Question

You measure the Km for hexokinase derived from different tissues and find it differ, why?

Answer 1

Ans) Compared to hexokinase, glucokinase has a higher Km (lower binding affinity) and a higher Vmax (increased capacity). Hexokinase is found in most tissues.

- Hexokinase is present in most all mammalian cells in low concentrations. The low Km for hexokinase (10−5M or 0.9 mg%) means that the enzyme is working at maximal velocity (Vmax) at extremely low glucose concentrations.

- But this enzyme is powerfully inhibited by its product, Glc-6-P. Hence, when Glc-6-P is not rapidly utilized, that is to say, no longer needed by the cell, hexokinase activity is reduced. This enzyme, also, is not inducible, and although insulin increases GLUT 4 transporter activity in plasma membranes of muscle fibers and adipocytes, neither its presence nor absence has any direct effect on hexokinase activity. Insulin increases cell membrane permeability of muscle cells and adipocytes to glucose, therefore, glucose entry into these cells becomes limited by the presence or absence of insulin, as well as by the cell’s ability to metabolize Glc-6-P.

- When muscle contraction stops, for example, the build-up of intracellular Glc-6-P will limit further phosphorylation of glucose, which in turn should limit further “trapping” of glucose inside inactive cells. Hexokinase will also phosphorylate fructose, but at a much slower rate than glucose.