Internal substrate concentrations during biotransformation of octanoic acid into 2-heptanone by spores ofPenicillium roquefortii

Internal substrate concentrations were monitored during biotransformation of octanoic acid into 2-heptanone by spores ofPenicillium roquefortii. The fatty acid rapidly enters the spores in its undissociated form, and a Collander-type relation shows that it strongly accumulates in the spore wall and membrane; this accumulation is reversible. The reaction takes place with cytoplasmic substrate concentrations that quickly fall to zero, and the process is limited by octanoic acid penetration into the cells. This entry is accompanied by proton efflux and involves an active transport process with a H⁺-ATPase system that exhibits Michaelian behavior. The driving force is postulated to be pH, which takes a value set by the initial substrate concentration through the stoichiometry of the H⁺/octanoic acid exchange.