The presence of citrate-stabilized gold nanoparticles (cit-Au NPs) significantly enhanced the specific activity of α-amylase, which could also be modulated by varying the concentration of the enzyme, while keeping the NP concentration constant. Also, the lowest concentration (0.175 μg mL⁻¹) at which the Michaelis–Menten behavior of the enzyme could clearly be observed in the presence of NPs was much less than that in their absence. At higher protein concentrations the activity decreased monotonically until it reached nearly the same as that of free enzyme, while exhibiting Michaelis–Menten kinetics at all concentrations. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) based particle size analyses indicated increased agglomeration of the NPs with increased protein concentration. The results have been explained based on a model which considered the presence of enzyme bound to NP and that available for enhanced catalysis, enzyme bound to NP but unavailable due to being buried inside the agglomerate and the free enzyme.