  In this work we demonstrate both experimentally and theoretically that the
analog noise generation by a single enzymatic logic gate can be dramatically
reduced to yield gate operation with virtually no input noise amplification.
This is achieved by exploiting the enzyme's specificity when using a
co-substrate that has a much lower affinity than the primary substrate. Under
these conditions, we obtain a negligible increase in the noise output from the
logic gate as compared to the input noise level. Experimental realizations of
the AND logic gate with the enzyme horseradish peroxidase using hydrogen
peroxide and two different co-substrates,
2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS) and ferrocyanide,
with vastly different rate constants confirmed our general theoretical
conclusions.
