- Hexoses are a class of monosaccharides composed of six carbon atoms, with the general molecular formula C₆H₁₂O₆. They are fundamental units of carbohydrates and among the most important simple sugars in nature.
- Hexoses can be divided into two main types: aldohexoses, which contain an aldehyde functional group at the first carbon (e.g., glucose, mannose, galactose), and ketohexoses, which contain a ketone functional group typically at the second carbon (e.g., fructose, sorbose). Their structural diversity arises from stereoisomerism, as small changes in the spatial arrangement of hydroxyl groups yield distinct sugars with unique biological roles.
- In biological systems, hexoses are central to energy metabolism and cellular structure. Glucose, the most prominent hexose, is the primary energy source for most living organisms. It undergoes glycolysis to produce ATP, fuels the tricarboxylic acid (TCA) cycle, and is a key precursor in biosynthetic pathways. Fructose, another important hexose, is widely found in fruits and honey and enters glycolysis after conversion to intermediates like fructose-6-phosphate. Galactose is a critical component of lactose in milk and contributes to the synthesis of glycolipids and glycoproteins. Mannose participates in protein glycosylation and cell recognition processes. The variety of functions among hexoses reflects their essential roles in sustaining life.
- Structurally, hexoses can exist in both linear (open-chain) and cyclic (ring) forms. In aqueous solutions, the cyclic forms—furanose (five-membered ring) and pyranose (six-membered ring)—predominate due to intramolecular hemiacetal or hemiketal formation. This ring closure generates additional isomeric forms called anomers (α and β), which differ in the orientation of the hydroxyl group at the anomeric carbon. Such structural versatility underpins the ability of hexoses to form diverse oligosaccharides and polysaccharides, including starch, glycogen, cellulose, and chitin. These macromolecules provide energy storage, structural support, and functional modifications to biomolecules.
- Beyond their roles in metabolism and structure, hexoses also serve as building blocks for more complex biological molecules. They are incorporated into nucleotides (e.g., ribose derived from glucose metabolism), glycosaminoglycans in connective tissues, and as part of the sugar moieties in glycoproteins and glycolipids, which are critical for cell–cell communication, immunity, and pathogen recognition. Their regulation in blood and tissues is tightly controlled, as imbalances in hexose metabolism are associated with metabolic disorders such as diabetes mellitus, galactosemia, and fructose intolerance.
- From an applied perspective, hexoses have broad significance in food, fermentation, and biotechnology. Glucose and fructose are major dietary sugars and sweeteners, while mannose and galactose find use in pharmaceuticals, cosmetics, and prebiotic formulations. Microbial fermentation of hexoses underpins industries producing bread, beer, wine, bioethanol, and organic acids. Advances in metabolic engineering are increasingly harnessing hexose utilization pathways for the production of biofuels, bioplastics, and high-value biochemicals from renewable biomass.
