Wikipedia
L-Glucose is an organic compound with formula CHO or H–(C=O)–(CHOH)–H, specifically one of the aldohexose monosaccharides. As the L-isomer of glucose, it is the enantiomer of the more common D-glucose.
L-Glucose does not occur naturally in higher living organisms, but can be synthesized in the laboratory. L-Glucose is indistinguishable in taste from D-glucose, but cannot be used by living organisms as source of energy because it cannot be phosphorylated by hexokinase, the first enzyme in the glycolysis pathway. One of the known exceptions is in Burkholderia caryophylli, a plant pathogenic bacterium, which contains the enzyme D-threo-aldose 1-dehydrogenase which is capable of oxidizing L-glucose
Like the D-isomer, L-glucose usually occurs as one of four cyclic structural isomers — α- and β-L-glucopyranose (the most common, with a six-atom ring), and α- and β-L-glucofuranose (with a five-atom ring). In water solution, these isomers interconvert in matters of hours, with the open-chain form as an intermediate stage.