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F420 and FO are phenolic 5-deazaflavin cofactors that complement nicotinamide and flavin redox coenzymes in biochemical oxidoreductases and photocatalytic systems. Specifically, these 5-deazaflavins lack the single electron reactivity with O2 of riboflavin-derived coenzymes (FMN and FAD), and, in general, have a more negative redox potential than NAD(P)(+). For example, F420-dependent NADP(+) oxidoreductase (Fno) is critical to the conversion of CO2 to CH4 by methanogenic archaea, while FO functions as a light-harvesting agent in DNA repair. The preparation of these cofactors is an obstacle to their use in biochemical studies and biotechnology. Here, a convenient synthesis of FO was achieved by improving the redox stability of synthetic intermediates containing a polar, electron-rich aminophenol fragment. Improved yields and simplified purification techniques for FO are described. Additionally, Fno activity was restored with FO in the absence of F420. Investigating the FO-dependent NADP(+)/NADPH redox process by stopped-flow spectrophotometry, steady state kinetics were defined as having a Km of 4.00 ± 0.39 μM and a kcat of 5.27 ± 0.14 s(-1). The preparation of FO should enable future biochemical studies and novel uses of F420 mimics.

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Here, we report the very first example of half-site reactivity and negative cooperativity involving an important F420 cofactor-dependent enzyme. F420H2:NADP(+) oxidoreductase (Fno) is an F420 cofactor-dependent enzyme that catalyzes the reversible reduction of NADP(+) through the transfer of a hydride from the reduced F420 cofactor. These catalytic processes are of major significance in numerous biochemical processes. While the steady-state kinetic analysis showed classic Michaelis-Menten kinetics with varying concentrations of the F420 redox moiety, FO, such plots revealed non-Michaelis-Menten kinetic behavior when NADPH was varied. The double reciprocal plot of the varying concentrations of NADPH displays a downward concave shape, suggesting that negative cooperativity occurs between the two identical monomers. The transient state kinetic data show a burst prior to entering steady-state turnover. The burst suggests that product release is rate-limiting, and the amplitude of the burst phase corresponds to production of product in only one of the active sites of the functional dimer. These results suggest either half-site reactivity or an alternate sites model wherein the reduction of the cofactor, FO occurs at one active site at a time followed by reduction at the second active site. Thus, the data imply that Fno may be a functional regulatory enzyme.

This Working Paper should not be reported as representing views of the IMF. The views expressed in this Working Paper are those of the author(s) and do not necessarily represent those of the IMF or IMF policy. Working Papers describe research in progress by the author(s) and are published to elicit comments and to further debate. This paper examines the channels through which external debt affects growth in low-income countries. Our results suggest that the substantial reduction in the stock of external debt projected for highly indebted poor countries (HIPCs) would directly increase per capita income growth by about 1 percentage point per annum. Reductions in external debt service could also provide an indirect boost to growth through their effects on public investment. If half of all debt-service relief were channeled for such purposes without increasing the budget deficit, then growth could accelerate in some HIPCs by an additional 0.5 percentage point per annum.

The residue of antibiotics in the water has led to increased antibioticresistant bacteria, harm to human health, and damage to health-beneficial healthy bacteria. An idea of constructing S-scheme α-Fe 2 O 3 /g-C 3 N 4 nanocomposites is studied toward a photocatalysis application for an efficient resolution of commercial antibiotics in wastewater. Outstanding S-scheme Fe 2 O 3 /g-C 3 N 4 nanocatalysts are synthesized by a straightforward method and could easily improve the recycling property, thanks to magnetic materials. Empirical results indicate that S-scheme Fe 2 O 3 /g-C 3 N 4 photocatalysts can degrade commercial cefalexin and amoxicillin (20 mg L −1 ) under visible light, with five and nine times higher performance than that of g-C 3 N 4 , respectively. Furthermore, the detailed evidence to propose S-scheme Fe 2 O 3 /g-C 3 N 4 heterojunctions and comparison of photocatalytic performance in antibiotic degradation have also been mentioned in this study. KEYWORDS: α-Fe 2 O 3 , g-C 3 N 4 , photocatalysis, α-Fe 2 O 3 /g-C 3 N 4 , S-scheme, antibiotic degradation

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Sphingosine-1-phosphate (S1P) is a potent lipid mediator that exerts its activity via activation of five different G protein–coupled receptors, designated as S1P1–5. This potent lipid mediator is synthesized from the sphingosine precursor by two sphingosine kinases (SphK1 and 2) and must be exported to exert extracellular signaling functions. We recently identified Mfsd2b as the S1P transporter in the hematopoietic system. However, the sources of sphingosine for S1P synthesis and the transport mechanism of Mfsd2b in erythrocytes remain to be determined. Here, we show that erythrocytes efficiently take up exogenous sphingosine and that a de novo synthesis pathway in part provides sphingosines to erythrocytes. The uptake of sphingosine in erythrocytes is facilitated by the activity of SphK1. By converting sphingosine into S1P, SphK1 indirectly increases the influx of sphingosine, a process that is irreversible in erythrocytes. Our results explain for the abnormally high amount of sphingosine accumulation in Mfsd2b knockout erythrocytes. Furthermore, we show that Mfsd2b utilizes a proton gradient to facilitate the release of S1P. The negatively charged residues D95 and T157 are essential for Mfsd2b transport activity. Of interest, we also discovered an S1P analog that inhibits S1P export from erythrocytes, providing evidence that sphingosine analogs can be used to inhibit S1P export by Mfsd2b. Collectively, our results highlight that erythrocytes are efficient in sphingosine uptake for S1P production and the release of S1P is dependent on Mfsd2b functions.