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PROTEIN,

any of the group of highly complex organic compounds found in all living cells and comprising the most abundant class of all biological molecules. Protein comprises approximately 50% of cellular dry weight.



Hundreds of protein molecules have been isolated in pure, homogeneous form; many have been crystallized. All contain carbon, hydrogen, and oxygen, and nearly all contain sulfur as well. Some proteins also incorporate phosphorous, iron, zinc, and copper. Proteins are large , any one of a class of simple organic compounds containing carbon, hydrogen, oxygen, nitrogen, and in certain cases sulfur.


These compounds are the building blocks of proteins. ..









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. Click the link for more information. , which in the intact protein are united through covalent chemical linkages called peptide organic compound composed of amino acids linked together chemically by peptide bonds. The peptide bond always involves a single covalent link the amino nitrogen of a second amino acid. linked together, form linear unbranched polymeric structures called polypeptide chains; such chains may contain hundreds of amino-acid residues; these are arranged in specific order for a given species of protein.




DEGRADATION



As with many other macromolecular components of the organism, most body proteins are in a dynamic state of synthesis and degradation amino acids to each other is hydrolyzed, and free amino acids are released. The process is carried out by a diverse group of enzymes called proteases. During proteolysis, the energy invested in Distinct proteolytic mechanisms serve different physiological requirements. Proteins can be divided into extracellular and intracellular, and the two groups are degraded by two distinct mechanisms. Extracellular proteins such as the plasma immunoglobulins and albumin are degraded in a process known as receptor-mediated endocytosis.










Ubiquitin-mediated proteolysis of a variety of cellular proteins plays an important role in many basic cellular processes such as the regulation of cell cycle and division, differentiation, and development; DNA repair; regulation of the immune and inflammatory responses; and biogenesis of organelles.

OCCURRENCE



Proteins are of importance in all biological systems, playing a wide variety of structural and functional roles. They form the primary organic basis of structures such as hair, tendons, muscle, skin, and cartilage. All of the enzymes, the catalysts in biochemical transformations, are protein in nature. Many hormones, such as insulin and growth hormone, are proteins.


The substances responsible for as the prosthetic group. Chromosomes are highly complex nucleoproteins, that is, proteins conjugated with nucleic acid. Viruses are also nucleoprotein in nature. Of the more than 200 amino acids that have been discovered either in the free state or in small

PROTEIN STRUCTURE



Every protein molecule has a characteristic three-dimensional shape, or conformation. Fibrous proteins, such as collagen and keratin, consist of polypeptide chains arranged in roughly parallel fashion along a single linear axis, thus forming tough, usually water-insoluble, fibers or sheets.


Globular proteins, e.g., many of the known enzymes, show a tightly folded structural geometry approximating the shape of an ellipsoid or sphere. Because the physiological activity of most proteins is closely linked to their three-dimensional architecture, specific terms are used to refer to different aspects of protein structure. The term primary structure denotes the precise linear sequence of amino acids that constitutes the polypeptide chain of the protein molecule.


Automated techniques for amino-acid sequencing have made possible the determination of the primary structure of hundreds of proteins. The physical interaction of sequential amino-acid subunits results in a so-called secondary structure, which often can either be a twisting of the polypeptide chain approximating a linear helix globular proteins also undergo extensive folding of the chain into a complex three-dimensional geometry designated as tertiary structure. Many globular protein molecules are easily crystallized and have been examined by X-ray diffraction, a technique that allows the visualization of the precise three-dimensional positioning of atoms in relation to each other in a crystal.







The tertiary structure of several protein molecules has been determined from X-ray diffraction analysis. Two or more polypeptide chains that behave in many ways as a single structural and functional entity are said to exhibit quaternary structure.


The separate chains are not linked through covalent chemical bonds but by weak forces of association. The precise three-dimensional structure of a protein molecule is referred to as its native state and appears, in almost all cases, to altered, e.g., by such physical factors as extremes of temperature, changes in pH, or variations in salt concentration, the protein is said to be denatured; it usually exhibits reduction or loss of biological activity.

PROPERTIES



The properties of proteins are determined in part by their amino acid composition.


As macromolecules that contain many side chains that can be protonated and unprotonated depending upon the pH of the medium, proteins are excellent buffers. The fact that the pH of blood varies only very slightly in spite of the numerous metabolic processes in which it participates is due to the very large buffering capacity of the blood proteins.

TYPES OF PROTEINS



A protein molecule that consists of but a single polypeptide chain is said to be monomeric; proteins made up of more than one polypeptide chain, as many of the large ones are, are called oligomeric. Based upon chemical composition, proteins are divided into two major classes: simple proteins, which are composed of only amino acids, and conjugated proteins, which are composed of amino acids and additional organic and inorganic groupings, certain of which are called non-amino acid portions of certain protein molecules. The key part of activity, especially when the prosthetic group is .


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. Conjugated proteins , organic compound composed of both a protein and a carbohydrate joined together in covalent chemical linkage. These structures occur in many life forms; they are prevalent and important in mammalian tissues. ..


... Click the link for more information. , which contain , any organic compound that is composed of both protein and the various fatty substances classed as lipids, including fatty acids and steroids such as cholesterol.


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MOLECULAR CHAPERONES



Molecular chaperones are specialized cellular proteins that bind nonnative forms of other proteins and assist them to reach a functional conformation. The role of chaperone proteins under conditions of stress, such as heat shock, is to protect proteins by binding to misfolded conformations when they are just starting to form, preventing aggregation; then, following return of normal conditions, they allow refolding to occur. Chaperones also play essential roles in folding under normal conditions, providing kinetic assistance to the folding process, and thus improving the overall rate and extent of productive folding.

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