  We report the first systematic study of designed two-input biochemical
systems as information processing gates with favorable noise-transmission
properties accomplished by modifying the gate's response from convex shape to
sigmoid in both inputs. This is realized by an added chemical "filter" process
which recycles some of the output back into one of the inputs. We study a
system involving the biocatalytic function of the enzyme horseradish
peroxidase, functioning as an AND gate. We consider modularity properties, such
as the use of three different input chromogens that, when oxidized yield
signal-detection outputs for various ranges of the primary input, hydrogen
peroxide. We also examine possible uses of different filter-effect chemicals
(reducing agents) to induce the sigmoid-response. A modeling approach is
developed and applied to our data, allowing us to describe the enzymatic
kinetics in the framework of a formulation suitable for evaluating the
noise-handling properties of the studied systems as logic gates for information
processing steps.
