Key Difference – D vs L Glucose
The “D” and “L” letter indication in the names of D-glucose and L-glucose are used to distinguish the structural differences in the glucose molecule. D-glucose and L-glucose are called enantiomers because their molecular structures are mirror images of each other. Therefore, the key difference between D and L glucose lies in their structure. The differences in their shapes can be best explained using the Fisher projection model; it is one of the ways of drawing organic molecules.
What is D Glucose?
D-glucose is the enantiomer of the L-glucose and it is also known as dextrose. Unlike L-glucose, it occurs widely in nature. Moreover, D-glucose is the most extensively used aldohexose in living organisms. For example, it is used as an energy source in most living organisms ranging from bacteria to human. These organisms get energy from D-glucose through aerobic or anaerobic respiration or fermentation.
What is L Glucose?
L-glucose is an organic compound and its IUPAC name is (2S,3R,4S,5S)-2,3,4,5,6-pentahydroxyhexanal. Its molecular formula and molecular weight are C6H12O6 and 180.16 gmol-1 respectively. L-glucose naturally occurs in fruits and other parts of plants in its free state. However, it is not found in higher living organisms. But, it can be synthetically produced in the laboratory. L-glucose has a similar taste which is equivalent to the taste of D-glucose. L-glucose cannot be consumed by living organisms as their energy source because it is not phosphorylated by hexokinase, which is the first enzyme in the glycolysis pathway.
What is the difference between D and L Glucose?
D and L glucose molecules have the same atoms although they only differ in their structure. The structural difference between D and L glucose molecules can be best identified by observing their flat mirror images rather than the two-dimensional Fischer projections.
D Glucose: In D-glucose, three hydroxyl groups and one hydrogen group are attached to the right. All the sugars derived from D-glyceraldehyde are D-sugars, including D-glucose and likewise L-glucose is derived from L-glyceraldehyde.
L Glucose: In L-glucose, the three hydroxyl groups and one hydrogen group are oriented to the left.
D Glucose: D-glucose can exist in both linear form and cyclic form. It has four different cyclic structures. In solutions, it is in an equilibrium mixture of α-D-glucopyranose and and β-D-glucopyranose.
L Glucose: L-glucose also exist in an equilibrium mixture of α-L-glucopyranose and β-L-glucopyranose.
D Glucose: D-glucose is the main energy source in most of the living organisms. It is consumed as the energy source in living cells by the processes such as aerobic respiration, anaerobic respiration or fermentation. The energy required for the human brain is taken from D-glucose. Therefore, D-glucose influences all thepsychological processes in the human body.
L Glucose: L-Glucose is a low-calorie sweetener which can be suggested for diabetes patients. But, it is not manufactured for sale due to its high manufacturing costs. In addition, the acetate derived from L-glucose, L-glucose pentaacetate is able to stimulate insulin release; that it might therefore be beneficial for type 2 diabetes too. Since L-glucose is laxative; it is proposed as a colon-cleansing agent.
The introduction of a phosphate group to a molecule or a compound.