Diabetics are Carb intolerant

[Yorksman]

Actually there arn't that many.
Dietary carbohydrates would be glucose, fructose, galactose, maltose, sucrose, lactose, maltodextrin, galactan, amylose & amylopectin.
On the enzyme front there is alpha-amylase, glaucoamylase, sucrase & lactase.

But that is a very small subsection and doesn't even contain the most important oligosaccharides, raffinose and stachyose which are to be found in legumes. Carbohydrates are molecules consisting of carbon, hydrogen and oxygen atoms. There are hundreds if not thousands of ways of combining them. The way they are bonded to each other is of major importance. The reason why humans don't digest beta carbs, such as those found in grass or trees or leaves is that the bonds in beta carbs are stronger than those in alpha carbs and we lack enzymes to break the sugars apart. Lactose is the only beta carb that some humans, but by no means all, can digest. Not all humans produce the enzyme lactase.

The family of digestible alpha carbs fall into monosaccharides, disaccharides, oligosaccharides, polysaccharides and nucleotides. Each category contains many different types and different human biologies handle them in different ways. The list of enzymes to handle them all is considerable. The Carbohydrate-Active Enzyme (CAZy) database lists enzymes that build and breakdown complex carbohydrates and glycoconjugates. It describes the present knowledge on 113 glycoside hydrolase, 91 glycosyltransferase, 19 polysaccharide lyase, 15 carbohydrate esterase and 52 carbohydrate-binding module families.


I think you may be thinking more in terms of the basic molecular building blocks, the simplest being the monosaccharides, ie the single sugar units but even glucose can exist in straight chain or ring form, see image below. The way the atoms are arranged in the single units however makes a huge difference. The extreme variety of monosaccharide structures, the variety intersugar linkages and the fact that virtually all types of molecules can be glycosylated (from sugars themselves, to proteins, lipids, nucleic acids, antibiotics, etc.), the large variety of enzymes acting on these glycoconjugates, oligo- and polysaccharides probably constitute one of the most structurally diverse set of substrates on Earth. Wiki has a page on the growing list Carbohydrate Active Enzymes with a link to the CAZy database.