We have found a new kind of structural motif that appears to be highly conserved among the pyridine nucleotide-linked alpha-amino acid dehydrogenases. This feature is comprised of four atoms closely packed in a planar form. Two of the atoms are donated by the enzyme, one is provided by the substrate (or reactive intermediate), a bound water molecule constitutes the fourth. This tetrad, by virtue of its spatial connectivity, constitutes a two-dimensional machine in contrast to the one-dimensional charge-relay system commonly observed at enzyme active sites. As such, this new motif is capable of more complex behavior permitting a wide variety of possible bonding patterns. Modulation of these potentially variable patterns can lead to qualitative differences in mechanism between structurally similar enzymes, and, in the case of a given enzyme, may constitute the core of its catalytic machinery. We offer a conjecture as to how such a structure may participate in enzymatic catalysis.