MetaCyc Enzyme: phosphohexose isomerase

Synonyms: PHI

Species: Triticum aestivum

An amyloplastic protein with phosphohexose isomerase activity was partially purified from the immature endosperm of wheat (Triticum aestivum) grains collected 26 days after anthesis [Sangwan90]. The enzyme was purified 133-fold, and its kinetic characteristics were determined. Although the enzyme showed a pH optimum around 8.6, it retained a relatively high activity upto pH 10.0. The enzyme displayed typical hyperbolical kinetics with both β-D-glucose 6-phosphate and β-D-fructofuranose 6-phosphate. pH affected the Km (F6P): whereas the Km (F6P) remained the same at pH 7.0 and 8.6, its value was much higher (390 μM) at pH 9.5 [Sangwan90]. The purified enzyme was stable at 4 C for more than a month and did not lost any activity when incubated at 40 C for 15 min, but lost 50% activity at 55 C. The F6P/G6P ratio was determined as approximately 3:1, suggesting that this enzyme is involved in starch biosynthesis. A number of inhibitors were tested, out of which only D-gluconate 6-phosphate (Ki 180 μM), D-ribose 5-phosphate (Ki 140 μM) and D-ribulose 5-phosphate (Ki 130 μM) were physiologically significant. A number of pentose phosphate pathway intermediates did show inhibitory activity towards the enzyme, which was however not deemed physiologically significant [Sangwan90].

Locations: amyloplast

Molecular Weight of Polypeptide: 130 kD (experimental) [Sangwan90 ]

Gene-Reaction Schematic: ?

Gene-Reaction Schematic

GO Terms:

Cellular Component: GO:0009501 - amyloplast

Created 18-Jan-2007 by Tissier C , TAIR

Enzymatic reaction of: phosphohexose isomerase

β-D-glucose 6-phosphate <=> β-D-fructofuranose 6-phosphate

The reaction direction shown, that is, A + B ↔ C + D versus C + D ↔ A + B, is in accordance with the direction in which it was curated.

This reaction is reversible.

In Pathways: starch biosynthesis

Inhibitors (Competitive): D-gluconate 6-phosphate [Sangwan90] , D-ribose 5-phosphate [Sangwan90] , D-ribulose 5-phosphate [Sangwan90]

Inhibitors (Unknown Mechanism): Cd2+ [Sangwan90] , Zn2+ [Sangwan90] , Sr2+ [Sangwan90] , Ni2+ [Sangwan90] , Hg2+ [Sangwan90]

Primary Physiological Regulators of Enzyme Activity: D-gluconate 6-phosphate , D-ribose 5-phosphate , D-ribulose 5-phosphate

Kinetic Parameters:

Km (μM)
β-D-fructofuranose 6-phosphate
β-D-glucose 6-phosphate

pH(opt): 8.6 [Sangwan90]


Sangwan90: Sangwan RS, Singh R (1990). "Characterization of amyloplastic phosphohexose isomerase from immature wheat (Triticum aestivum L.) endosperm." Indian J Biochem Biophys 27(1);23-7. PMID: 2341161

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Please cite the following article in publications resulting from the use of MetaCyc: Caspi et al, Nucleic Acids Research 42:D459-D471 2014
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