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Non-Michaelis-Menten Kinetics

When the kinetic behaviour does not conform to Eqn (10) or Eqn (11) the reaction is said to exhibit non-Michaelis-Menten kinetics. If the Michaelis-Menten equation is obeyed approximately over a restricted range of substrate concentrations it may be convenient to regard this behaviour as a deviation from this equation rather than as an unrelated phenomenon. For example, a reaction may obey an equation of the following form

........ (12)

in which the constants V', K' _mA and K _iA are used for illustration without any implication of universal or standard definitions. If K _iA is large compared with K mA the behaviour predicted by Eqn (12) will approximate to that predicted by Eqn (11), with V and K _mA replaced by V' and K _mA, in the lower range of substrate concentrations. However, with Eqn (12) the rate passes through a maximum as the concentration increases, and there is said to be inhibition by substrate, and the constant K _iA, which has the dimensions of a concentration, is called the substrate inhibition constant.

When more complex kinds of non-Michaelis-Menten behaviour occur it is usually unhelpful to use terminology and symbolism suggestive of the Michaelis-Menten equation; instead the approach discussed in Section 10 is appropriate. In all cases it is advisable to avoid the term Michaelis constant and the symbol K _m when the Michaelis-Menten equation is not obeyed, because it is defined as a parameter of that equation. The symbol [A]_0.5 or [A]_1/2, not K _mA, may be used for the value of [A] at which v = 0.5 V.

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