 different amino acids occur in most proteins, the amino acids present, their position in the molecule, and the spatial arrangement of the molecule all determine the proteins and characteristics of the proteins. In turn the function of a protein depends, in large measure, on its structure.
Amino acids
The amino acids are the fundamentals units of protein structure. All amino acids contain at least one amino group (-NH2) in the alpha position and one carboxyl, and all (except Glycine) contain an asymmetric carbon atom. For this reason, they may exist as isomers. Most naturally occurring amino acids are of the L-configurations, although D-amino acids are not uncommon in some microorganisms. The presence of a D-amino acid oxidase in mammalian tissue, however, suggests that the D-forms may play some yet unrecognized role in mammalian protein metabolism.
Proteins and Amino Acid
