We present a comprehensive study of the direct electron transfer reaction of soluble PQQ-GDH from Acinetobacter calcoaceticus. Wild-type PQQ-sGDH nonspecifically adsorbed on carbon cryogel electrodes retained its enzymatic activity for glucose and maltose oxidation at pH 7.2 and 37 °C. The cyclic voltammograms in the absence of enzymatic substrate showed 2 redox peaks that suggest a two-step, one-electron oxidation/reduction of PQQ. Calibration curves showed a linear amperometric response for a wide glucose concentration range, including the values normally found in blood. At saturation, the catalytic current reached 0.93 mA cm(-2). Altogether the experimental results suggest that the amperometric output of the electrodes and the shape of the calibration curves represent a combination of the intrinsic enzyme kinetics, the maximum rate of heterogeneous electron transfer and the substrate accessibility to the enzyme's active center caused by the confinement of the enzyme into the mesoporous structure. A new mutant enzyme, N428C, developed in our group that shows almost twice the maximum catalytic activity in homogeneous experiments in solution, also showed a DET signal on carbon cryogel electrodes for glucose electro-oxidation. The higher activity for the mutant enzyme was also verified on the electrode surface.