Occurrence, metabolism, transport and function of seven-carbon sugars

Seven-carbon (7-C) sugars and sugar alcohols are common in higher plants, algae, fungi and bacteria. The biochemical origin and physiological function of these monosaccharides in plants and algae however is not well understood and has not been fully investigated. Here the occurrence, metabolism, and transport of heptuloses, heptitols, and heptoses are integrated in accordance with function to emphasise the importance of these apparently neglected sugars. This therefore is the first comprehensive synthesis of knowledge about 7-C sugar biochemistry, a relatively underexplored area of carbohydrate biology that needs to be integrated into mainstream sugar research. Available information on the metabolism of heptuloses, heptitols, and heptoses in Medicago sativa (alfalfa), Persea americana (avocado), Primula sp., Kalanchoe pinnata, and the red alga Porphyridium sp. was thoroughly investigated and evaluated. Results indicate that 7-C sugars share a common precursor and are products of a TKT-dependent heptulose shunt in which Suc-derived Fru 6-P is converted either to Sed 7-P or mannoheptulose 7-P or both in competent tissues and species. In plants, free heptuloses probably arise as a consequence of phosphatase activity, whereas heptoses appear to be formed by isomerisation of the corresponding phloem translocated heptuloses following import into non-photosynthetic tissue. It is proposed that the major physiological function of 7-C sugars and heptitols, in addition to serving as a carbon sink, involves metal ion chelation, translocation and remobilisation to fulfil nutrient requirements essential for growth and development.