Enzymes, Kinetics, And Regulation Practice Test

Phosphofructokinase-1 (PFK-1) is a key regulatory enzyme in glycolysis, catalyzing the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. Its activity is tightly controlled to match the cell's energy needs. In a cell with high energy charge, certain metabolic intermediates accumulate and bind to PFK-1 at a site distinct from the active site, causing a conformational change that decreases the enzyme's affinity for its substrate.

A 2-day-old male infant develops lethargy, vomiting, and poor feeding. He rapidly becomes comatose. Laboratory studies show a plasma ammonia level of 1500 µmol/L (normal < 50 µmol/L). The plasma citrulline level is very low, but urinary orotic acid is markedly elevated. This constellation of findings suggests a defect in the urea cycle.