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المساهمات : 36 تاريخ التسجيل : 02/10/2008
| موضوع: allosteric الإثنين أكتوبر 06, 2008 9:26 pm | |
| An allosteric protein is one which has an active (catalytic) site and an allosteric (effector) site. In an allosteric enzyme, the active site binds to the substrate of the enzyme and converts it to a product. The allosteric site is occupied by some small molecule which is not a substrate. However, when the allosteric site is occupied by the effector molecule, the configuration of the active site is changed so that it is now unable to recognize and bind to its substrate (Figure 1). If the protein is an enzyme, when the allosteric site is occupied, the enzyme is inactive, i.e., the effector molecule decreases the activity of the enzyme. There is an alternative situation, however. The effector molecule of certain allosteric enzymes binds to its allosteric site and consequently transforms the enzyme from an inactive to an active state (Figure 2). Some multicomponent allosteric enzymes have several sites occupied by various effector molecules that modulate enzyme activity over a range of conditions
allosteric enzymes can exist in more than one conformation, dependent on the presence or absence of a ligand. Therefore, in addition to the active site of allosteric enzymes (the site where the substrate reaction takes place) there is a ligand binding site which, when occupied, changes the conformation and hence the properties of the protein. For example, if an amino acid is present in the growth medium, this molecule may act as the ligand to an enzyme within the biosynthetic pathway for that amino acid, effectively turning off production. Consequently the ligand may be completely unrelated to the substrate on which the enzyme works. Allosteric enzymes may either have their activity enhanced in the presence of their ligand, or have it decreased.
Allosteric enzymes change their conformation in response to the binding of a regulating molecule. The regulator, called a modulator or effector, can have a positive or negative effect on enzyme activity. Allosteric enzymes have two conformations, one that is active, catalytically, and the other that is inactive. Positive modulators (activators) bind to an allosteric site on the enzyme and stabilise the active conformation. Negative modulators (inhibitors) bind to an allosteric site that stabilises the inactive conformation of the molecule. Enzymes with allosteric sites usually have two or more subunits. The principle of allosteric control of enzyme activity is illustrated in Figure 13.
Allosteric control • Enzymes at or near the branch-points of metabolic pathways are regulated by allosteric mechanism. • Oligomeric proteins • Small molecules called effectors bind at a site other than the active site and regulate the activity of allosteric enzymes. • Two types of effectors: Positive and negative • Homotropic effectors: Substrate itself is the effector • Results in a sigmoidal as opposed to hyperbolic velocity versus [S] curve. This effect is also called positive cooperativity (analogous to binding of oxygen to hemoglobin).
Properties of Allosteric enzymes 1. Catalyze irreversible reactions; are rate limiting 2. Generally contain more than one polypeptide chain 3. Do not follow Michaelis-Menten Kinetics 4. Can be upregulated by allosteric activators at constant [S] 5. Can be down regulated by allosteric inhibitors at constant [S] 6. Activators and Inhibitors need not have any structural resemblance to substrate structure
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المساهمات : 325 تاريخ التسجيل : 05/10/2007 العمر : 37
| موضوع: رد: allosteric الثلاثاء أكتوبر 07, 2008 12:59 pm | |
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المساهمات : 70 تاريخ التسجيل : 20/07/2007
| موضوع: رد: allosteric الأربعاء أكتوبر 22, 2008 9:26 pm | |
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