What is the difference between beta and alpha glucose?

Answer 1

First of all: "Beta glucose" and "Alpha glucose" aren't actually the correct, formal terms. If you were taking a biochemistry test using those names would be marked wrong.

You see: glucose wraps into a ring.

The molecule of glucose is often explained as a chain of carbon molecules with hydroxyl and hydrogen groups attached, but whenever it is dissolved in water it spontaneously loops around to form a ring - the same way that you can take a rope and take the two free ends and bring them together and tie them into a knot to make a loop.

Well, just like tying a knot, there are two ways to do it: in a rope you can either start with laying the right end over the left end, or you can start with the left end lying over the right end (okay - to make a full knot that holds the loop together you have to do it twice, so there are really 4 ways to do it with the rope - but in a glucose molecule you only need to do it once).

In a rope the result may look the same, but actually these two ways of looping together are mirror images of each other. And looping glucose together one way or the other may not seem like a big deal either, but glucose, like most organic molecules, is asymmetric, and because of this there are certain properties of the way the molecule behaves that are goverened by the exact orientation of the molecules. In biolochemistry this is important because many enzymes that build or break down organic molecules can only work on a molecule of one specific orientation or the other, but not both.

In glucose when you make this loop what you are really doing is linking carbon number 1 (in a straight-chain glucose carbon 1 would be linked to an oxygen atom in an aldehyde configuration with a double bond between the two atoms while all the other oxygen atoms in the molecule were only as hydroxyl groups) to the hydroxyl group from carbon number 5 (and in doing so the aldehyde group converts into another hydroxyl group). If you do this in a way that carbon number 6 is swiveled around to end up OPPOSITE from the new hydroxyl group on carbon 1 (meaning if you think of this 6-atom ring as a flat ring with the hydroxyl groups and carbon 6 sticking up or down - it isn't, but it's easier to visualize that way and the geometry of the bent ring isn't any different - these 2 structures would be on opposite sides of the ring), then it is an alpha configuration.

Formally speaking, the 6-atom ring structure which is formed is "Glucopyranose" -- Alpha-glucopyranose if carbon number 6 is opposite to the carbon-1 hydroxyl group, and Beta-glycopyranose if carbon number 6 is on the same side as the carbon-1 hydroxyl group.

And while I have been writing that "you make" these structures - they're not really made: glucose molecules in water spontaneously flip back and forth between glycopyranose and the straight chain constantly. In typically-dilute water solutions seen in living organisms more than 99% of glucose molecules will be in either alpha or beta glucopyranose form.

You can also make 5-atom ring structures (with carbons 5 and 6 hanging off like a tail) which are also alpha and beta - but this form is called a glucofuranose. In some chemistry labs this is important, but in biological systems the percentage of glucofuranose present, either alpha or beta, is negligible.

But now the big question: WHY is alpha vs. beta glucofuranose important?

Short answer: They are biochemically different, and used differently. Most importantly:

Alpha-glucofuranose chemically linked together in chains is either starch or glycogen; either way: humans and most animals have the enzymes to break this back down to glucose to burn for fuel.

Beta-glucofuranose chemically linked together in chains is called cellulose - wood! And no animal on earth has the enzymes to break this down to glucose again - only fungi or microbes (including the microbes in the guts of cows or termites; that's how cows live on grass - the animals can't do it themselves).