An enzyme inhibitor that acts in the initial phases of enzyme action as a substrate, but does not complete the catalytic cycle to regenerate an active enzyme; e.g. organophosphate ester anti-cholinesterases, which mimic acetylcholine in that the phosphoric acid moiety of the hemisubstrate acts like the acetly group of acetylcholine to form an ester with the enzyme's active-site serine residue but, unlike the acetyl-enzyme intermediate of the normal enzymic action, the phosphoryl-enzyme is only very slowly hydrolysed to regenerate an active enzyme.