Question

In to enzymes, turnover number means
A. The number of substrate molecules that a molecule of an enzyme converts into products per hour.
B. The number of substrate molecules that a molecule of an enzyme converts into products per second.
C. The number of substrate molecules that a molecule of an enzyme converts into products per minute.
D. The number of substrate molecules that a molecule of an enzyme converts into products per day.

Answer 1

Hint: An enzyme is a biocatalyst that increases the rate of a chemical reaction without itself being changed in the overall process. Turnover number is one of the remarkable properties of enzymes. The efficiency of a catalyst is measured in terms of turnover number.

Complete answer:
The turnover number of an enzyme refers to the number of molecules of substrate converted into products by a molecule of enzyme per second. Enzymes are very efficient catalysts. The turnover number is represented as Kcat, kinetic efficiency of the enzyme. Its unit is 1/sec.
$$K_{cat}=\frac{V_{max}}{E_t}$$
$$K_{cat}$$ = Turnover number,
$$V_{max}$$= Maximum rate of reaction
$$E_t$$= Total enzyme concentration
The concentration of enzyme is taken into consideration in the above equation because the turnover number does not vary with the amount of enzyme used and therefore remains constant for the enzyme. It depends upon the number of molecules of product made per enzyme per unit time.

Hence, the correct answer is option (B).

Additional information:
Enzymes are organic compounds, produced in the living cells to speed up chemical reactions in biological systems. Enzymes have several unique properties:-
-Most enzymes are proteins.
-Enzymes are highly specific.
-They initiate and accelerate chemical reactions.
-They act actively at optimum temperature. Enzyme activity decreases at low temperature and they become inactive at high temperature.
-They work on substances called substrates. The enzyme and substrate combine to form a complex called enzyme-substrate complex. The enzyme-substrate complex breaks down to give products and the enzymes remain unchanged.
-There are three diverse models that represent the enzyme-substrate binding:
a. Lock and key model
b. Induced-fit model
C. Transition-state model
-The activity of the enzyme depends upon the acidity of the medium.

Note: There are many factors which influence the activity of enzymes. The important factors are:- temperature, hydrogen ion concentration (pH), the concentration of enzyme, concentration of substrate and cofactors (non-protein components). Enzymes can be divided into two general classes:-simple enzymes and conjugated enzymes.