Cooperative Kinetics of the Glucan Phosphatase Starch Excess4

Claudia A. Mak, Kenyon Weis, Tiffany Henao, Andrea Kuchtova, Tiantian Chen, Savita Sharma, David A. Meekins, Matthias Thalmann, Craig W. Vander Kooi, Madushi Raththagala

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5 Scopus citations


Glucan phosphatases are members of a functionally diverse family of dual-specificity phosphatase (DSP) enzymes. The plant glucan phosphatase Starch Excess4 (SEX4) binds and dephosphorylates glucans, contributing to processive starch degradation in the chloroplast at night. Little is known about the complex kinetics of SEX4 when acting on its complex physiologically relevant glucan substrate. Therefore, we explored the kinetics of SEX4 against both insoluble starch and soluble amylopectin glucan substrates. SEX4 displays robust activity and a unique sigmoidal kinetic response to amylopectin, characterized by a Hill coefficient of 2.77 ± 0.63, a signature feature of cooperativity. We investigated the basis for this positive kinetic cooperativity and determined that the SEX4 carbohydrate-binding module (CBM) dramatically influences the binding cooperativity and substrate transformation rates. These findings provide insights into a previously unknown but important regulatory role for SEX4 in reversible starch phosphorylation and further advances our understanding of atypical kinetic mechanisms.

Original languageEnglish
Pages (from-to)2425-2435
Number of pages11
Issue number31
StatePublished - Aug 10 2021

Bibliographical note

Funding Information:
This study was supported by the National Science Foundation award MCB-2012074 to M.R., CHE-1808304 to C.W.V.K., and Skidmore College start-up funds provided to M.R. Support was also provided by the Schupf Scholar Program to C.A.M. and Skidmore Summer Collaborative Research Fellowship to T.H. This work also used the support provided by the Swiss National Science Foundation project number P2ZHP3_178037 to M.T.

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ASJC Scopus subject areas

  • Biochemistry


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