NH: Hill Coefficient

The value of the parameter n_H of the Hill saturation function for protein-ligand binding:

Y = L^{^n_H} / (K_D + L^{^n_H}),

where Y is the degree of saturation (i. e., fraction of the maximum binding), L is the concentration of the ligand, K_D is the dessociation constant.

In certain ranges of ligand concentrations, the Hill function can be a good approximation to the real saturation curve. These are the ranges of linearity of the experimental curve plotted in Hill co-ordinates [log L, log( Y / (1 - Y )].

The Hill coeficient, n_H, determined by the slope of the binding saturation curve in Hill copordinates, can be used as a quantitative measure of the degree of cooperativity. The saturation kinetics displays a positive cooperativity if n_H > 1 and a negative cooperativity if n_H < 1. A unity Hill coefficient corresponds to non-cooperative binding, or, in terms of the enzyme kinetics, hyperbolic Michaelis-Menten rate law (RL). Cooperative binding kinetics is always characterized by sigmoidal saturation curve Y(L) and rate law v(L).

Example (bovine heart 2.4.1.11):

AC KT NH ADD
D-glucose 6-phosphate S 1.58 10(mmol/l) P_i
D-glucose 6-phosphate H 1.0
D-glucose 6-phosphate S 1.5
10(mmol/l) P_i
10(mmol/l) Mg²⁺

D-glucose 6-phosphate H 1.07 10(mmol/l) Mg²⁺
D-glucose 6-phosphate S 1.63 10(mmol/l) ATP
D-glucose 6-phosphate S 1.12
10(mmol/l) ATP
10(mmol/l) Mg²⁺

AC	KT	NH	ADD
D-glucose 6-phosphate	S	1.58	10(mmol/l) P_i
D-glucose 6-phosphate	H	1.0
D-glucose 6-phosphate	S	1.5	10(mmol/l) P_i 10(mmol/l) Mg²⁺
D-glucose 6-phosphate	H	1.07	10(mmol/l) Mg²⁺
D-glucose 6-phosphate	S	1.63	10(mmol/l) ATP
D-glucose 6-phosphate	S	1.12	10(mmol/l) ATP 10(mmol/l) Mg²⁺