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Understanding the structural basis of specificity and promiscuity of paralogous enzymes is important for deciphering molecular mechanisms and is a necessary step towards designing enzyme-specific modulators. The main objective of this thesis is to provide structural insights that relate protein local sequences to their observed binding and activity profiles through the study of two human protein families – cytosolic sulfotransferases (SULTs) and sirtuins (SIRTs). This was achieved by comparing the family-wide ligand binding fingerprints of these two enzyme families with the structural details of their corresponding enzyme-ligand co-crystal structures. The hSULT enzyme family was profiled against a focused library through binding and activity assays. This suggested a number of novel compounds that bind to the less well-characterized SULT members (SULT1C3 and SULT4A1), and revealed additional broad-spectrum hSULT inhibitors. Based on the profiling data, three enzyme/co-factor/ligand complex structures were solved using X-ray crystallography. The structure of SULT1C2•PAP(3'-phosphoadenosine 5'-phosphate)•pentacholorphenol(PCP) provided a rationale for a novel SULTs inhibition mechanism that depends on substrate acidity. The SULT1B1•PAP•resveratrol structure suggested that the hydrogen-bonding coordination of the 5-OH group on resveratrol is the structural determinant for the observed substrate preference towards resveratrol. SULT2A1•PAP•lithocholic acid(LCA) ternary complex structure confirms that the specificity of SULT2A1 for lithocholic acid derives from its high hydrophobicity in the substrate binding pocket. The same approach was used to interrogate the interaction of the sirtuins with their peptide substrates. The binding and enzymatic assays for human sirtuins have suggested that SIRT1 and SIRT2 are generally less discriminate against substrates while class IV sirtuins - SIRT6 and SIRT7 might be highly specific enzymes. Three different biochemical and kinetic assays showed that SIRT6-dependent histone deacetylation is about 1,000 times slower than for other highly active sirtuins. To understand the molecular basis for the specificity and low activity of SIRT6, I determined the first set of crystal structures for SIRT6 in complex with ADPr (ADP ribose) and the non-hydrolyzable analog of OAADPr (2’-O-acetyl-ADP ribose) – NAADPr (2’-N-acetyl-ADP ribose). The structures revealed human SIRT6 has unique structural features including a splayed zinc-binding domain, lacks a helix bundle and the conserved, highly flexible, NAD(+)-binding loop, which contribute to its observed biochemical behavior.

Understanding the structural basis of specificity and promiscuity of paralogous enzymes is important for deciphering molecular mechanisms and is a necessary step towards designing enzyme-specific modulators. The main objective of this thesis is to provide structural insights that relate protein local sequences to their observed binding and activity profiles through the study of two human protein families – cytosolic sulfotransferases (SULTs) and sirtuins (SIRTs). This was achieved by comparing the family-wide ligand binding fingerprints of these two enzyme families with the structural details of their corresponding enzyme-ligand co-crystal structures. The hSULT enzyme family was profiled against a focused library through binding and activity assays. This suggested a number of novel compounds that bind to the less well-characterized SULT members (SULT1C3 and SULT4A1), and revealed additional broad-spectrum hSULT inhibitors. Based on the profiling data, three enzyme/co-factor/ligand complex structures were solved using X-ray crystallography. The structure of SULT1C2•PAP(3'-phosphoadenosine 5'-phosphate)•pentacholorphenol(PCP) provided a rationale for a novel SULTs inhibition mechanism that depends on substrate acidity. The SULT1B1•PAP•resveratrol structure suggested that the hydrogen-bonding coordination of the 5-OH group on resveratrol is the structural determinant for the observed substrate preference towards resveratrol. SULT2A1•PAP•lithocholic acid(LCA) ternary complex structure confirms that the specificity of SULT2A1 for lithocholic acid derives from its high hydrophobicity in the substrate binding pocket. The same approach was used to interrogate the interaction of the sirtuins with their peptide substrates. The binding and enzymatic assays for human sirtuins have suggested that SIRT1 and SIRT2 are generally less discriminate against substrates while class IV sirtuins - SIRT6 and SIRT7 might be highly specific enzymes. Three different biochemical and kinetic assays showed that SIRT6-dependent histone deacetylation is about 1,000 times slower than for other highly active sirtuins. To understand the molecular basis for the specificity and low activity of SIRT6, I determined the first set of crystal structures for SIRT6 in complex with ADPr (ADP ribose) and the non-hydrolyzable analog of OAADPr (2’-O-acetyl-ADP ribose) – NAADPr (2’-N-acetyl-ADP ribose). The structures revealed human SIRT6 has unique structural features including a splayed zinc-binding domain, lacks a helix bundle and the conserved, highly flexible, NAD(+)-binding loop, which contribute to its observed biochemical behavior.

No description available.

Flagellate protozoa from the order Trypanosomatida have developed a range of strategies to survive in their mammalian hosts. A consequence is that the glycolytic pathway has assumed an important role, especially in bloodstream-form Trypanosoma brucei, where it is essential as the sole producer of ATP. The seventh enzyme in the pathway, 2,3-bisphosphoglycerate-independent phosphoglycerate mutase (iPGAM) is particularly attractive as a drug target because it shares no common properties with the corresponding enzyme in humans. This enzyme catalyses the conversion of 3PGA to 2PGA, with the requirement for metal ions to assist the catalytic function. In this study, two important biochemical and structural aspects of the enzyme were investigated: i) The in vitro and in vivo requirements for biologically relevant metal ions to support the activity of iPGAM, and ii) The ability of trypanosomatid iPGAM to exist in multiple conformations and oligomeric states in solution. The maximum activity of iPGAM in vitro requires Co2+, but this cannot be the case in vivo where ICP-OES analyses confirmed that Co2+ was essentially undetectable in T. brucei cytosolic fractions. The activity of iPGAM in vivo is therefore one of the lowest among the glycolytic enzymes. By contrast, Mg2+ and Zn2+ were found to be the most abundant metals in both cytosolic fractions and in purified bacterially expressed iPGAM. Our newly-developed multimode-plate reader discontinuous assay further revealed that of the biologically relevant metals, only Mg2+ can support iPGAM activity, but at less than 50% of the level of Co2+. By contrast, Zn2+ strongly inhibits iPGAM. This assay which was developed with minimal metal interference on the coupling enzymes, also showed that in solution, the ratio of the concentrations of 3PGA:2PGA (substrate:product) at equilibrium is not 1:1 as observed in the crystal structure, but is in fact 12:1, which may be due to the tighter binding of 2PGA to the enzyme. A series of biophysical analyses, notably by SEC-MALS showed that iPGAM from Leishmania mexicana, another trypanosomatid protozoan parasite exists in different forms and oligomeric states in solution, either as the closed-form monomer, openiii form monomer, or closed/open-form dimer which can be successfully separated by ion-exchange chromatography. The open-form LmiPGAM is particularly relevant for drug development, as the catalytic site in the closed-form structure is poorly inaccessible. Both virtual and high-throughput screening approaches were used to identify novel potential inhibitors. Out of a collection of 11 compounds tested at 1 mM, two showed substantial inhibition with 49% and 14% remaining activity. Taken together, the findings from this study demonstrated the potential of iPGAM to be a key modulator in controlling glycolytic flux in trypanosomes, and thus further validated it as an important drug target.