Question

Glucose-6-phosphate dehydrogenase catalyzesthe first step of the Pentose Phosphate Pathway. This enzyme is highly specific for NADP+NADP+; the KmKm​for NAD+NAD+is about a thousand times that of NADP+NADP+. What does this mean in terms of affinity of the enzyme for NAD+NAD+and NADP+NADP+?

Answer 1

Enzymes have varying tendencies to bind their substrates (affinities). The Michaelis constant (KM) is defined as the substrate concentration at which the reaction rate is half of its maximal value (or in other words it defines the substrate concentration at which half of the active sites are occupied). It indicates the affinity of an enzyme for a given substrate: the lower the KM value, the higher the affinity of the enzyme for the substrate. Here, the Km of NAD+ is higher and for NADP+ is lower. HIgher the Km lower the affinity and vice versa. Therefore, the affinity of G6P dehydrogenase is lower for NAD+ and higher for NADP+.