The Lys residue at position 108 of human beta-carotene 15,15'-monooxygenase is located on the outside surface of the active tunnel of the enzyme. Hydrophobic mutations (K108F and K108L) at this position substantially decreased the affinity of the enzyme for substrates with ionone rings at both ends, such as alpha-carotene, beta-carotene, and beta-cryptoxanthine. In contrast, these mutations had little effect on the affinity of the enzyme for substrates with one ionone ring and one open-chain end, such as beta-apo-4'-carotenal and beta-apo-8'-carotenal. The residue 108 may be related to the indirect interaction with the second ionone ring of the substrates with two ionone rings.