Description
Proteins are large biological molecules consisting of one or more
chains of amino acids. Proteins perform a vast array of functions
within living organisms, including catalyzing metabolic reactions,
replicating DNA, responding to stimuli, and transporting molecules
from one location to another. Proteins differ from one another
primarily in their sequence of amino acids, which is dictated by
the nucleotide sequence of their genes, and which usually results
in folding of the protein into a specific three-dimensional
structure that determines its activity.
A polypeptide is a single linear polymer chain of amino acids
bonded together by peptide bonds between the carboxyl and amino
groups of adjacent amino acid residues. The sequence of amino acids
in a protein is defined by the sequence of a gene, which is encoded
in the genetic code. In general, the genetic code specifies *0
standard amino acids; however, in certain organisms the genetic
code can include selenocysteine and—in certain
archaea—pyrrolysine. Shortly after or even during synthesis, the
residues in a protein are often chemically modified by
posttranslational modification, which alters the physical and
chemical properties, folding, stability, activity, and ultimately,
the function of the proteins. Sometimes proteins have non-peptide
groups attached, which can be called prosthetic groups or
cofactors. Proteins can also work together to achieve a particular
function, and they often associate to form stable protein
complexes.
Like other biological macromolecules such as polysaccharides and
nucleic acids, proteins are essential parts of organisms and
participate in virtually every process within cells. Many proteins
are enzymes that catalyze biochemical reactions and are vital to
metabolism. Proteins also have structural or mechanical functions,
such as actin and myosin in muscle and the proteins in the
cytoskeleton, which form a system of scaffolding that maintains
cell shape. Other proteins are important in cell signaling, immune
responses, cell adhesion, and the cell cycle. Proteins are also
necessary in animals' diets, since animals cannot synthesize all
the amino acids they need and must obtain essential amino acids
from food. Through the process of digestion, animals break down
ingested protein into free amino acids that are then used in
metabolism.
Proteins may be purified from other cellular components using a
variety of techniques such as ultracentrifugation, precipitation,
electrophoresis, and chromatography; the advent of genetic
engineering has made possible a number of methods to facilitate
purification. Methods commonly used to study protein structure and
function include immunohistochemistry, site-directed mutagenesis,
nuclear magnetic resonance and mass spectrometry.
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