Enzyme Kinetics

__**Enzymes Part 2 - Kinetics**__

**Application of Km and Vmax: Glucokinase and Hexokinase**

* Hexokinase (most tissue) contrasts w/ glucokinase (liver) by the following:
* Hex has lower Vmax, lower Km, greater affinity for glucose
* Turned off by high concentrations of glucose 6-P
* Both HexoK and GlucoK catalyze Glucose → Glucose 6-P
* Glycolysis breaks down glucose
* Glycogenesis stores glucose
* Glucokinase higher Km tells us it has lower affinity for glucose, compared to hexokinase
* Hexokinase is active during fasting
* Greater affinity, can convert even small amounts of glucose into energy
* Glucokinase only becomes active after a high-carb meal
* Lower affinity, needs a lot of glucose to be activated
* Main role in glycogenesis because you want to store this excess
* Liver
* Where GK is found
* Nutrients absorbed from intestines go here first; GK converts excess glucose from a meal to glycogen
* Other tissues cant convert excess glucose to glycogen because Glucose 6-P inhibits Hexokinase (product inhibition)
* Hexokinase only works until you have made enough G6-P to serve body’s energy needs
* HK feeds into glycolysis; explains low Km because even though you have little glucose, HK binds to it because it has a greater affinity
* Makes sense that GK has a higher Km because we do not want it storing glucose unless we have already satisfied our energy needs
* HK is inhibited by G6-P because it is mediating glycolysis; once you’ve made enough G6-P to fulfill you energy needs, HK turns off and GK turns on to store as glycogen for later use. Once glucose levels fall, GK turns off because it has a low affinity for glucose
* Vmax
* **High Vmax** = high capacity to turn substrate into product
* Allows GK to phosphorylate glucose to glucose 6-P after a meal to store as glycogen
* Adding P to glucose means it gets trapped in the cell
* Km
* Helps determine enzyme efficiency
* How well it binds (affinity) and how quick it converts to product
* Kcat/Km
* Kcat measures speed of product formation once ES has been made
* Km measures binding affinity of E and S to make ES
* E + S