hexokinase

[benneh]

plain question really, can anyone help?

does the hexokinase enzyme have an active site complimentary to ONLY glucose, meaning you have to have glucose (not other sugars) to start respiration?

cheers

[Dr Noodle]

http://en.wikipedia.org/wiki/Hexokinase

Says glucose is the most important substrate not the only one. Type it into google, you'll find lots of things. Nobody has answered you probably because they don't agree with you asking for answers. The internet is a vital tool for research, learn how to use it.

[benneh]

thanks for reply but no need for the caustic tone. im doing coursework and have already been looking for 2 hours on the internet (trying my best to learn how to use it of course) and found no evidence of how the enzyme works on other substrates.

i know hexokinase A, B and C have high affinity for glucose, and that hexokinase occurs in different isozymes with different Km values but that doesnt really tell me anything

[chicken66]

searching for 'hexokinase' and 'substrate' brings up this paper
http://cancerres.aacrjournals.org/cg...t/32/5/948.pdf
search within the paper for 'hexokinase', and there's a bit in there that may answer your question...if that's of help

[Pammy]

I strongly disagree with the advice to students to rely strongly on the internet. I usually tell my students: don't touch the internet, at least not until you've covered the basics in a reputable textbook.
There is so much rubbish on the internet...And if you know nothing about a topic, you have no means to distinguish what is good and what is bad. You google it up, click on the first page, and memorise what you read, taking it at face value...that's not a way of working you want to cultivate, especially as a doctor. The number of times I have been confronted with utterly wrong, out of context statements in answers by my students is astronomical - it's invariably due to their trying to save time by using the web.
Take some basic biochemistry textbooks, or a metabolism textbook, and read the chapter on glycolysis, and take it from there.

And read the chapter on enzymes! Then you will find out that enzyme active sites are generally not complementary to the substrate, but to the transition state of a reaction...That way, you lower the energy barrier which means that the reaction is faster. The most important thing about proteins is that they are dynamic, not static, molecules. Active sites close and open, and change shape during the course of a reaction. In hexokinase, upon binding of glucose, the active site changes shape to bring ATP (which donates a phosphate) in close proximity to the glucose, ready to react away!

Most enzymes can take alternative substrates. Hexokinase included! The types of studies where you systematically search for alternative substrates are very useful as they allow you to find out about the characteristics of a substrate that is crucial for it to be allowed by the enzyme.
The exact substrates for each hexokinase depend on where it is from! In general they will of course be things similar to glucose, e.g. mannose, deoxyglucose, glucosamine etc.
Hope this helps!
Pammy

[Dr Noodle]

My apologies Benneh, it was a bit bitchy.

Pammy isn't it a good idea to learn how to sift through the rubbish, I thought that's a big part of learning how to research?

[benneh]

thanks pammy. i didnt suceed on the internet so asked my chemistry teacher. he's given me a biochemistry book for the weekend and i will certainly read about glycolysis and enzymes from a deeper perspective

this is all for my bio coursework on how different sugars affect rate of respiration. not particularily enjoying it!

cheers

[Pammy]

Dr Noodle, I agree that it is important to learn to use the web appropriately...However if you know nothing about a topic, you can't judge what is rubbish and what isn't!
Mostly, later at uni, you end up using medical search engines (e.g. PubMed, Medline, which give publications) rather than the web as such...The problem remains the same: lots of scientific papers have flaws, but if you don't know anything about a topic it's hard to tell what these are...So I usually tell students to start from the basics, get an overview first (or at a higher level, read some reviews - not just one, as they can be quite biased as well).

Regarding the coursework: I am not sure what sugars in particular you are looking at and I don't want to help too much :-) But it might be worth considering that some sugars can be broken down to or converted to glucose - the biochemistry textbook will give you an overview of metabolic pathways for the different sugars (respiration with these substrates will then be slower). It might be way too detailed for what you need, but in the metabolism section of most biochem textbooks there is a reasonable introductory chapter. A relatively simple textbook is by Elliott & Elliott, and also the baby Voet & Voet (think it's called "principles of biochemistry" or something like that, don't take the large version which is just called "biochemistry" and comes in two volumes). While I hate to recommend them, there's also Instant Notes in Biochemistry!
Pammy

[Kyle2008]

Biochemistry is the study of the chemical processes in living organisms. It deals with the structure and function of cellular components, such as proteins, carbohydrates, lipids, nucleic acids, and other biomolecules.
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Kyle
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[Christa]

A hexokinase is an enzyme that phosphorylates a six carbon sugar, a hexose, to a hexose phosphate. In most tissues and organisms, glucose is the most important substrate of hexokinases, and glucose 6 phosphate the most important product. Hexokinases have been found in every organism checked, ranging from bacteria, yeast, and plants to humans and other vertebrates. They are categorized as actin fold proteins, sharing a common ATP binding site core surrounded by more variable sequences that determine substrate affinities and other properties. Several hexokinase isoforms or isozymes providing different functions can occur in a single species.
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Christa


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