Interaction of Feedback Loops. Simple Feedback Loops: Simulation of threonine (Thr) biosynthetic pathway starting from aspartate with three feedback inhibition loops. Interaction of Feedback Loops: Threonine biosynthetic pathway connected with the downstream BCAA pathways. The graphical insets show the approach (minutes) to steady state (μM) synthesis and utilization of the substrates, intermediates, and end-products of the pathways. Where available, the ranges of reported values for pathway intermediate and end-product levels in cells growing in a glucose minimal salts medium are shown in parentheses (μM) in the inset graphs. Dashed lines indicate feedback regulation. Plus sign is positive feedback and minus sign is negative feedback.