Definitions:

Isomer: of a compound is a separate compound in which each molecule contains the same constituent atoms as the first compound, but with those atoms arranged differently. Thus isomers are different compounds having identical chemical formula.

Glucose, fructose and galactose are isomers with empirical formul (C6H12O6). A structural isomer of glucose, such as fructose, has identical chemical groups bonded to different carbon atoms. A sterioisomer of glucose such as galactose, has identical chemical groups bonded to the same carbon atoms but in different orientations. 

Enzymes that act on different sugars can distinguish both the structural and steroioisomers of this basic 6 carbon skeleton. 

–Structural Isomers: If there are differences in the actual structure of carbon skeleton of for example organic molecules, the molecuels are called structural isomers. Glucose and fructose, for example, are structural isomers of C6H12O6. Fructose is a structural isomer that differs in the position of the carbonyl carbon C≈O. Your taste buds can differentiate fruactose which tasts much sweeter than glucose, despite the fact that both sugars have identical chemical compositions. 

–Stereoisomer: is an isomer in which the same atoms are bonded to the same other atoms, but where the configuration of those atoms in three dimensions differs. For example, a dashed triangle leading from a marked carbon to a H atom indicates that the H lies below the planes of the two five-sided rings of which the carbon atom might be a part. If the H atom lies above the planes of the rings, then the resulting structure is a stereoisomer.

Thus steroisomers have the same carbon skeleton but difffer in how the groups attached to this skeleton are arranged in space. Enzymes in biologcial systems usually recognize only a single, specific steroisomer. 

—-Enantiomers: are stereoisomers (spatial isomers) wherein the isomeric compounds have the same chemical formula and the same chemical structure, but differ in their orientation in three-dimensional space. Such stereoisomers can exist for all molecules that contain an asymmetric carbon atom. An “asymmetric carbon” is a C atom to which four different substituents are attached, whereby, due to the tetrahedral structure of C bonds in 3 dimensions, the spatial orientation of substituents attached to a C atom varies. When there is only one asymmetric C atom in the molecule and thus only 2 stereoisomers, these isomers are called enantiomers.

Enantiomers are thus mirror images of each other. A molecule that has mirror image versions is called a chiral molecule. The two molecules have the same groups but cannot be usperimposed much like your two hands. 

Enantiomers are stereoisomers that are nonsuperimposable mirror images of each other like left and right hands. Enantiomers are identified and distinguished by their optical characteristics when a purified solution of the separated isomers is exposed to plane-polarized light. Enantiomers accordingly exhibit different optical activity; the enantiomer that roates a plan of polarized light in the clockwise direction is the (+)- enantiomer (also called the destrorotaory of d- isomer); the enantiomer that rotates a plane of polarized light in the counterclockwise direction is the (-)-enantiomer (also called the levorotatory or l- isomer).

Enantiomers may also be designated as the S-enantiomer and the R-enantiomer according to a different criterion relating to the location of the chiral centers. “Chiral” is defined as describing asymmetric molecules that are mirror immages of each other (i.e., like right and left hands).

Although enantiomers h ave nearly identical physical properties, they often have very different biological activities. This is due to the chirality of biological molecules, such as proteins, and the resulting affect on, for example, enzyme active sites. Whereas an enzyme may recognize and catalyze a reaction with one enatiomer due to physical and chemcial complementarity with the enzyme’s active site, the same enzyme may not recognize the other enantiomer, due to noncomplementarity.

In vivo, one enantiomer may be converted to the other enantiomer through racemization which is a process whereby a compound consisting of a single enantiomer is converted to a one to one mixture of that enantiomer and its opposit (i.e., the racemate) by the cleavage and reformation of a chemcial bond at the chiral center of the molecule. Racemates are mixtures of equal amounts of enantiomers and are denoted as (d,l) or (+/-_ pairs for the steroisomerism of a given chiral center. Since there are 2 enantiomers in a racemate of a compound with a single chiral center, a racemate potentially compirses a very small genus of two species. As the number of chiral centers increases, however, the number of species within a racemate also increase. This increase would be 2n where n is the number of chiral centers.

Diastereomers: are stereoisomers that are not enantiomers.