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lecture_3 - Classes of Proteins Enzymes The largest class...

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Classes of Proteins Enzymes - The largest class of proteins. These proteins are catalysts. They accelerate the rates of the various biological reactions that take place in the cell. Almost every reaction that occurs in biochemistry is facilitated by some sort of enzyme. They are typically named based on the reaction that they catalyze, and have suffixes with the letters -ase. Examples include proteases ( trypsin , chymotrypsin , carboxypeptidases ), lactase (hydrolyzes milk sugar), Regulatory Proteins - Don’t perform any chemical transformation. They regulate cellular processes based on their abilities to bind to other macromolecules, such as receptors, DNA, RNA, etc. A well known example of course is insulin , the hormone that regulates glucose metabolism. Insulin is a fairly small protein (5.7 kD), and is composed of two identical polypeptide chains that are linked via a disulfide bond. Transport Proteins - Well known example is hemoglobin . Many others are membrane bound proteins which aid in shuttling nutrients or metabolites (glucose, vitamins, amino acids) across the cell membrane. Storage Proteins - Just polypeptides of amino acids that can be broken down to the individual amino acid in order to extract nitrogen. Nitrogen is frequently a limiting nutrient for growth. Proteins can also store metal ions. Metal ions, which are positively charged, can bind to negatively charged side chains, or side chains that are very polar. Ferritin , is the common animal storage protein for iron. Reading, Chap. 3 (57-63), Chap 5 (115-126).
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Classes of Proteins II Contractile and Motile Proteins - Endow cells with the capacity for special forms of movement. Actin and Myosin (muscle fibers). Tubulin , major protein component of microtubules, which facilitate cell division. Structural Proteins - Provide strength and protection to cells. They are typically insoluble. a - keratins are the major proteins of the skin, hair, and fingernails. Collagen is the major protein in bone, connective tissue, tendons, and cartilage. Silk is also a polymer of a structural protein, fibroin, which is a b -keratin. Scaffold Proteins - These proteins act as adaptors. They bind to other proteins, and form a scaffold upon which certain protein complexes or protein-DNA complexes e.g. can be built. They can also help to mediate certain reactions between two proteins. Protective and Exploitive Proteins - Immunoglobulins or antibodies , which are made by lymphocytes, and function in the immune system by locating and neutralizing molecules that are not intrinsic to the host cell. Thrombin and fibrinogen , which are blood clotting proteins, which help to prevent severe loss of blood upon injury. Antifreeze proteins help to prevent the blood of arctic and antarctic fish from freezing. Also include toxins, like those from poisonous snakes, or even bacteria.
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Amino Acid Structure Amino acids are the building blocks of peptides and proteins, the majority of which are composed of 20 commonly occurring ones.
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