The enzyme PRPP amidotransferase displays hyperbolic kinetics (Fig ure 13.14) with respect to glutamine. With respect to the second sub- strate, PRPP, the enzyme shows sigmoidal kinetics. The presence of nu cleotides further exaggerates the sigmoidal nature of the v (velocity) vs (PRPP) plot. The intracellular concentrations of glutamine are approxi mately equal to the Km of the enzyme for glutamine. On the other hand the PRPP levels in the cell, which do fluctuate, can be 10 to 100 times less than the Km of the enzyme for PRPP. Consequently, the PRPP concentrations in the cell play an important role in controlling de novo purine nucleotide synthesis.