The tautomerase superfamily is composed of a group of proteins characterized by two key features: the N-terminal proline and a beta-alpha-beta-motif. This superfamily has been divided into five families represented by 4-oxalocrotonate tautomerase (4-OT), 5-(carboxymethyl)-2-hydroxymuconate isomerase (CHMI), cis-3-chloroacrylic acid dehalogenase (cis-CaaD), malonate semialdehyde decarboxylase (MSAD), and macrophage migration inhibitory factor (MIF). Cg10062 is a homologue of cis-CaaD, but has several distinct biochemical properties from cis-CaaD. For example, Cg10062 can be irreversibly inhibited by (R)- or (S)-oxirane-2-carboxylate, whereas cis-CaaD can only be irreversibly inhibited by (R)-oxirane-2-carboxylate. FG41MSAD is a homologue of MSAD, with comparable decarboxylase activity but missing Arg-73 known to be crucial for the MSAD activity. In order to understand the unique biochemical characteristics of Cg10062 and FG41MSAD, we have solved five crystal structures. These crystal structures have established a solid structural basis for understanding the mechanisms of their activities. The eukaryotic protein phosphatases are composed of a group of proteins that are responsible for reversible phosphorylation. The eukaryotic protein phosphatases have been divided into three families, the phosphoprotein phosphatase (PPP) family, the protein phosphatase Mg2+- or Mn2+-dependent (PPM) family and the protein Tyr phosphatase (PTP) family. PDP1 is a member of PPM family. PDP1 is also an important component of the large pyruvate dehydrogenase complex (PDC) which catalyzes the decarboxylation of pyruvate to yield acetyl-CoA with the accompanying reduction of NAD+. In order to understand the mechanism in which it dephosphorylates its target protein we have solved the structure of the catalytic domain of PDP1. Analysis of these structures in the light of their evolutionary contexts enables us to appreciate the unity and diversity of the biological systems at the chemical level and help us solve interesting problems, such as the possible physiological functions for some members within the tautomerase superfamily. / text