  This paper considers the capacity of a diffusion-based molecular
communication link assuming the receiver uses chemical reactions. The key
contribution is we show that enzymatic reaction cycles, which is a class of
chemical reactions commonly found in cells consisting of a forward and a
backward enzymatic reaction, can improve the capacity of the communication
link. The technical difficulty in analysing enzymatic reaction cycles is that
their reaction rates are nonlinear. We deal with this by assuming that the
amount of certain chemicals in the enzymatic reaction cycle is large. In order
to simplify the problem further, we use singular perturbation to study a
particular operating regime of the enzymatic reaction cycles. This allows us to
derive a closed-form expression of the channel gain. This expression suggests
that we can improve the channel gain by increasing the total amount of
substrate in the enzymatic reaction cycle. By using numerical calculations, we
show that the effect of the enzymatic reaction cycle is to increase the channel
gain and to reduce the noise, which results in a better signalto- noise ratio
and in turn a higher communication capacity. Furthermore, we show that we can
increase the capacity by increasing the total amount of substrate in the
enzymatic reaction cycle.
