title: Involvement of lysine-193 of the conserved "K-T-G-G" motif in the catalysis of maize starch synthase IIa.
abstract: It has been suggested that the lysine residue in the conserved K-T-G-G motif could be the substrate ADP-glucose binding site of Escherichia coli glycogen synthase (GS). Since the K-X-G-G motif is highly conserved between E. coli GS and all the maize starch synthase (SS) isozymes, it has become widely accepted that the lysine in the conserved K-T-G-G motif may also function as the ADPGlc binding site of maize SS. We have used chemical modification and site-directed mutagenesis to study the function of lysine residues in SS. Pyridoxal-5'-phosphate inactivated maize SSIIa activity in a time and concentration dependent manner. ADPGlc completely protected SSIIa from inactivation by pyridoxal-5'-phosphate, indicating that lysine residue(s) could be important for ADPGlc binding and enzyme catalysis. In contrast to E. coli GS, mutation of conserved lysine193 (K-T-G-G) in maize SS did not alter the ADPGlc binding while significantly changing the enzyme activity toward different primers. Our results suggest that lysine-193 (K-T-G-G) is not directly involved in ADPGlc binding, instead mutation in the conserved lysine position affected the primer preference.
authors.affiliation: Interdepartmental Plant Physiology Program and Agronomy Department, Iowa State, University, Ames, IA, USA.
authors.forename: Zhong
authors.initials: Z
authors.lastname: Gao
authors.forename: Peter
authors.initials: P
authors.lastname: Keeling
authors.forename: Richard
authors.initials: R
authors.lastname: Shibles
authors.forename: Hanping
authors.initials: H
authors.lastname: Guan
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D001483:Base Sequence; D002384:Catalysis; D017124:Conserved Sequence; D017931:DNA Primers; D007700:Kinetics; D008239:Lysine / Q000378:metabolism*; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010940:Plant Proteins; D013214:Starch Synthase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D003313:Zea mays / Q000201:enzymology*
chemical_list: D017931:DNA Primers; D010940:Plant Proteins; C111377:starch synthase II; D013214:Starch Synthase; D008239:Lysine