Interesting trends were observed by investigating the distribution of intra-and intermolecular DBs in the seven types of enzymes (Table 3; disaggregated **data** for each of the 14 subsets of protein structures examined in the present paper are given in the Supplementary Materials, Table S6). Table 3. Average percentages (estimated standard deviations in parentheses) of various types of proteins (non-enzyme and the seven types of enzymes) that contain both intra-and intermolecular disulfide bonds (Both), only intramolecular disulfide bonds (Intra-only) or only intermolecular disulfide bonds (Inter-only). Disulfide bonds are identified with the FIND or with the GREP method. About 40-50% of the proteins that contain DBs-only intramolecular, only intermolecular or both-are enzymes, and nearly all of them are hydrolases when intramolecular DBs are present; when there are only intermolecular-and no intramolecular-DBs, different types of enzymes can be observed-mainly oxidoreductases, transferases and hydrolases but also, with lower frequency, lyases, isomerases and ligases.