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Fructose (/ ˈ f r ʌ k t oʊ s,-oʊ z /), or fruit sugar, is a ketonic simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose.It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed by the gut directly into the blood of the portal vein during digestion.
Wavy single bonds represent unknown or unspecified stereochemistry or a mixture of isomers. For example, the adjacent diagram shows the fructose molecule with a wavy bond to the HOCH 2 - group at the left. In this case the two possible ring structures are in chemical equilibrium with each other and also with the open-chain structure.
A classic example of ring opening and contraction is the isomerization of glucose (an aldehyde with a six-membered ring) to fructose (a ketone with a five-membered ring). The conversion of D-glucose-6-phosphate to D-fructose-6-phosphate is catalyzed by glucose-6-phosphate isomerase, an intramolecular oxidoreductase.
The reaction creates a ring of carbon atoms closed by one bridging oxygen atom. The resulting molecule has a hemiacetal or hemiketal group, depending on whether the linear form was an aldose or a ketose. The reaction is easily reversed, yielding the original open-chain form. In these cyclic forms, the ring usually has five or six atoms.
The following other wikis use this file: Usage on bn.wikipedia.org ফ্রুক্টোজ; Usage on da.wikipedia.org Fruktose; Usage on el.wikipedia.org
The pyranose ring is formed by the reaction of the hydroxyl group on carbon 5 (C-5) of a sugar with the aldehyde at carbon 1. This forms an intramolecular hemiacetal.If reaction is between the C-4 hydroxyl and the aldehyde, a furanose is formed instead. [1]
If the sugar is an aldohexose, with the carbonyl in position 1, the reaction may involve the hydroxyl on carbon 4 or carbon 5, creating a hemiacetal with five- or six-membered ring, respectively. If the sugar is a 2-ketohexose, it can only involve the hydroxyl in carbon 5, and will create a hemiketal with a five-membered ring.
Finally, the ring is closed by rotating the substrate about the C3-C4 bond again and deprotonating the C5 hydroxyl with Lys518. [ 11 ] When going from fructose-6-phosphate toward glucose-6-phosphate, the result could be mannose -6-phosphate if carbon C2 is given the wrong chirality , but the enzyme does not permit that result except at a very ...