An L-stereoisomer of glucose molecule, which is comparatively less biologically active than D-glucose, and one that rotates the plane polarized light in an anticlockwise direction (as opposed to D-glucose that rotates it in clockwise direction)
Glucose is a hexose monosaccharide since it is comprised of six carbon atoms. One of the carbon atoms is part of an aldehyde functional group. Hence, it is member of the aldohexose group. Depending on the position of the hydroxyl (-OH) groups, it may either be dextro- (as in D-glucose) or laevo- (as in L-glucose). This nomenclature based on Fischer projection designates D- when it rotates the plane polarized light in the clockwise direction. L- is when it rotates the plane polarized light in an anticlockwise direction.
L-glucose occurs more abundantly in nature than L-glucose. L-glucose is a short form of Levorotatory-glucose. It is one of the two stereoisomers of glucose (the other is D-glucose). Compared with D-glucose, the L-glucose is less biologically active and less common. In higher forms of organisms, the L-glucose is not produced naturally. It is synthesized artificially in a laboratory.
L-glucose cannot be used as a source of energy in cellular respiration. That is because hexokinase cannot phosphorylate it during glycolysis. Burkholderia caryophylli, though, has the enzyme D-threo-aldose 1-dehydrogenase that enables the oxidation of L-glucose.1
1 Sasajima, K., & Sinskey, A. (1979). "Oxidation of l-glucose by a Pseudomonad". Biochimica et Biophysica Acta (BBA) – Enzymology, 571: 120–126.