Monosaccharides are the monomeric structures of carbohydrates. In other words, they are the most basic structures that are present in the carbohydrate group. Let’s understand its structure then. The most common skeletons are composed of unbranched carbons, all joined by simple bonds, as if it were a little carbon train. We know that one of the carbons is double linked to an oxygen, thus forming a carbonyl group. The other carbons are not left alone, and each is linked to a hydroxyl group . Now, when the carbonyl group is leading the train, that is, it is at the end of the chain, we call the monosaccharide aldose. Ças the carbonyl group is present in any other wagon, in this case, in any carbon other than the end, we say that the monosaccharide is a ketosis .
Number of Carbons:
Regarding the number of carbons, monosaccharides can be classified into trioses , tetroses , pentoses , hexoses and heptoses , containing three, four, five, six or seven carbons, respectively.
It is worth mentioning that there are several stereoisomers – isomers of similar structure, but with different spatial arrangement – of saccharides . And why is that? All monosaccharides, with the exception of only one, dihydroxyacetone , have asymmetric carbons called chiral carbons , which receive this classification because they are saturated with four bonds with different chemical units . In addition, they are also central carbons, so they are never at the end of the molecule.
But what does this have to do with stereoisomery ?
Units that bind to chiral carbon can alter their arrangement, forming different stereoisomers. Below is an example of a glucose molecule:
First thing to be identified in the molecule is to know its chiral carbon. In monosaccharides, we know that chiral carbon is the one that is located the furthest from the carbon of the carbonyl group. In the case of glucose, it is the 5th “top to bottom” carbon in the image. Also note in this example, which we call D-glucose, when the hydroxyl group of the chiral carbon is located to the right of the molecule, that’s why D ( right ). In L-glucose, on the other hand, we find the hydroxyl group on the left side, so L ( levo ).
It is important to understand the stereoisomerism of monosaccharides, as each arrangement has a biologically active form . In addition, the action of enzymes is strictly stereospecific , that is, even though it is a glucose molecule, an enzyme made to act on D-glucose does not work in the L-glucose arrangement.