Figure 4.42. The ethylene synthesis pathway and Yang cycle. The initial precursor is the amino acid methionine and key regulatory enzymes in the pathway are ACC synthase and ACC oxidase which are positively and negatively modulated by a number of factors (see box). Hydrogen cyanide, formed in the last step of ethylene synthesis, is detoxified by cysteine. The ACC pool can be diminished via conversion to NBmalonyl ACC. After synthesis ethylene may bind to receptors and through a series of steps instigate a physiological response or simply diffuse out of the tissue.