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The rate of a reaction is the concentration of substrate disappearing (or product produced) per unit time (mol L −1 s −1).. The % purity is 100% × (specific activity of enzyme sample / specific activity of pure enzyme).
The enzyme unit, or international unit for enzyme (symbol U, sometimes also IU) is a unit of enzyme's catalytic activity. [ 1 ] 1 U (μmol/min) is defined as the amount of the enzyme that catalyzes the conversion of one micro mole of substrate per minute under the specified conditions of the assay method .
Another example comes from enzymes in the liver called cytochrome P450 oxidases, which are important in drug metabolism. Induction or inhibition of these enzymes can cause drug interactions. [94] Enzyme levels can also be regulated by changing the rate of enzyme degradation. [1]: 30.1.1 The opposite of enzyme induction is enzyme repression.
Hexokinase-I (HK-I) is an enzyme activator because it draws glucose into the glycolysis pathway. Its function is to phosphorylate glucose releasing glucose-6-phosphate (G6P) as the product. HK-I not only signals the activation of glucose into glycolysis but also maintains a low glucose concentration to facilitate glucose diffusion into the cell.
As shown on the right, enzymes with a substituted-enzyme mechanism can exist in two states, E and a chemically modified form of the enzyme E*; this modified enzyme is known as an intermediate. In such mechanisms, substrate A binds, changes the enzyme to E* by, for example, transferring a chemical group to the active site, and is then released.
in which e is the concentration of free enzyme (not the total concentration) and x is the concentration of enzyme-substrate complex EA. Conservation of enzyme requires that [28] = where is now the total enzyme concentration. After combining the two expressions some straightforward algebra leads to the following expression for the concentration ...
When used to model enzyme rates in vivo , for example, to model a metabolic pathway, this representation is inadequate because under these conditions product is present. As a result, when building computer models of metabolism [ 1 ] or other enzymatic processes, it is better to use the reversible form of the Michaelis–Menten equation.
where [A] is the fixed concentration of agonist and EC 50 is the concentration of agonist that results in half maximal activation of the receptor. Whereas the IC 50 value for a compound may vary between experiments depending on experimental conditions, (e.g. substrate and enzyme concentrations) the K i is an absolute value.