The GGG(A) X motif has been reported to determine the activity of esterases or lipases towards tertiary alcohols. The gamma-isoenzyme of pig liver esterase (gamma-PLE, PLE 1) contains a GGGX motif; this motif was systematically mutated to the corresponding AGGX, GAGX, GGAX, AAAX, AAGX, AGAX and GAAX motifs to investigate its influence on the activity and especially enantioselectivity in the kinetic resolution of three acetates of tertiary alcohols. None of the mutants showed higher enantioselectivity than the starting enzyme (E = 17), and mutation of the central glycine to alanine completely destroyed activity and selectivity. Hence, only subtle changes of this crucial motif lead to substantial changes in esterase properties. These findings were analyzed and interpreted by computer modeling.