Did you say during last Tuesday (9/15) evening's lecture that
alpha glucoses only bond with other alphas and betas only bond with betas? The result
being cellulose and glycogen (starch) and there are no inbetweens.For example, There
aren't glucose polymers that are partially dense and linear (cellulose) and partially
helical. Is that what you meant?
The homogeneity of bonds (all alpha or all beta) to which I referred in lecture was
meant to apply only to the specific polymers under discusson, i.e, glycogen/starch and
cellulose. In the former it is all alphas and in the latter all betas. But polysaccharides
exist that contain a mixture of alphas and betas (dermatan sulfate). However, even here,
that pattern is repetitious and thus still monotonous, i.e., a,b,a,b,a,b,a,b,a,b, and NOT
In class yesterday, you were speaking about the formation of
disaccharides. During the lecture, I thought I heard you say that disaccharides could only
have alpha-alpha links or beta-beta links. I thought that the formation of sucrose was an
alpha-beta link. Am I wrong to think that you can have all three links? Please clarify.
The homogeneity of linkage type to which I was referring in lecture was in starch/glycogen
(1,4's all alpha) and cellulose (1,4's all beta), specifically. I did not mean to rule out
linkage of more than one type in other polysaccharides. On the other hand, even if such
diversity were present it would not change the basically monotonous character of most
polysaccharides, the linkage pattern would be repeated, eg., a,b,a,b,a,b,a,b and not
a,b,b,b,a,a,b,a,a,a,a,b. As we will soon see, such monotony is NOT characteristic of most
cellular polymers (i.e., proteins and nucleic acids).
In relation to the two questions about the uniformity (or at
least repetitiveness) of bonds in the sugar polymers: this uniformity isn't a result of
anything inherent to the sugars themselves, right? It's a function of the enzymes that put
the macromolecules together, that they're 'programmed' to make only that kind (or series)
Right. There are only a few enzymes to do this job, and an enzyme is usually designed to
do just one job (and do it well, as we shall see). Thus the repetitiveness.
We've seen alpha-glucose->alpha-glucose, and
beta-glucose->beta-glucose: so what does alpha-glucose->beta-glucose make?
The alpha or beta conformation is locked in only where a glycosidic bond is made. The C1
that is not involved in bonding will be in equilibrium between alpha and beta (mixture)
just as with the free monomer. Thus we can only talk about a glucose-alpha-glucose or