pentose

Meaning: A pentose is a five-carbon monosaccharide. Aldopentoses such as ribose have an aldehyde at C1. Ketopentoses such as ribulose have a ketone at C2. Ribose is the sugar in RNA nucleotides. 2-Deoxyribose, which has H instead of OH on C2, is the sugar in DNA. In a nucleotide the sugar is in a five-membered (furanose) ring. The base attaches at C1′ by an N-glycosidic bond, and the phosphate attaches at C5′. Cells make ribose-5-phosphate through the pentose phosphate pathway in the cytosol. The oxidative phase of this pathway converts glucose-6-phosphate to ribulose-5-phosphate and produces NADPH.

Equation: (CH₂O)n, with n = 5, gives C₅H₁₀O₅. Here n is the number of carbons (a pure number, no unit).

On the MCAT: Expect structure questions that ask you to tell ribose from deoxyribose, count carbons, or use the primed numbering (1′ to 5′) in nucleotides. Pathway questions ask what the pentose phosphate pathway supplies. NADPH is used for reductive biosynthesis such as fatty acid synthesis and for keeping glutathione reduced. Ribose-5-phosphate is used for nucleotides. Glucose-6-phosphate dehydrogenase is the regulated first enzyme.

Common confusion: Pentose names the number of carbons, not the ring size. Ribose in nucleotides is a furanose ring, but free pentoses can also form six-membered pyranose rings. The pentose phosphate pathway is not mainly an ATP-producing pathway. Its main outputs are NADPH and pentose sugars.