Table I.

Comparison of apparent affinity for substrates of potato tuber wild-type and mutant ADPGlc PPases

ADPGlc PPaseSynthesis DirectionPyrophosphorolysis Direction
S0.5 for Glc-1-PVmaxS0.5for ADPGlcVmax
mm units mg−1 mm units mg−1
Wild type0.057  ± 0.003 (1.1)48  ± 10.20  ± 0.01 (1.3)55  ± 1
SK198RLwt 7.7  ± 0.1 (1.3)24  ± 10.49  ± 0.02 (1.1)16  ± 1
SK198ALwt 22.0  ± 2.5 (1.5)46  ± 31.3  ± 0.1 (1.0)20  ± 1
SK198ELwt 31.1  ± 2.7 (1.8)1.7  ± 0.12.1  ± 0.2 (1.0)1.4  ± 0.1
SwtLK213R 0.044  ± 0.002 (1.1)27  ± 10.36  ± 0.03 (1.8)37  ± 1
SwtLK213A 0.037  ± 0.001 (1.0)25  ± 10.46  ± 0.02 (1.7)31  ± 1
SwtLK213E 0.036  ± 0.001 (0.9)31  ± 10.68  ± 0.01 (1.8)45  ± 1
SK198RLK213R 5.6  ± 0.1 (1.5)24  ± 10.72  ± 0.01 (1.9)14  ± 1

Reactions were performed in either the synthesis (assay II) or the pyrophosphorolysis direction (assay I) as described in Methods. Data represent the average of two identical experiments ± the average difference of the duplicates. The values in parentheses are the Hill interaction coefficients (n H). One unit of enzyme activity is expressed as the amount of enzyme required to form 1 mol of ADPGlc per minute at 37°C assayed in either the synthesis or the pyrophosphorolysis direction.