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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Heat Stability of Maize Endosperm ADP-Glucose Pyrophosphorylase Is Enhanced by Insertion of a Cysteine in the N Terminus of the Small Subunit¹

Program in Plant Molecular and Cellular Biology and Horticultural Sciences (C.R.L.L., J.S., L.C.H.) and Department of Biochemistry and Molecular Biology (S.K.B., A.K.S.), University of Florida, Gainesville, Florida 32610–0245

ADP-glucose pyrophosphorylase (AGPase) is a key regulatory enzyme in starch biosynthesis. However, plant AGPases differ in several parameters, including spatial and temporal expression, allosteric regulation, and heat stability. AGPases of cereal endosperms are heat labile, while those in other tissues, such as the potato (Solanum tuberosum) tuber, are heat stable. Sequence comparisons of heat-stable and heat-labile AGPases identified an N-terminal motif unique to heat-stable enzymes. Insertion of this motif into recombinant maize (Zea mays) endosperm AGPase increased the half-life at 58°C more than 70-fold. K_m values for physiological substrates were unaffected, although K_cat was doubled. A cysteine within the inserted motif gives rise to small subunit homodimers not found in the wild-type maize enzyme. Placement of this N-terminal motif into a mosaic small subunit containing the N terminus from maize endosperm and the C terminus from potato tuber AGPase increases heat stability more than 300-fold.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: L. Curtis Hannah (hannah{at}mail.ifas.ufl.edu).

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.067637.

^* Corresponding author; e-mail hannah{at}mail.ifas.ufl.edu; fax 352–392–6957.

Received June 24, 2005; returned for revision September 11, 2005; accepted September 13, 2005.

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