  We discuss the shear viscosity of a Newtonian solution of catalytic enzymes
and substrate molecules. The enzyme is modeled as a two-state dimer consisting
of two spherical domains connected with an elastic spring. The enzymatic
conformational dynamics is induced by the substrate binding and such a process
is represented by an additional elastic spring. Employing the Boltzmann
distribution weighted by the waiting times of enzymatic species in each
catalytic cycle, we obtain the shear viscosity of dilute enzyme solutions as a
function of substrate concentration and its physical properties. The substrate
affinity distinguishes between fast and slow enzymes, and the corresponding
viscosity expressions are obtained. Furthermore, we connect the obtained
viscosity with the diffusion coefficient of a tracer particle in enzyme
solutions.
