  The rate-limiting step of some enzymatic reactions is a physical step, i.e.
diffusion. The efficiency of such reactions can be improved through an increase
in the arrival rate of the substrate molecules, e.g. by a directed passage of
substrate (ligand) to active site after its random encounter with the enzyme
surface. Herein, we introduce a cellular automata model simulating the ligand
passage over the protein surface to its destined active site. The system is
simulated using the lattice gas automata with probabilistic transition rules.
Different distributions of amino acids over the protein surface are examined.
For each distribution, the hydration pattern is achieved and the mean number of
iteration steps needed for the ligand to arrive at the active site calculated.
Comparison of results indicates that the rate at which ligand arrives at the
active site is clearly affected by the distribution of amino acids outside the
active side. Such a process can facilitate the ligand diffusion towards the
active site thereby enhancing the efficiency of the enzyme action.
