Enzymes are accurate and specific molecular machines---an enzyme usually catalyzes onlyone particular type of reaction and can utilize no more than a few substrates.This propertyis of special importance to living organisms, enabling them to achieve high precision in metabolic reactions.Despite extensive theoretical and experimental studies, how enzymesachieve their specificity at the molecular level remains elusive for a large number of them.Inthis study, we use 2 plant enzymes in the flavanone synthesis pathway to study the structural basis and mechanism of their specificity.The two enzymes are highly similar in 3D structures, but possess distinct specificity to substrates differing by only one hydroxyl group.