  We consider a realization of the XOR logic gate in a system involving two
competing biocatalytic reactions, for which the logic-1 output is defined by
these two processes causing a change in the optically detected signal. A model
is developed for describing such systems in an approach suitable for evaluation
of the analog noise amplification properties of the gate and optimization of
its functioning. The initial data are fitted for gate quality evaluation within
the developed model, and then modifications are proposed and experimentally
realized for improving the gate functioning.
