BackgroundThe yellow fever mosquito, Aedes aegypti, is the primary vector for the viruses that cause yellow fever, mostly in tropical regions of Africa and in parts of South America, and human dengue, which infects 100 million people yearly in the tropics and subtropics. A better understanding of the structural biology of olfactory proteins may pave the way for the development of environmentally-friendly mosquito attractants and repellents, which may ultimately contribute to reduction of mosquito biting and disease transmission.MethodologyPreviously, we isolated and cloned a major, female-enriched odorant-binding protein (OBP) from the yellow fever mosquito, AaegOBP1, which was later inadvertently renamed AaegOBP39. We prepared recombinant samples of AaegOBP1 by using an expression system that allows proper formation of disulfide bridges and generates functional OBPs, which are indistinguishable from native OBPs. We crystallized AaegOBP1 and determined its three-dimensional structure at 1.85 Å resolution by molecular replacement based on the structure of the malaria mosquito OBP, AgamOBP1, the only mosquito OBP structure known to date.ConclusionThe structure of AaegOBP1 ( = AaegOBP39) shares the common fold of insect OBPs with six α-helices knitted by three disulfide bonds. A long molecule of polyethylene glycol (PEG) was built into the electron-density maps identified in a long tunnel formed by a crystallographic dimer of AaegOBP1. Circular dichroism analysis indicated that delipidated AaegOBP1 undergoes a pH-dependent conformational change, which may lead to release of odorant at low pH (as in the environment in the vicinity of odorant receptors). A C-terminal loop covers the binding cavity and this “lid” may be opened by disruption of an array of acid-labile hydrogen bonds thus explaining reduced or no binding affinity at low pH.

Pteridine reductase (PTR1) is a target for drug development against Trypanosoma and Leishmania species, parasites that cause serious tropical diseases and for which therapies are inadequate. We adopted a structure-based approach to the design of novel PTR1 inhibitors based on three molecular scaffolds. A series of compounds, most newly synthesized, were identified as inhibitors with PTR1-species specific properties explained by structural differences between the T. brucei and L. major enzymes. The most potent inhibitors target T. brucei PTR1, and two compounds displayed antiparasite activity against the bloodstream form of the parasite. PTR1 contributes to antifolate drug resistance by providing a molecular bypass of dihydrofolate reductase (DHFR) inhibition. Therefore, combining PTR1 and DHFR inhibitors might improve therapeutic efficacy. We tested two new compounds with known DHFR inhibitors. A synergistic effect was observed for one particular combination highlighting the potential of such an approach for treatment of African sleeping sickness.

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Crystal structures of recombinant Lactococcus lactis 6‐phosphogluconate dehydrogenase (LlPDH) in complex with substrate, cofactor, product and inhibitors have been determined. LlPDH shares significant sequence identity with the enzymes from sheep liver and the protozoan parasite Trypanosoma brucei for which structures have been reported. Comparisons indicate that the key residues in the active site are highly conserved, as are the interactions with the cofactor and the product ribulose 5‐phosphate. However, there are differences in the conformation of the substrate 6‐phosphogluconate which may reflect distinct states relevant to catalysis. Analysis of the complex formed with the potent inhibitor 4‐phospho‐d‐erythronohydroxamic acid, suggests that this molecule does indeed mimic the high‐energy intermediate state that it was designed to. The analysis also identified, as a contaminant by‐product of the inhibitor synthesis, 4‐phospho‐d‐erythronamide, which binds in similar fashion. LlPDH can now serve as a model system for structure‐based inhibitor design targeting the enzyme from Trypanosoma species.

In mammals, adenine phosphoribosyltransferase (APRT, EC 2.4.2.7) is present in all tissues and provides the only known mechanism for the metabolic salvage of adenine resulting from the polyamine biosynthesis pathway or from dietary sources. In humans, APRT deficiency results in serious kidney illness such as nephrolithiasis, interstitial nephritis, and chronic renal failure as a result of 2,8-dihydroxyadenine (DHA) precipitation in the renal interstitium. To address the molecular basis of DHA-urolithiasis, the recombinant human APRT was crystallized in complex with adenosine 5'-monophosphate (AMP). Refinement of X-ray diffraction data extended to 2.1 A resolution led to a final crystallographic R(factor) of 13.3% and an R(free) of 17.6%. This structure is composed of nine beta-strands and six alpha-helices, and the active site pocket opens slightly to accommodate the AMP product. The core of APRT is similar to that of other phosphoribosyltransferases (PRTases), although the adenine-binding domain is quite different. Structural comparisons between the human APRT and other "type I" PRTases of known structure revealed several important features of the biochemistry of PRTases. We propose that the residues located at positions corresponding to Leu159 and Ala131 in hAPRT are responsible for the base specificities of type I PRTases. The comparative analysis shown here also provides structural information for the mechanism by which mutations in the human APRT lead to DHA-urolithiasis.

Adenine phosphoribosyltransferase (APRT) is an important enzyme component of the purine recycling pathway. Parasitic protozoa of the order Kinetoplastida are unable to synthesize purines de novo and use the salvage pathway for the synthesis of purine bases rendering this biosynthetic pathway an attractive target for antiparasitic drug design. The recombinant human adenine phosphoribosyltransferase (hAPRT) structure was resolved in the presence of AMP in the active site to 1.76 A resolution and with the substrates PRPP and adenine simultaneously bound to the catalytic site to 1.83 A resolution. An additional structure was solved containing one subunit of the dimer in the apo-form to 2.10 A resolution. Comparisons of these three hAPRT structures with other 'type I' PRTases revealed several important features of this class of enzymes. Our data indicate that the flexible loop structure adopts an open conformation before and after binding of both substrates adenine and PRPP. Comparative analyses presented here provide structural evidence to propose the role of Glu104 as the residue that abstracts the proton of adenine N9 atom before its nucleophilic attack on the PRPP anomeric carbon. This work leads to new insights to the understanding of the APRT catalytic mechanism.

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In this work, glucoamylase was produced by