Proteinkristallographie und Computer-Graphik – auf dem Weg zu einer planvollen Arzneimittelentwicklung

Expression, purification, biochemical characterization and inhibition of recombinant Plasmodium falciparum aldolase

The energy metabolism of the blood stage form of the human malaria parasite Plasmodium falciparum is adapted to the host cell. Like erythrocytes, P. falciparum merozoites lack a functional citric acid cycle. Generation of ATP depends therefore fully on the glycolytic pathway. Aldolase is a key enzyme of this pathway and a high degree of sequence diversity between parasite and host makes it a potential drug target. We have expressed the enzyme in its tetrameric form in Escherichia coli and the catalytic constants Vmax and Km of the recombinant enzyme correspond to the constants of parasite-derived aldolase. Rabbit antibodies against the recombinant P. falciparum aldolase inhibit the natural enzyme and no cross-reaction with human aldolase is detectable. Both the recombinant and the natural protein bind to the cytosolic domain of the band 3 membrane protein in vitro. A 19-residue synthetic peptide corresponding to the sequence of the binding domain of band 3 is an inhibitor when included in the binding assay. In addition, this peptide inhibits the catalytic activity of recombinant P. falciparum aldolase when assayed in a buffer system devoid of anions such as chloride or phosphate. The band 3-derived peptides compete with the aldolase substrate fructose-1,6-diphosphate for binding, suggesting that both reagents have a high affinity for the substrate pocket. A similar sequence motif exists in P. falciparum actin II. A 19-residue peptide corresponding to this sequence is also an inhibitor which could suggest that the P. falciparum aldolase can associate with the cytoskeleton of the parasite or of the host.

Preliminary crystallographic studies of glycosomal glyceraldehyde phosphate dehydrogenase from Trypanosoma brucei brucei

Crystals of glyceraldehyde phosphate dehydrogenase from the glycosome of Trypanosoma brucei brucei have been grown, and a partial data set has been collected using synchrotron radiation. The crystals diffract initially to 2.3 A resolution. The space group is P2(1)2(1)2, with cell dimensions a = 135 A, b = 255 A, c = 115 A, so there are probably at least two tetramers in the asymmetric unit.

Trypanothione reductase from Trypanosoma cruzi. Purification and characterization of the crystalline enzyme

The structural differences between trypanothione reductase of Trypanosoma cruzi and human glutathione reductase, an enzyme of known three-dimensional structure, offer an opportunity for rational drug design against Chagas' disease. As a first step in the analysis of the parasite enzyme we report its purification and characterization. 2.2 mg trypanothione reductase was extracted from 33 g wet weight of cultured epimastigotes or from 4 g lyophilized cells. The flavoenzyme was purified 2400-fold to homogeneity in three steps with an overall yield of 45%. The enzyme is a dimer with a subunit Mr of 50,000. Using NADPH (Km = 5 microM) and trypanothione disulfide (Km = 45 microM) as substrates, a turnover number of 14,200 min-1 was estimated. Trypanothione reductase, the parasite enzyme, and glutathione reductase, the host enzyme, exhibit mutually exclusive specificities for their respective disulfide substrates. When screening cell cultures or column eluates for the presence of trypanothione reductase, a microassay based on Ellman's reagent as indicator was used. A mixture of regioisomeric glutathionylspermidine disulfides isolated from Escherichia coli served as substrate in this microassay. Experimentally, the catalytic cycle of the enzyme can be subdivided into the half-reactions Eox + NADPH + H+----EH2 + NADP+, and EH2 + trypanothione disulfide----Eox + dihydrotrypanothione. This is also true for the crystallized enzyme in the presence of 2 M (NH4)2SO4. The spectral properties of trypanothione reductase both in the oxidized form (Eox) and in the two-electron-reduced form (EH2) closely resemble those of human glutathione reductase. Both proteins contain a flavin and a redox-active disulfide at the catalytic site. After reduction of Eox to EH2, trypanothione reductase can be inactivated by specifically alkylating one of the nascent active-site thiols.