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W. K. Yip is the first author of a 1990 study probing the active site of 1-aminocyclopropane-1-carboxylate synthase, an enzyme involved in Ethylene production in plants. The enzyme uses pyridoxal phosphate and acts on S-adenosylmethionine, but its own substrate can also inactivate it. The study investigated the chemistry underlying that apparent paradox.^[ref]

The researchers labeled purified apple enzyme, cut it into peptides and separated the products by high-performance liquid chromatography. Peptide sequencing and mass spectrometry located a modified lysine residue in an active site peptide. Their results connected the same lysine residue with binding pyridoxal phosphate and with covalent attachment of a fragment derived from the substrate during inactivation. Rather than merely observing that an enzyme stopped working, the experiments identified the modified site and constrained the chemical explanation.^[ref]