Number of reactions and ORFs included in the S. cerevisiaemetabolic network arranged by enzyme categories. For comparison, data for an E. coli (Edwards and Palsson 2000) metabolic network is shown. (EC 1) Oxidoreductases; (EC 2) transferases; (EC 3) hydrolases; (EC 4) lyases; (EC 5) isomerases; (EC 6) ligases.