66
1
General Principles
3.6 Occurrence of D-Erythrose, D-Ribose, and D-Xylose
Of the two D-tetroses, only 4-phospho-D-erythrose is found in any quantity, as an intermediate
in the photosynthetic reactions. Of the four pentoses, only D-ribose and D-xylose occur to any
extent. The phospho-aldo-D-pentoses, D-ribose-5-phosphate and D-xylose-5-phosphate, are
found in the photosynthetic reactions. Three phospho-keto-D-pentoses are also found as intermediates in the photosynthetic reactions: D-ribulose-5-phosphate, D-xylulose-5-phosphate,
and D-ribulose-1,5-bis-phosphate, the latter carbohydrate being directly involved in the fixing
of CO 2 in photosynthesis.
⊡ Scheme 3
Important naturally occurring four- and five-carbon sugar phosphates
Nicotinamide adenine dinucleotide (NAD + and NADH), coenzymes containing D-ribofuranose-diphosphate, are involved in many oxidation and reduction reactions, respectively, of
carbohydrate metabolism. D-Ribofuranose is also the main component of the universal energy
donor and energy carrier, adenosine triphosphate (ATP), which is one of the primary products
of the light reactions of photosynthesis and is responsible for providing the energy for the
formation of the carbon–carbon bond in the fixation of CO 2 . It is also important in the transfer and utilization of energy in the metabolism of nonphotosynthesizing organisms. Another
important D-ribofuranose coenzyme that is formed as a primary product of the light reactions
of photosynthesis is the reducing coenzyme, nicotinamide adenine dinucleotide-phosphate,
1
General Principles
3.6 Occurrence of D-Erythrose, D-Ribose, and D-Xylose
Of the two D-tetroses, only 4-phospho-D-erythrose is found in any quantity, as an intermediate
in the photosynthetic reactions. Of the four pentoses, only D-ribose and D-xylose occur to any
extent. The phospho-aldo-D-pentoses, D-ribose-5-phosphate and D-xylose-5-phosphate, are
found in the photosynthetic reactions. Three phospho-keto-D-pentoses are also found as intermediates in the photosynthetic reactions: D-ribulose-5-phosphate, D-xylulose-5-phosphate,
and D-ribulose-1,5-bis-phosphate, the latter carbohydrate being directly involved in the fixing
of CO 2 in photosynthesis.
⊡ Scheme 3
Important naturally occurring four- and five-carbon sugar phosphates
Nicotinamide adenine dinucleotide (NAD + and NADH), coenzymes containing D-ribofuranose-diphosphate, are involved in many oxidation and reduction reactions, respectively, of
carbohydrate metabolism. D-Ribofuranose is also the main component of the universal energy
donor and energy carrier, adenosine triphosphate (ATP), which is one of the primary products
of the light reactions of photosynthesis and is responsible for providing the energy for the
formation of the carbon–carbon bond in the fixation of CO 2 . It is also important in the transfer and utilization of energy in the metabolism of nonphotosynthesizing organisms. Another
important D-ribofuranose coenzyme that is formed as a primary product of the light reactions
of photosynthesis is the reducing coenzyme, nicotinamide adenine dinucleotide-phosphate,