So two protein molecules that are different are said to be isozymes if they essentially catalyze that same, similar type of process. And the isozyme to the hexokinase that is found in liver cells but is not found in skeleton muscle cells, is glucokinase. So, unlike in skeleton muscle tissue, there is an isozyme to hexokinase present liver cells known as glucokinase. Now, what is the major difference between glucokinase and hexokinase? Well, there are two important differences. Number one, glucokinase has a much lower affinity for that glucose substrate molecule than hexokinase.