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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

Inferred from experiment

EC Number:

D-glucopyranose 6-phosphate ⇄ β-D-fructofuranose 6-phosphate

The direction shown, i.e. which substrates are on the left and right sides, 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:
Substrate Km (μM) Citations
β-D-fructofuranose 6-phosphate 120.0 [Sangwan90]
D-glucopyranose 6-phosphate 440.0 [Sangwan90]

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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