No evidence of hemoglobin damage by SARS-CoV-2 infection

SARS-CoV-2 disease (COVID-19) has affected over 22 million patients worldwide as of August 2020. As the medical community seeks better understanding of the underlying pathophysiology of COVID-19, several theories have been proposed. One widely shared theory suggests that SARS-CoV-2 proteins directly interact with human hemoglobin (Hb) and facilitate removal of iron from the heme prosthetic group, leading to the loss of functional hemoglobin and accumulation of iron. Herein, we refute this theory. We compared clinical data from 21 critically ill COVID-19 patients to 21 non-COVID-19 ARDS patient controls, generating hemoglobin-oxygen dissociation curves from venous blood gases. This curve generated from the COVID-19 cohort matched the idealized oxygen-hemoglobin dissociation curve well (Pearson correlation, R2 = 0.97, P<0.0001; CV(RMSD) = 7.3%). We further analyzed hemoglobin, total bilirubin, lactate dehydrogenase, iron, ferritin, and haptoglobin levels. For all analyzed parameters, patients with COVID-19 had similar levels compared to patients with ARDS without COVID-19. These results indicate that patients with COVID-19 do not exhibit any hemolytic anemia or a shift in the normal hemoglobin-oxygen dissociation curve. We therefore conclude that COVID-19 does not impact oxygen delivery through a mechanism involving red cell hemolysis and subsequent removal of iron from the heme prosthetic group in hemoglobin.

A Stable Bacterial Peroxidase with Novel Halogenating Activity and an Autocatalytically Linked Heme Prosthetic Group

Reconstructing the phylogenetic relationships of the main evolutionary lines of the mammalian peroxidases lactoperoxidase and myeloperoxidase revealed the presence of novel bacterial heme peroxidase subfamilies. Here, for the first time, an ancestral bacterial heme peroxidase is shown to possess a very high bromide oxidation activity (besides conventional peroxidase activity). The recombinant protein allowed monitoring of the autocatalytic peroxide-driven formation of covalent heme to protein bonds. Thereby, the high spin ferric rhombic heme spectrum became similar to lactoperoxidase, the standard reduction potential of the Fe(III)/Fe(II) couple shifted to more positive values (−145 ± 10 mV at pH 7), and the conformational and thermal stability of the protein increased significantly. We discuss structure-function relationships of this new peroxidase in relation to its mammalian counterparts and ask for its putative physiological role.

Expression, Purification, and Characterization of Aspergillus fumigatus Sterol 14-α Demethylase (CYP51) Isoenzymes A and B

ABSTRACT Aspergillus fumigatus sterol 14-α demethylase (CYP51) isoenzymes A (AF51A) and B (AF51B) were expressed in Escherichia coli and purified. The dithionite-reduced CO-P450 complex for AF51A was unstable, rapidly denaturing to inactive P420, in marked contrast to AF51B, where the CO-P450 complex was stable. Type I substrate binding spectra were obtained with purified AF51B using lanosterol (Ks , 8.6 μM) and eburicol (Ks , 22.6 μM). Membrane suspensions of AF51A bound to both lanosterol (Ks , 3.1 μM) and eburicol (Ks , 4.1 μM). The binding of azoles, with the exception of fluconazole, to AF51B was tight, with the Kd (dissociation constant) values for clotrimazole, itraconazole, posaconazole, and voriconazole being 0.21, 0.06, 0.12, and 0.42 μM, respectively, in comparison with a Kd value of 4 μM for fluconazole. Characteristic type II azole binding spectra were obtained with AF51B, whereas an additional trough and a blue-shifted spectral peak were present in AF51A binding spectra for all azoles except clotrimazole. This suggests two distinct azole binding conformations within the heme prosthetic group of AF51A. All five azoles bound relatively weakly to AF51A, with Kd values ranging from 1 μM for itraconazole to 11.9 μM for fluconazole. The azole binding properties of purified AF51A and AF51B suggest an explanation for the intrinsic azole (fluconazole) resistance observed in Aspergillus fumigatus.

Control of DegP-Dependent Degradation of c-Type Cytochromes by Heme and the Cytochrome c Maturation System in Escherichia coli

ABSTRACT c-type cytochromes are located partially or completely in the periplasm of gram-negative bacteria, and the heme prosthetic group is covalently bound to the protein. The cytochrome c maturation (Ccm) multiprotein system is required for transport of heme to the periplasm and its covalent linkage to the peptide. Other cytochromes and hemoglobins contain a noncovalently bound heme and do not require accessory proteins for assembly. Here we show that Bradyrhizobium japonicum cytochrome c 550 polypeptide accumulation in Escherichia coli was heme dependent, with very low levels found in heme-deficient cells. However, apoproteins of the periplasmic E. coli cytochrome b 562 or the cytosolic Vitreoscilla hemoglobin (Vhb) accumulated independently of the heme status. Mutation of the heme-binding cysteines of cytochrome c 550 or the absence of Ccm also resulted in a low apoprotein level. These levels were restored in a degP mutant strain, showing that apocytochrome c 550 is degraded by the periplasmic protease DegP. Introduction of the cytochrome c heme-binding motif CXXCH into cytochrome b 562 (c-b 562) resulted in a c-type cytochrome covalently bound to heme in a Ccm-dependent manner. This variant polypeptide was stable in heme-deficient cells but was degraded by DegP in the absence of Ccm. Furthermore, a Vhb variant containing a periplasmic signal peptide and a CXXCH motif did not form a c-type cytochrome, but accumulation was Ccm dependent nonetheless. The data show that the cytochrome c heme-binding motif is an instability element and that stabilization by Ccm does not require ligation of the heme moiety to the protein.

A Kinetic Study of the Cytochrome c-Hydrogen Peroxide Reaction

The kinetics of the reaction of cytochrome c with H2O2 was studied in aqueous phosphate media near physiological pH (5.7-7.2). The UV-VIS spectra indicated that the heme prosthetic group of the protein suffered an oxidative degradation during the reaction. The experimental rate law was v = kexp [cytochrome c] [H2O2], with kexp = (A + B [H+])/(1 + C [H+]). The apparent activation parameters associated with kexp were Ea = 46 ± 2 kJ mol-1, ∆Ho≠ = 43 ± 2 kJ mol-1 and ∆So≠ = -111 ± 5 J K-1 mol-1. The reaction showed base catalysis and was also catalyzed by both CrO42- and acrylamide. It was inhibited by the oxidants WO42-, MoO42-, VO3- and [Fe(CN)6]3- as well as by the reductants [Fe(CN)6]4- and D-mannitol, whereas Zn2+ and superoxide dismutase had no appreciable effect. The results are consistent with initial reduction of iron from Fe(III) to Fe(II) (supported by inhibition caused by most oxidants), followed by a hydroxyl-radical mediated oxidative degradation of the heme prosthetic group (supported by the inhibition caused by both [Fe(CN)6]4- and D-mannitol).