James Richard Fromm
The term protein, coined by the Swedish chemist Jons J.Berzelius in 1838, is taken from the Greek word proteios, meaning "of the first rank". Proteins are the single most important class of molecules in living organisms. A protein is a polypeptide, generally more than 100 amino acids in length. Proteins may consist of one polypeptide chain or of several polypeptide chains.
A polypeptide chain consists of a linear sequence of peptide linkages with the remaining R groups of the amino acids branching from the chain like leaves from a twig. Although these R groups contain other organic functional groups within them, under physiological conditions the different R groups do not generally react with each other to form covalent bonds which link two polypeptide chains together or form cross-links from one part of a polypeptide chain to another part of the same chain. As a consequence, the primary structure of a protein, which is the sequential order of the amino acids that make up its polypeptide chains, is the most important factor in establishing the three-dimensional structure of the protein molecule.
One group on one of the common amino acids--the -SH sulfhydryl group of cysteine--reacts with itself to form disulfide bridges between polypeptide chains. The formation of a cysteine bridge is a reduction reaction.
Proteins may consist of a single polypeptide chain, as myoglobin does, or of multiple chains linked by disulfide bonds; the two chains of insulin are joined by two disulfide bonds. More complex proteins may consist of multiple chains held together by noncovalent forces. Some protein molecules contain organic structures which are not polypeptide chains. Hemoglobin, for example, includes the additional iron-containing heme group which is essential for its transport of oxygen. The four polypeptide chains of hemoglobin (two of one kind and two of another) are held together by noncovalent forces. The effect of the noncovalent forces, particularly hydrogen bonding, is often to form local regions of ordered structures within the protein. One common type of local order is the alpha helix structure discovered by the American chemist Linus Pauling.
The effects of local ordering, of disulfide bridge formation, and of additional organic structures establish the secondary structure of a protein, which is its overall three-dimensional configuration. The detailed three-dimensional structure of a protein molecule, called its tertiary structure, can be established for many proteins by use of x-ray crystallography. Knowledge of the tertiary structure is often necessary to understand the chemical and physiological actions of protein molecules, since their most significant actions may involve only a single small active site on a very large molecule.
Most of the structure of living tissue, including our own, is made up of proteins. In living organisms these proteins are not totally static structures but in many cases are being built up and taken down continually. In a standard 70 kg man, the protein turnover is about 400 g/day; of this about 300 g/day are recycled to new protein and 30 to 100 g/day are degraded from amino acids to ammonia, NH3, and from there to urea, H2N-CO-NH2, in which form the ammonia is excreted. The lower level of 30 g/day excreted is obtained in the absence of continued food intake.
All of the proteins in all known living organisms consist of 20 different common amino acids, of which humans can synthesize about half and the other half must be taken in in food. The rare occurrence of others in proteins is due to modification of one of the twenty common amino acids. There are at least 150 amino acids known to exist, but most of them are not found in protein material although they may occur elsewhere in biological material.
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