Hexokinase Information

Enzyme Name

Hexokinase

Reaction Catalyzed

ATP dependent phosphorylation of Glucose

Reaction Type

Group Transfer

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

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"); to C1 (the aldehyde on the right)? Why or why not?

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

DG^o'

-16 kJ/M

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.

K_eq

~600=

Comments

Why is ATP required? What would the DG^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

DG for these conditions

-27 kJ/M

Mechanism for Chemistry

Mechanism for Enzyme

Pictures of Enzyme

1. Open Ribbon Hexokinase without substrates bound is in an "Open" conformation. Here only the main chain is represented by these ribbons.
2. Open Spheres colored Hexokinase without substrates bound is in an "Open" conformation. All of the atoms in the protein are represented by atom colored spheres. C=Gray; O=red; S=yellow.
3. Open Spheres green Hexokinase without substrates bound is in an "Open" conformation. All of the atoms in the protein are represented by green colored spheres. (this is to compare with selections 7 and 8 below)
4. Closed Ribbon Hexokinase with substrates bound is in an "Closed" conformation. Here only the main chain is represented by these red ribbons.
5. Closed Ribbon + substrates Hexokinase with substrates bound is in an "Closed" conformation. Here the main chain is represented by these red ribbons. The substrates are added as the atom colored spheres.
6. Closed Spheres colored Hexokinase with substrates bound is in an "Closed" conformation. All of the atoms in the protein are represented by atom colored spheres. C=Gray; O=red; S=yellow.
7. Closed Spheres red Hexokinase with substrates bound is in an "Closed" conformation. All of the atoms in the protein are represented by red colored spheres. (this is for direct comparison of items 3 above and 8 below)
8. Closed red - Open green Comparison of the two conformations of hexokinase. "Open" in green and "Closed" in red.