509-335-1047, hwangs@wsu.edu

Assistant Research Professor, Institute of Biological Chemistry

Ph.D. 1995, Kyungpook National University, South Korea

Research Interests

Starch is the major energy reserve in most plants.  A portion of the photoassimilates generated by photosynthesis is converted into starch as a transitory intermediate in source (also transient sink) organs or as a stable final product in sink organs.  Several enzymes play pivotal roles in the course of starch metabolism.  The ultimate goal of this research is to significantly improve important agronomic parameters that impact plant productivity (biomass) by understanding and customizing specific enzymes in these processes. Our current research is focusing on the enzymes that directly control starch synthesis: ADP-glucose pyrophosphorylase and starch phosphorylase.  Computational and biochemical approaches such as structural modeling, mutagenesis, and kinetic analyses, have uncovered significant insights on the roles of the two AGPase subunits in enzyme’s regulation and catalysis.  Several allosterically up-regulated AGPase mutants have been introduced into model plants (Arabidopsis and rice) and studies are undergoing to determine the relationship between enzymatic properties and starch synthesis/plant productivity. Starch phosphorylases (OsPHOL and OsPHOH) are also being studied as the candidates for further understanding of starch metabolism since recent results indicate that a mutation on the OsPHOL gene significantly reduced starch content in rice endosperm.  

Selected Publications

Sato, H., Shibahara, K., Tokunaga, T., Nishi A., Tasaki, M., Hwang, S.K., Okita, T.W., et al. (2008) Mutation of plastidic α-glucan phosphorylase affects the synthesis and structure of starch in rice endosperm. Plant Cell (in press).

Hwang, S.K., Nagai, Y., Kim, D., Okita, T.W. (2008) Direct appraisal of the potato tuber ADP-glucose pyrophosphorylase large subunit in enzyme function by study of a novel mutant form. Journal of Biological Chemistry 283, 6640-6647.

Chung, K.J., Hwang, S.K., Hahn B.S., Kim, K.H., Kim, J.B., Kim, Y.H., Yang, J.S., Ha, S.H. (2008) Authentic seed-specific activity of the Perilla oleosin 19 gene promoter in transgenic Arabidopsis. Plant Cell Reports 27, 29-37.

Kim, D., Hwang, S.K., and Okita, T.W. (2007) Subunit interactions specify the allosteric properties of potato tuber ADP-glucose pyrophosphorylase. Biochem. Biophys. Res. Comm. 362, 301-306.

Lee, S.K., Hwang, S.K., Han, M., Eom, J.S., Kang H.G., Han, Y., Choi, S.B., Cho, M.H., Bhoo, S.H., An, G., Hahn, T.R., Okita, T.W., Jeon, J.S. (2007) Identification of the ADP-glucose pyrophosphorylase isoforms essential for starch synthesis in the leaf and seed endosperm of rice (Oryza sativa L.). Plant Molecular Biology 65, 531-546.

Hwang, S.K., Hamada, S., Okita, T.W. (2007) Catalytic implications of the higher plant ADP-glucose pyrophosphorylase large subunit. Phytochemistry 68, 464-477.

Hwang, S.K., Hamada, S., Okita, T.W. (2006) ATP binding site in the plant ADP-glucose pyrophosphorylase large subunit. FEBS Letters 580, 6741-6748.

Hwang, S.K., Salamone. P.R., Okita. T.W. (2005) Allosteric regulation of the higher plant ADP-glucose pyrophosphorylase is a product of synergy between the two subunits. FEBS Letters 579, 983-990.

Hwang, S.K., Salamone. P.R., Kavakli. H., Slattery. C.J., and Okita. T.W. (2004) Rapid purification of the potato ADP-glucose pyrophosphorylase by polyhistidine-mediated chromatography. Protein Expression & Purification 38, 99-107. 

Sakulsingharoj, C., Choi, S.B., Hwang, S.K., Edwards, G.E., Bork, J., Meyer, C.R, Preiss, J., and Okita, T.W. (2004) Engineering starch biosynthesis for increasing rice seed weight: the role of the cytoplasmic ADP-glucose pyrophosphorylase. Plant Science 167, 1323-1333. 

Singh, S., Slattery, C.J., Choi, S.B., Hwang, S.K., and Okita, T.W. (2003) Expression, kinetics and regulatory properties of native and recombinant ADP-glucose pyrophosphorylase isoforms from chickpea. Plant Physiology and Biochemistry 41, 399-405.