Wolfram - 21-12-2003 at 17:52
The enzymes in glycolysis, do they fit the ring-closed forms of the sugars or the open? The enymes can not fit both forms ofcourse, and I have seen
both in the books so which is the right?
chemoleo - 21-12-2003 at 18:46
The first reactions definitely use the ring form. I.e. hexokinase, which phosphorylates glucose on C6, works on the ring form. For the next reaction,
again, the ring form is recognised, howevver, as isomerisation to fuctose 6 phosphate happens here, you can imagine that the ring has to be opened,
and closed in a different manner. One of the transition state of this reaction will probably be in the linear form.
Anyway, as far as I remember from carbohydrate chemistry lectures, moste aldo/keto sugars occur in the ring form (due to an intramolecular reaction).
There is an equilibrium between the alpha and beta forms, and both can exist in the trough or chair kind of way (viewed sideways). The preferred
shape of the glucose ring is determined by the axial or equitorial positions of the hydroxyls, particularly on C1/C2 or wherever the aldo/keto groups
are 
btw, there are sugars that dont form rings, such as mannitol (H2COH-(HCOH)4-H2COH). That's because there is no carbonyl that is needed for the
intramolecular reaction!
[Edited on 22-12-2003 by chemoleo]
PHILOU Zrealone - 24-5-2004 at 17:13
Normaly sugars means osides or polyosides...in the swetener family you get sugars and polyols (like sorbitol, manitol, inositol,...) and other
molecules (sulfamates, sacharin, some big proteins...).
Confusion arise from the fact sugars and polyols are both carbohydrates...the later name comes from the property that upon contact with oleum or upon
heating water takes off and leaves carbon.
But the very difference between sugars and polyols is the intra or extramolecular hemicetal/cetal or hemiacetal/acetal part of the osides...the carbon
that holds that specific bond is called anomeric carbon.
Saccharose (note the end of the word) is common table suggar and the part of the molecule that gives the sweet taste is the open chain..this can only
happen in water system and the equilibrium set its % to less tha 3%...so in suggar 97% of the weight doesn't account for the sweet taste...not
surprising that other molecules outperforms it on the sweetener scale.
Cl-CH2-CH2-Cl has kind of a sweet taste and so many different molecules can give the sweet taste. Also glycol, glycerol...the apprent similitude of
all those is the proximity of two electronegative atoms (2 or 3 atoms distant) with free non binding electronic doublet.
Suggar baby love, suggar baby love....
Oh suggar ...honey honey....you are my candy girl...
I want candy ....
