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Encyclopedia > Competitive inhibitor

In biochemistry there are three ways in which certain chemical substances may reduce or prevent the activities of enzymes: competitive, non-competitive and uncompetitive inhibition. Biochemistry is the study of the chemistry of life, a bridge between biology and chemistry that studies how complex chemical reactions give rise to life. ...

In competitive inhibition, the molecule binds to the same active site as the normal enzyme substrate, but doesn't necessarily activate it. The substrate molecules cannot enter the active site while the inhibitor is there, and inhibitors cannot enter the site when the substrate is there. This is classical competitive inhibition. Non-classical inhibition occurs when the inhibitor binds remote to the active site, creating a conformational change in the enzyme such that the substrate can no longer bind to it. Characteristic for this mode of inhibition is that increasing the concentration of substrate reduces the effect of the inhibitor, and vice-versa. In this case, the maximum speed of the reaction is unchanged because though the substrate's affinity for the site is decreased, eventually the substrate concentration will increase to match the concentration of the inhibitor. This is almost complete inhibition. In biochemistry, a substrate is a molecule which is acted upon by an enzyme. ...

In noncompetitive inhibition, the inhibitor works by occupying some other site on the enzyme, as in non-classical competitive inhibition. In this case, the enzyme will bind with the inhibitor creating an E-I complex. The substrate will still be allowed to bind to the enzyme, thus creating an E-I-S complexe thereafter. The reaction will still take place when the inhibitor leaves the enzyme, but the speed will be decreased. The Michaelis-Menten constant, Km, which is a measure of the substrate's affinity for the site will remain the same, even though the maximum speed of the reaction decreases. In this mode of inhibition, there is no competition between the inhibitor and the substrate, so increasing the concentration of the substrate still does not allow the maximum enzyme activity rate to be reached.

In uncompetitive inhibition, the substrate will bind to the active site, forming the E-S complexe, but will then form an E-I-S complexe with the inhibitor. The inhibitor can only bind once the substrate has. The maximum enzyme activity rate decreases because the reaction takes longer due to the inhibitor forming a complexe with the enzyme and the substrate. The affinity of the substrate to the site seems to increase because it takes longer for the product to form, thus the Km increases. The maximum speed of the reaction and the Km values change proportionally to one another.

The kinetics of these activities is described by the Michaelis-Menten equations. The constant Km is the concentration of the substrate at the point when the speed is at half its maximum value. Michaelis-Menten kinetics describe the rate of enzyme mediated reactions for many enzymes. ...

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Definitions (659 words)
A competitive inhibitor is a ligand which binds to the same site on the receptor as the agonist or drug that your are interested studying.
is the equilibrium dissociation constant for a competitive inhibitor of the receptor.
It is refereed to as a competitive inhibitor because its value is determined by measuring the ability of the unlabeled drug to compete with a radiolabeled drug for the receptor.
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