Question

Explain why it is beneficial for regulation of fasting blood glucose concentrations that the liver express

Glucokinase (Type IV hexokinase) rather than hexokinase type I or II?

Answer 1

There are 4 types of Hexokinase found in mammalian tissue. Among them hexokinase I,II and II are isoforms whereas hexokinase IV can also be termed as glucokinase GK which varies widely from the former. Both hexokinase and glucokinase are enzymes which phophorylate glucose to G-6-P thereby it enters the cell.

Hexokinase I is relatively ubiquitous. It is expressed in skeletal muscle and β-cells but the brain and kidney has the highest levels of expression of kexokinase I. Hexokinase II is found in skeletal muscle and adipose tissues and plays key role in glucose uptake of insulin-responsive skeletal muscle.

Glucokinase is seen in hepatocytes of liver and beta cells of pancreas. These tissues needs to respond rapidly to change in blood glucose level. Glucokinase also controls the rate of entry of glucose into glycolytic pathway and its subsequent metabolism and so serves as a glucose sensory. It also acts in key role for storing glucose as glycogen especially in the postprandial state. In such way, it seems to be be beneficial to regulates fasting blood glucose level.  

By comparing, glucokinase has higher Km and higher Vmax which means it has lower affinity and higher capacity and can be induced by insulin. Whereas, hexokinase has lower Km (higher binding affinity) and lower V max(lower capacity) and cannot be induced by insulin but regulated via negative feed back inhibition by G-6-P.