Fig. 1.

Heterosis for the flux (J) through a linear metabolic pathway of Michaelian enzymes far from saturation. The flux is represented as a function of the activities of two enzymes, with the same arbitrary values of kinetic parameters. P1 and P2, and P3 and P4, are two pairs of parents. The hybrids have mid-parental concentration/activity for both enzymes (points in the middle of the curves relating the parental points). In the P1*P2 cross, there is only mid-parent heterosis for the flux because parent P2 has a flux close to the maximum due to high concentration/activity of both enzymes. In the P3*P4 cross, the hybrid displays best-parent heterosis because the parents have low flux values due to low concentration/activity of enzyme 2 (parent P3) or enzyme 1 (parent P4)