BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 7. ENZYMOLOGY

7.5. Fundamentals of Enzymatic Reaction Kinetics

7.5.1. Dependence of Enzymatic Reaction Rate on Enzyme Concentration

When carrying out an enzymatic reaction under conditions of substrate excess, the reaction rate depends directly on the Enzyme Concentration. The graphical representation of this relationship is a straight line (Fig. 7.16).

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Fig. 7.16. Dependence of the enzymatic reaction rate on enzyme concentration

However, the absolute quantity of an enzyme is often impossible to determine; therefore, in practice, relative units characterizing enzyme activity are used: one International Unit (IU) corresponds to The amount of enzyme that catalyzes The conversion of 1 µmol of substrate per 1 min under optimal conditions for the enzymatic reaction. Optimal conditions are specific to each enzyme and depend on the ambient Temperature and solution pH in the absence of activators and inhibitors:

The number of activity units (IU) is determined by the formula

IU _ amount of converted substrate (µmol)

time (min)

In 1973, a new unit of enzyme activity was adopted: 1 katal (kat), which corresponds to the amount of catalyst that converts 1 mol of substrate per 1 s. The number of katals is determined by the formula

The International Unit of enzymatic activity (IU) is related to the katal by the following equations:

1 kat = 1 mol S/s = 60 mol S/min = 60·10⁶ µmol/min = 6·10⁷ IU,

1 IU = 1 µmol/min = 1/60 µmol/s = 1/60 µkat = 16.67 nkat.

In medical and pharmaceutical practice, International Units (IU) are frequently used to assess enzyme activity. To estimate the proportion of enzyme molecules relative to other Proteins in a given tissue, the specific activity (SA) of the enzyme is determined, which is numerically equal to the number of enzyme activity units (IU) in a tissue sample per mass [mg] of protein in that tissue:

Specific activity is used to evaluate the degree of Enzyme Purification: the fewer extraneous proteins present, the higher the specific activity.