General Properties, Occurrence, and Preparation
1.2
65
⊡ Figure 3
Haworth structures for α- and β-D-glucopyranose and their C1 and 1C conformations
in the C1-conformation, with only the hemiacetal hydroxyl group in the axial position. This is
the most likely reason that D-glucose is the predominant carbohydrate on the earth, as it is the
only D-hexose that can have all of its hydroxyl groups (exclusive of the hemiacetal hydroxyl
group) equatorial, and at equilibrium in solution β-D-glucopyranose has a ratio of ∼2:1 to
α-D-glucopyranose, which has its hemiacetal hydroxyl group axial. D-Xylose, a pentose, can
also form a six-membered ring by its terminal hydroxyl group reacting with the aldehyde group
and form a C1 conformation and place all of its bulky hydroxyl groups equatorial, but because
it has five carbons, it would have to be split into a 2-carbon fragment and a 3-carbon fragment
for metabolism and would require two separate pathways for further metabolism. D-Glucose
has 6-carbons and is split into two 3-carbon fragments, D-glyceraldehyde-3-phosphate and
dihydroxy acetone-phosphate that are interconvertible and requires only a single metabolic
pathway for further metabolism. This is an additional plausible reason that D-glucopyranose
is the predominant carbohydrate on the earth.
The D-pentoses will also exist in solution as the six-membered ring structure, but both D-xylose
and D-ribose often have their C-5 hydroxyl groups phosphorylated and the C-5 hydroxyl group
cannot react with the aldehyde group to form the six-membered ring, and therefore they do the
next best thing, with the C-4 hydroxyl group reacting with the aldehyde group to form the
five-membered, furanose ring, hemiacetal structure.
1.2
65
⊡ Figure 3
Haworth structures for α- and β-D-glucopyranose and their C1 and 1C conformations
in the C1-conformation, with only the hemiacetal hydroxyl group in the axial position. This is
the most likely reason that D-glucose is the predominant carbohydrate on the earth, as it is the
only D-hexose that can have all of its hydroxyl groups (exclusive of the hemiacetal hydroxyl
group) equatorial, and at equilibrium in solution β-D-glucopyranose has a ratio of ∼2:1 to
α-D-glucopyranose, which has its hemiacetal hydroxyl group axial. D-Xylose, a pentose, can
also form a six-membered ring by its terminal hydroxyl group reacting with the aldehyde group
and form a C1 conformation and place all of its bulky hydroxyl groups equatorial, but because
it has five carbons, it would have to be split into a 2-carbon fragment and a 3-carbon fragment
for metabolism and would require two separate pathways for further metabolism. D-Glucose
has 6-carbons and is split into two 3-carbon fragments, D-glyceraldehyde-3-phosphate and
dihydroxy acetone-phosphate that are interconvertible and requires only a single metabolic
pathway for further metabolism. This is an additional plausible reason that D-glucopyranose
is the predominant carbohydrate on the earth.
The D-pentoses will also exist in solution as the six-membered ring structure, but both D-xylose
and D-ribose often have their C-5 hydroxyl groups phosphorylated and the C-5 hydroxyl group
cannot react with the aldehyde group to form the six-membered ring, and therefore they do the
next best thing, with the C-4 hydroxyl group reacting with the aldehyde group to form the
five-membered, furanose ring, hemiacetal structure.