In the apicoplast of apicomplexan parasites, plastidic-type ferredoxin and ferredoxin-NADP + reductase (FNR) form a short electron transport chain that provides reducing power for the synthesis of isoprenoid precursors. These proteins are attractive targets for the development of novel drugs against diseases such as malaria, toxoplasmosis, and coccidiosis. We have obtained ferredoxin and FNR of both Toxoplasma gondii and Plasmodium falciparum in recombinant form, and recently we solved the crystal structure of the P. falciparum reductase. Here we report on the functional properties of the latter enzyme, which differ markedly from those of homologous FNRs. In the physiological reaction, P. falciparum FNR displays a k cat five-fold lower than those usually determined for plastidic-type FNRs. By rapid kinetics, we found that hydride transfer between NADPH and proteinbound FAD is slower in the P. falciparum enzyme. The redox properties of the enzyme were determined, and showed that the FAD semiquinone species is highly destabilized. We propose that these two features, i.e. slow hydride transfer and unstable FAD semiquinone, are responsible for the poor catalytic efficiency of the P. falciparum enzyme. Another unprecedented feature of the malarial parasite FNR is its ability to yield, under oxidizing conditions, an inactive dimeric form stabilized by an intermolecular disulfide bond. Here we show that the monomer-dimer interconversion can be controlled by oxidizing and reducing agents that are possibly present within the apicoplast, such as H 2 O 2 , glutathione, and lipoate. This finding suggests that modulation of the quaternary structure of P. falciparum FNR might represent a regulatory mechanism, although this needs to be verified in vivo.

The NADP-binding site of Plasmodium falciparum ferredoxin-NADP(+) reductase contains two basic residues, His286 and Lys249, conserved within the Plasmodium genus, but not in other plant-type homologues. Previous crystal studies indicated that His286 interacts with the adenine ring and with the 5'-phosphate of 2'-P-AMP, a ligand that mimics the adenylate moiety of NADP(H). Here we show that replacement of His286 with aliphatic residues results both in a decrease in the affinity of the enzyme for NADPH and in a decrease in k(cat), due to a lowered hydride-transfer rate. Unexpectedly, the mutation to Gln produces an enzyme more active than the wild-type one, whereas the change to Lys destabilizes the nicotinamide-isoalloxazine interaction, decreasing k(cat). On the basis of the crystal structure of selected mutants complexed with 2'-P-AMP, we conclude that the His286 side chain plays a dual role in catalysis both by providing binding energy for NADPH and by favoring the catalytically competent orientation of its nicotinamide ring. For the latter function, the H-bonding potential rather than the positively charged state of the His286 imidazole seems sufficient. Furthermore, we show that the Lys249Ala mutation decreases K(m)(NADPH) and K(d) for NADP(+) or 2'-P-AMP by a factor of 10. We propose that the Lys249 side chain participates in substrate recognition by interacting with the 2'-phosphate of NADP(H) and that this interaction was not observed in the crystal form of the enzyme-2'-P-AMP complex due to a conformational perturbation of the substrate-binding loop induced by dimerization.

Objectives Sepsis is a time-dependent and life-threating condition. Despite several biomarkers are available, none of them is completely reliable for the diagnosis. This study aimed to evaluate the diagnostic utility of monocyte distribution width (MDW) to early detect sepsis in adult patients admitted in the Emergency Department (ED) with a five part differential analysis as part of the standard clinical practice. Methods A prospective cohort study was conducted on 985 patients aged from 18 to 96 and included in the study between November 2019 and December 2019. Enrolled subjects were classified into four groups based on sepsis-2 diagnostic criteria: control, Systemic Inflammatory Response Syndrome (SIRS), infection and sepsis. The hematology analyzer DxH 900 (Beckman Coulter Inc.) provides the new reportable parameter MDW, included in the leukocyte 5 part differential analysis, cleared by Food and Drug administration (FDA) and European Community In-Vitro-Diagnostic Medical Device (CE IVD) marked as early sepsis indicator (ESId). Results MDW was able to differentiate the sepsis group from all other groups with Area Under the Curve (AUC) of 0.849, sensitivity of 87.3% and specificity of 71.7% at cut-off of 20.1. MDW in combination with white blood cell (WBC) improves the performance for sepsis detection with a sensitivity increased up to 96.8% when at least one of the two biomarkers are abnormal, and a specificity increased up to 94.6% when both biomarkers are abnormal. Conclusions MDW can predict sepsis increasing the clinical value of Leukocyte 5 Part Differential analysis and supporting the clinical decision making in sepsis management at the admission to the ED.

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