Talk:Hexokinase

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Tito4000, i agree that Mg²⁺ is essential for the reaction, but you should have not mentioned it because then to keep it fair we have to mention it in ALL articles about enzymes, classical and signaling, that use ATP. In every biochemistry textbook hexokinase is the first enzyme that is discussed, as glycolysis is always the first pathway in the "metabolism" part of the book. So mention it there in such way makes sence, but here on Wikipedia - no. Also Mg²⁺ is not a cofactor - cofactors by definition are the non-protein part of the enzymes, as you probably know cofactors are either prostetic groups - tightly bonded (most of the time covalently), and coenzymes - loosly. Regardless of that they all share one thing in common - they all undergo chemical transformation during the reaction, i'm trying to think about an exeption but i can't recall, while Mg²⁺ does not - it just "shields" the negative charges of ATP as to allow an easier access to the ATP's γ phosphate by the attacking nucleophil group of the phosphorylated compound, which is -CH₂-OH of Glc in our case. Boris 00:25, 8 December 2005 (UTC)

Hi Boris. Thank you for your comments. If MgATP is the true substrate in most reactions where ATP is a substrate, well it should be mentioned as such. The fact that it's a general feature of ATP should not preclude our mentioning it.

On the other hand, it is important to note that for the case of hexokinase in particular, non-complexed ATP is a compound that acts upon hexokinase as an agent that releases it from its binding to Porin on the outer mitochondrial membrane (Bustamante,E., and Pedersen, P.L.(1980). "Mitochondrial Hexokinase of Rat Hepatoma Cells in Culture: Solubilization and Kinetic Properties". Biochemistry 19: 4972-4977). Free ATP, unchelated by Mg ions, induces the release of the enzyme from the membrane, whereas the MgATP complex is ineffective. The bound and the ATP-solubilized forms of mitochondrial hexokinase from hepatoma cells are kinetically different.

Consequently, I believe that it is key to mention that MgATP is the true substrate for the hexokinase reaction whilst perhaps it should be worth mentioning the fact that unchelated ATP is actually an agent that releases the protein from the external mitochondrial membrane.--Tito4000 15:54, 8 December 2005 (UTC)

Tito, you made the same change about Mg being the "true substrate" to glucokinase. What do you mean by "true substrate"? This seems strange, since the Mg remains unaltered other than the reversible electron change, while the hexose to hexose phosphate reaction is one of the most important steps in carbohydrate metabolism. "True substrate" strikes me as a nearly meaningless term-- are you claiming that hexose is not a substrate? I was going to delete the sentence in GK also as meaningless or misleading to a reader but wanted to ask you for an explanation first. alteripse 01:07, 9 December 2005 (UTC)

Hi Boris. I don't know if you got to read my previous comment on this page since you make no mention of it. At any rate, by using the word 'true' I'm not intending to imply that MgATP is more of a substrate than the hexose. I'm not saying that MgATP is the 'true' substrate to stress that glucose is a 'false' substrate. Not at all. My intention is to simply state that MgATP is the substrate and not ATP (which is fact is an agent that exerts kinetic change through release from the mitochondrial membrane). It is a fact that the Mg portion of MgATP doesn't change upon enzyme activity but neither does the oxygen residue on carbon 4. Please feel free to rephrase the statements or to simply delete them as you already did. --Tito4000 02:56, 9 December 2005 (UTC)

Oops!, I've just realized that the comment to which I was answering was written by Alteripse and not by Boris. Sorry about that, Alteripse. Nevertheless, my response stands. Boris and Alteripse, please read my two comments. Thank you. --Tito4000 03:05, 9 December 2005 (UTC)

ok thanks i will modify

[edit] Possible image

User:TimVickers produced the following image which I nominated for deletion last week because it wasn't being used anywhere. User:Deryck Chan was nice enough to look through my nominations at my request and thought that this one was worth saving, and might be useful on this article. ~ BigrTex 16:47, 5 February 2007 (UTC)