In vivo channeling of substrates in an enzyme aggregate for beta-carotene biosynthesis

• Published in: Proceedings of the National Academy of Sciences - PNAS 1991-06, Vol.88 (11), p.4936-4940
• Main Authors: Candau, Reyes, Bejarano, Eduardo R., Cerda-Olmedo, Enrique
• Format: Article
• Language: English
• Subjects: beta -carotene; Biochemistry; Biological and medical sciences; biosintesis; biosynthese; biosynthesis; carotenoide; carotenoides; carotenoids; Carotenoids - biosynthesis; Cell aggregates; Cell physiology; champignon; channeling; ciclo biogeoquimico; culture media; cycle biogeochimique; cycling; enzyme inhibitors; Enzyme substrates; Enzymes; Ethylamines - pharmacology; Fundamental and applied biological sciences. Psychology; fungi; Gene expression regulation; Heterokaryon; hongos; Imidazoles; Imidazoles - pharmacology; in vivo; inhibidores de enzimas; inhibiteur d' enzyme; Kinetics; medio de cultivo; Membrane and intracellular transports; Metabolism; metabolisme; metabolismo; milieu de culture; Models, Theoretical; Molecular and cellular biology; Molecules; Multienzyme Complexes - metabolism; Nicotine - pharmacology; Phycomyces; Phycomyces - drug effects; Phycomyces - enzymology; Phycomyces blakesleeanus; Picolines - pharmacology; substrates
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.88.11.4936

The existence and the mode of operation of certain enzyme aggregates may be established from the concentrations of intermediates measured in the presence of specific inhibitors. beta-Carotene, the most abundant carotenoid pigment in the fungus Phycomyces blakesleeanus, arises from ring formation at both ends of lycopene. The inhibitors nicotine, imidazole, alpha-picoline, and 2-(4-chlorophenylthio)triethylamine lead to the simultaneous accumulation of lycopene, beta-carotene, and the one-ring intermediate gamma-carotene. The quantitative analytical values obey precise mathematical relationships: those expected from the operation of an enzyme aggregate with two cyclases equally sensitive to the inhibitors. The intermediates lycopene and gamma-carotene rejected by chemically inhibited enzymes may be readmitted to other cyclases in the wild type but not in heterokaryons containing a carA mutation. We have calculated the fraction of inhibited cyclase under each condition, the affinity constant of each inhibitor for the cyclase, and the probability that a rejected intermediate molecule will be readmitted and further metabolized. The probabilities for lycopene and gamma-carotene are identical and independent of the inhibitor responsible for the rejection. Our calculations suggest that each rejected intermediate molecule is readmitted to the enzyme aggregates two or three times on the average.