Hexokinase

Enzyme Name	Hexokinase

Reaction Catalyzed	ATP dependent phosphorylation of Glucose
Reaction Type	Group Transfer
Pathway involvement	Glycolysis ONLY	Hexokinase is not part of the gluconeogenesis pathway. In gluconeogenesis a separate enzyme (Glucose-6-Phosphatase) hydrolyzes a phosphate from Glucose-6P and is not ATP dependent. Rather it performs a simple hydrolysis reaction to cleave off phosphate.
Cofactors/Cosubstrates	ATP is a cosubstrate; ADP is a coproduct	cosubstrate/coproduct = molecules that enter and leave the active site along with the substrate/product and are generally altered in the reaction as well cofactors = nonprotein molecules that are required for a reaction that are generally a permanent part of the enzyme. These remain unaltered after the reaction is complete.

Rationale

Most cellular metabolites are charged. The reason is to maintain them inside the cell. Uncharged molecules, like glucose for instance, are free to diffuse through the cell membrane, but charged molecules cannot diffuse through the membrane. Remember the "core" of the membrane is hydrophobic. Charged molecules and hydrophobic ones do not mix well. A good source of charge at pH 7 will be phosphate groups.

Once a cell begins the metabolic processes on a specific molecule it would be a shame if it left the cell and went somewhere else. Like money in your bank account, you sure do not want it "leaking" to other accounts in the bank!

So the first step in glycolysis is to put a phosphate on the glucose, in an ATP dependent group transfer. Why does it NEED to be an ATP dependent group transfer? why not just put a phosphate on using a condensation reaction (reverse of a hydrolysis)? Think thermodynamics here... What do we know about the thermodynamics of hydrolysis reactions

Question

How many places on glucose COULD the phosphate be transferred to? It is important to recall the mechanism of group transfer reactions here.

COULD it be transferred to C6 (the alcohol at the "top"); the C5 alcohol, the C1 (the aldehyde on the right)? Why or why not?

Propose a reason why the phosphate is transferred exclusively to the C6 alcohol of glucose by this enzyme.

ΔG^o'	-16 kJ/M
K_eq	~600=
Comments	Starting from standard state and allowing the reaction to come to equilibrium the product concentrations would end up ~600 times higher than the substrates. This extremely favorable Standard Free Energy is helpful to get the entire pathway rolling in the right direction. Why is ATP required? What would the ΔG^o' for putting a phosphate on glucose be if we used inorganic phosphate (reversal of a hydrolysis reaction) instead of ATP? It would be +15 kJ/M. Which direction would be favored then?
Substrate Concentrations^*
S₁=	Glucose	5mM
S₂=	ATP	2mM
P₁=	Glucose-6-phosphate	0.83 mM
P₂=	ADP	0.14 mM
ΔG for these conditions		-27 kJ/M
	Physiology of properly working bodies works very hard to maintain constantly high glucose and ATP concetrations. The very high negative ΔG ensures that the reaction overwhelmingly occurs in the direction of glycolysis. This does not mean that it ONLY proceeds in this direction only that the rate of reaction in the glycoysis direction is much faster than the "reverse" direction. While nothing can be indicated regarding the overall observed reaction rate we can say that under these conditions the "forward" rate is 50,000 times faster than the "reverse".

Mechanism for Chemistry
Mechanism for Enzyme	Hover your mouse over a step to reveal a description `Hexokinase. Animation of an Hexokinase reaction Blue: represents the enzyme. THe EA^- is the Aspartate from the enzyme active site in it's basic (deprotonated) state. "Start" begins an animation of the group transfer reaction. It proceeds through the reaction in the "forward" direction and then "backwards" again. Note how the enzyme is involved. "+" increases speed while "-" decreases the animation speed. You may also step through the reaction using "next" or "previous"`

Pictures of Enzyme		Oscillate between the "Open Ribbon" and the "Closed Ribbon" pictures to compare the two structures separately. This is a case where the observed enzyme structure changes somewhat depending on substrates binding to the active site. In the "open conformation" there is a lot of water in the active site. In the "closed form" on the other hand there is no room for both the substrates AND water. This is important since theoretically water could just as easily be deprotonated by the enzyme as glucose. If water is deprotonated and attacks the ATP the end result would be ATP hydrolysis (to ADP and PO₄) and no useful reaction would occur.


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Anaerobic Glucose Metabolism

Hexokinase Information

Hexokinase