ProteinsJAMA Any of the group of highly complex organic compounds found in all living cells. Protein is the most
abundant class of all biological molecules, comprising about 50% of cellular dry weight. Classified
by biological function, proteins include the enzymes, which catalyze cellular reactions; collagen, keratin,
and elastin, which are structural, or support, proteins; hemoglobin and other transport proteins; casein,
ovalbumin, and other nutrient proteins; antibodies, which are necessary for immunity; protein hormones,
which regulate metabolism; and proteins such as actin and myosin, the contractile muscle proteins that
perform mechanical work. Structurally, proteins are large molecules composed of one or more chains of
varying amounts of the same 22 amino acids, which are linked by peptide bonds. Each protein is
characterized by a unique and invariant amino acid sequence. Protein chains may contain hundreds of
amino acids; some proteins also incorporate phosphorus or such metals as iron, zinc, and copper. The
amino acid sequence also determines the molecule's three-dimensional structure; this so-called native state
is required for proper biological function. The information for the syntheses of the specific amino acid
sequences from free amino acids is carried by the cell's nucleic acid. See also Peptide; Ribonucleic Acid.
a large, complex organic compound consisting of sequences
of amino acids. Proteins are major components of living cells, and compose
hormones, enzymes and immunoglobulins; they are essential for growth,
bodily repair and most life functions.
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