Serum ACE2 Level is Associated With Severe SARS-CoV-2 Infection: A Cross-Sectional Observational Study.

Objectives: Angiotensin-converting enzyme 2 (ACE2) represents the primary receptor for SARS-CoV-2 to enter endothelial cells, causing coronavirus disease of 2019 (COVID-19). In this study, we investigate the association between circulating ACE2 levels with the severity of COVID-19. Methods: Serum ACE2 levels were measured in 144 COVID-19-positive subjects at hospital admission, and 123 COVID-19-negative control subjects. The association between ACE2 and clinical outcomes was analyzed. Results: About 144 COVID-19 patients and 123 healthy controls data were analyzed, the mean age of patients was 62 years and 50% of them were males. The mean age of the control group was 55 years and 63% were males. ACE-II level was measured and compared between COVID-19 patients and controls and revealed no significant differences (P > .05). ACE-II level was measured in COVID-19 patients and compared between different patient's subgroups, ACE II level was not dependent on gender, smoking, ACE intake, or comorbidities (P > .05), and was significantly correlated with cardiovascular diseases (CVS) (P-value = .046) ICU admission (P-value = .0007) and Death (P-value = .0082). Conclusion: There was no significant difference between the COVID-19 and Control group, however, ACE2 serum level was significantly higher in patients with COVID-19 who were critically ill or non-survivors, its increased level is also associated with length of stay. Elevated ACE2 level is associated with the severity of COVID-19 disease, and it has the potential to be a predictor of the severity of the disease.

Genotoxicity of Glyphosate on Cultured Human Lymphocytes.

Glyphosate-based herbicides are the most used herbicides in the world. Despite being widely used, a dispute exists whether glyphosate-based herbicides have a negative effect on human health, particularly genotoxic effects. Therefore, the aim of this study was to investigate glyphosate genotoxicity on cultured human lymphocytes. Cultured human lymphocytes were treated with different concentrations of glyphosate (20, 40, and 200 µmol/L). Four toxicity measures were examined: frequency of chromosomal aberrations (CAs), frequency of sister-chromatid exchange (SCE), production of 8-OHdG, and cell kinetics analysis. The results show that glyphosate induced significant (P < 0.05) increases in the levels of SCE at the highest used concentration (200 µmol/L). However, no significant elevation in SCE levels was observed at the lower examined concentrations (20 and 40 µmol/L). No significant changes in CA were detected at all examined concentrations (P = 0.86). Also, glyphosate did not induce changes to the normal level of 8-OHdG at all examined concentrations (P = 0.98). Last, no significant changes in either mitotic index or proliferative index were observed at any examined concentrations (P > 0.05). The results collectively indicate a lack of genotoxicity and cytotoxicity of glyphosate in cultured human lymphocytes when dealing with environmentally relevant concentrations (20 and 40 µmol/L). However, being exposed to higher concentrations (200 µmol/L) led to slightly higher level of SCE. Therefore, we recommend cautionary measures when dealing with glyphosate-based herbicides for individuals, such as farmers, who may be extensively exposed to high concentrations of these herbicides.

Knowledge about oral manifestations of systemic diseases among medical and dental students from Jordan: An interdisciplinary educational gap.

OBJECTIVE: To assess the ability of medical students to recognize oral manifestation of selected systemic diseases and compare their performance with dental students. MATERIALS AND METHODS: A total of 400 senior medical and dental students were approached to participate. The study protocol involved two parts; a self-administered survey and a direct interview involving clinical photographs for oral signs of systemic diseases. RESULTS: A total of 283 (70.8%) agreed to participate and completed the two- part study. The study population was made of 110 (38.9%) [82 females and 28 males] dental students, and 173(61.1%) [98 females and 75 males] medical students. The knowledge score regarding questions about the normal mouth and oral structures was 15.7 ± 6 out of 22. Dental students had a significantly higher knowledge score about normal mouth and oral structures (20.9 ± 4; range from 17 to 22) compared to medical students (10.6 ± 7; range from 4 to 21) (p = .029). The knowledge score regarding questions about oral manifestations of systemic diseases was 26.8 ± 6 out of 40. Dental students had a significantly higher knowledge score about oral manifestations of systemic diseases (30.8 ± 7; range from 15 to 37) compared to medical students (22.9 ± 4; range from 10 to 36) (p = .031). Only 24.3% (n = 42) medical students reported having adequate training to be able to distinguish between normal mouth and diseases. Nearly all medical students (91.3%; n = 158) felt that it is important to have more formal training in oral examination and disease diagnosis. CONCLUSION: Medical students lack adequate knowledge, diagnostic ability, and confidence with regard to diagnosis of oral signs of systemic diseases. To ensure that medical students have necessary skills in assessing oral diseases, curricula revisions and modifications are required, and specific oral health-related learning outcomes should be introduced and reinforced through clinical training.

Evaluating Awareness and Practices Towards Diabetes and Diabetic Retinopathy in Adult Patients Attending the Eye Clinic in a Tertiary Academic Hospital in Jordan.

AIM AND BACKGROUND: Awareness of diabetes mellitus (DM) and its complications, particularly diabetic retinopathy (DR), is one of the main factors of early detection and improved management. This study aims to assess the level of awareness of DM type 2 complications in a cohort of Jordanian patients, with special emphasis on DR. METHODS: A total of 176 consecutive patients with DM type 2 attending the ophthalmology clinic at Jordan University Hospital were included in the study. Each participant responded to a questionnaire which assessed their awareness and behaviors towards DM type 2 and DR. RESULTS: A total of 176 individuals with diabetes responded to the invitation to participate. They were 93 (52.8%) males and 83 (47.2%) females. Mean age (±SEM) for the study population was 60.6 (±0.82) years. Of all participants, 93.8% were aware that diabetes can affect the eyes. Only 4.5% reported that DR could occur without symptoms and/or loss of vision. Symptoms affecting the eyes were the main cause behind attending the ophthalmology clinic in 60.8% of the cases. The awareness score of participants for DM and DR ranged from 4 to 15 out of 20 with a mean score of 11.4/20. Statistically significant relationships of awareness mean score were observed with gender, educational level, employment status, insurance status, Hemoglobin (Hb) A1c level, and dyslipidemia as a co-morbidity (p<0.05). Binary logistic regression revealed disease duration and HbA1c as the main predictive factors of having DR. CONCLUSION: Among this cohort of Jordanian individuals with diabetes, awareness towards DM and DR was relatively low, and patient practices did not correlate with perceived awareness. Awareness scores correlated with HbA1c readings and higher educational levels among other variables. Emphasis on communication strategies and patient education is essential in establishing efficient screening programs and effective strategies to curtail visual impairment and other complications of the diabetes pandemic.

Association of Irisin Circulating Level with Diabetic Retinopathy: A Case-Control Study.

Obesity and obesity induced type 2 diabetes development and progression have been associated with sedentary lifestyle. Irisin, a newly discovered myokine, has been demonstrated at lower levels in obese and type 2 diabetes patients compared to controls. The main aim of this study is to explore association of Irisin with diabetic retinopathy (DR). A total of 233 healthy and adults participated in this study. Participants were divided into four categories: a healthy control group and an age-match subset of patients with type 2 diabetes; a positive control group of patients with type 2 diabetes not affected by DR (No DR); and patients with type 2 diabetes affected by DR (non-proliferative DR (NPDR) and proliferative DR (PDR)). Plasma samples were quantified for Irisin measurement, lipid profile and HbA1c. Comparison of the age-matched groups of healthy controls and patients with type 2 diabetes revealed lower Irisin plasma level in type 2 diabetes group. Analyses revealed negative correlations of Irisin to HbA1c and LDL levels and positive correlation to HDL level. Comparing Irisin level in No DR and DR groups revealed a higher level in No DR group and analysis per DR classification indicated higher Irisin level in NPDR group. Our results demonstrate not only correlation of plasma Irisin level with DR stages, but also significantly different Irisin level among them. This is promising in terms of researching Irisin as a potential associating marker for type 2 diabetes and DR development and progression.

A cross-sectional study of basic education influence on the clinical training: Attitudes and perception among Jordanian medical students.

BACKGROUND: Over the past century, the medical educational model has been static with no significant improvement. Studies show that students are leaning towards a more active, dynamic, learner-centered education model that fits their needs and encourages them to be more responsible for their learning. Thus, we conducted this study to investigate Jordanian medical students' perceptions and attitudes towards the value of basic sciences in their clinical training. METHODOLOGY: This was a cross-sectional study that utilized an online, self-administered questionnaire targeting medical students in their clinical years. The questionnaire comprises 5 domains targeting students' perceptions, attitudes, and suggestions of the medical educational system in general and basic sciences in specific. RESULTS: Overall, 578 medical students completed the survey with a male to female ratio of 0.7, and 56% of participants studied were studying at Mutah University, while 42% were at the University of Jordan. Approximately three-fourth (73.9%) of the students reported that basic medical sciences are critical to their development as physicians. Approximately, 82% believe that it is vital to integrate the clinical practice into basic science teaching. Besides, 82.4% of students agreed that faculty members' teaching style influences the educational content's delivery at the basic level. Moreover, 73% of students lean towards the inclusion of problem-based learning into their curriculums. On the other hand, 41.7% of students reject basic science questions in their written clinical exams. CONCLUSION: Our study highlights the positive attitudes of Jordanian medical students towards basic medical sciences. It also demonstrates that students are more comfortable with an active and dynamic educational model that fits their needs and qualifications. Thus, we recommend a student-centered medical educational model trail to maximize learning and teaching efficiency and develop competent medical practitioners.

Relationship between Serum Vascular Endothelial Growth Factor Levels and Stages of Diabetic Retinopathy and Other Biomarkers.

AIM: This study aims to measure serum vascular endothelial growth factor (VEGF) levels in a sample of Jordanian patients and to determine their relationship with the different stages of diabetic retinopathy. It also explores the correlation between VEGF concentrations and different biochemical and demographic findings. MATERIALS AND METHODS: A total of 167 adults participated in the study. Participants were divided into two main categories: patients with diabetes mellitus (DM) type 2 without diabetic retinopathy (DR) (N = 62) and patients with DM type 2 affected by DR (N = 105). DR patients were further subclassified into nonproliferative (N = 41) and proliferative (N = 64). Basic laboratory tests were measured to correlate with VEGF levels. Irisin, a hormone linked to diabetic retinopathy was also measured and correlated with VEGF. RESULTS: Serum VEGF was found to positively correlate with the severity of diabetic retinopathy. The means of VEGF serum concentrations were 60 pg/mL for controls, 133 pg/mL for nonproliferative DR patients, and 229 pg/mL for proliferative DR patients. We found a significant positive correlation with glycosylated hemoglobin (HbA1c), and a significant negative correlation with high-density lipoprotein (HDL) levels, age, and irisin. CONCLUSION: In this cohort of Jordanian diabetics, serum VEGF concentrations strongly correlated with the presence and stages of diabetic retinopathy, suggesting it as an appropriate indicator for diabetic retinopathy early detection and management in this society. VEGF levels also significantly correlated with HbA1c, HDL, and irisin levels. Further studies are encouraged to explore these relationships in other ethnic groups and with different diabetic complications.

The understanding of research ethics at health sciences schools in Jordan: a cross-sectional study.

BACKGROUND: Research ethics is required for high-quality research that positively influences society. There is limited understanding of research ethics in Middle Eastern countries including Jordan. Here, we aim to investigate the level of understanding of research ethics principles among health sciences faculty members in Jordan. METHODS: This is a cross sectional study where faculty members from the University of Jordan were surveyed for their knowledge and, attitude of research ethics principles. The study was conducted in the period between July 2016 to July 2017 using a customized-design questionnaire involving demographic data and participants' contributions toward research, and assessment of participants' knowledge, belief and attitude towards research ethics. Different question-formats have been used including multiple-choice, yes or no, and a four point Likert-type questions. Obtained responses were tabulated according to gender, academic-rank, and knowledge about research ethics principles. RESULTS: The study had a response rate of 51%. Among the 137 participants of this study, most (96%) were involved in human and animal research, yet, only 2/3 had prior training in research ethics. Moreover, 91% believed that investigators should have training in research ethics and 87% believed that there should be a mandatory postgraduate course on that. The average correct scores for correct understanding of researchers towards research ethics was 62%. Yet, there were some misconceptions about the major ethical principles as only 43% identified them correctly. Additionally, the role of research ethics committees was not well understood by most of the respondents. CONCLUSIONS: Although there is acceptable knowledge about research ethics, discrepancies in understanding in research ethics principles seems to exist. There is a large support for further training in responsible conduct of research by faculty in health sciences in Jordan. Thus, such training should be required by universities to address this knowledge gap in order to improve research quality and its impact on society.

Antioxidant capacity and biomarkers of oxidative stress in saliva of khat-chewing patients: a case-control study.

OBJECTIVES: The aim of this study was to investigate the antioxidant levels and biomarkers of oxidative stress in saliva from khat-chewing patients compared with controls. STUDY DESIGN: Saliva samples were collected from 51 chronic khat chewers and 46 age- and sex-matched controls. Concentrations of oxidative stress biomarkers (malonyl-dialdehyde [MDA], protein carbonyl, and 8-hydroxy-2'-deoxyguanosine) and antioxidant defense (total antioxidant capacity [TAC], superoxide dismutase, glutathione peroxidase, and catalase [CAT) were analyzed. RESULTS: Salivary MDA level in the khat users group (45 ± 9.2 nmol/mL) was significantly increased in comparison with controls (13 ± 2.1 nmol/mL; P < .001), but there were no significant differences between the 2 groups regarding the levels of salivary protein carbonyl and oxidized guanine species. Salivary TAC was significantly reduced in khat users (0.25 ± 0.028 mmol/L) in comparison with controls (0.34 ± 0.037 mmol/L). Salivary CAT level was significantly reduced in khat users (6.0 ± 0.47 U/mL) in comparison with controls (7.7 ± 0.43 units/mL; P <.05), but no significant differences were observed between the 2 groups with regard to salivary superoxide dismutase or glutathione peroxidase levels. CONCLUSIONS: Chronic khat chewing is associated with increased levels of salivary MDA and reduced levels of TAC and CAT among a population of adult men in comparison with non-khat-chewing controls. These findings suggest that the pro-oxidative effect of khat chewing may be a contributing mechanism for various oral diseases associated with khat use, including cancer, periodontitis, and caries.

PKM2 and HIF-1α regulation in prostate cancer cell lines.

Prostate cancer (PCA) is one of the most common cancer types in men, with cancer progression being linked to hypoxia and the induction of hypoxia-inducible factor (HIF).We investigated the expression of pyruvate kinase M2 (PKM2), its regulation by HIF isoforms 1α and 2α, and its role in HIF stabilization. We additionally examined cell survival in the prostate cancer cell lines PC3 and LNCaP under severe hypoxic (0.1% O2) and normoxic (20% O2) conditions. qRT-PCR showed higher up-regulation of PKM2 mRNA expression in LNCaP cells than in PC3 cells, while western blotting showed that PKM2 protein levels were up-regulated only in LNCaP cells. Inhibition of HIF-1α and HIF-2α by small interfering RNA (si-RNA) demonstrated HIF-1α dependent up-regulation of PKM2 at the mRNA and protein levels in LNCaP cells. PKM2 inhibition by si-RNA significantly decreased hypoxia-response element (HRE) activation in a gene reporter assay and down-regulated HIF-1α target vascular endothelial growth factor (VEGF) mRNA expression in PC3 cells, whereas HIF-1α protein levels were not significantly reduced. Additionally, PKM2 inhibition significantly reduced clonogenic survival in both cell lines in a colony formation assay. Prolyl hydroxylase 3 (PHD3) mRNA expression was up-regulated in both cell lines. It has been shown that PKM2 expression is regulated by HIF-1α and that PKM2 favors HIF-1α transactivation under mild (1% O2) but not severe (0.1% O2) hypoxic conditions, and some of our findings are consistent with these previous results. However, this mechanism was not fully observed in our studied cell lines, as PKM2 regulation and HIF-1α stabilization at the transactivation level occurred under severe hypoxic conditions. This discrepancy suggests that tumor tissue origin and cell type influence this model. Our findings expand the current knowledge of the mechanisms of PCA regulation, and would be important in developing novel therapeutic strategies.

N-Acetyl Cysteine Therapy Does Not Prevent Renal Failure in High-Risk Patients Undergoing Open-Heart Surgery.

BACKGROUND: Renal dysfunction is a common complication after cardiovascular surgery. Controversial issues have been discussed regarding the role of N-acetyl cysteine in the prevention of postoperative renal dysfunction. The purpose of this meta-analysis is to assess whether N-acetyl cysteine offers any protection against the development of acute renal dysfunction after cardiac surgery. METHODS: Multiple databases were searched for randomized trials comparing the role of N-acetyl cysteine and placebo in human patients undergoing cardiac surgery. End-points studied were: the incidence of acute renal failure, hemodialysis, early mortality, duration of hospital stay, and maximal change in creatinine values. Dichotomous variables were compared using the risk difference (RD) calculated with inverse weighting; continuous data was pooled as (standardized) mean difference. Results are presented with 95% confidence interval (P < .05 is significant); results are presented within 95% confidence interval. RESULTS: Thirteen randomized trials (713 and 707 patients in the N-acetyl cysteine and control groups, respectively) were included in the present analysis; nine dealing with patients at high-risk for acute renal failure. The incidence of postoperative acute renal dysfunction was 23% and 36% in the N-acetyl cysteine and control cohorts, respectively. N-acetyl cysteine therapy did not reduce acute renal dysfunction in the high-risk cohort [RD -0.03 (-0.09 to 0.02); P = .22; I2 = 24%]. Maximal change in creatinine levels after surgery was also comparable [standardized mean difference 0.07 (-0.23, 0.09); P = .39]. Early mortality was 2.9% and 3.7% in the N-acetyl cysteine and control cohorts respectively; [RD 0 (-0.03 to 0.02); P = .63; I2 = 20%]. Hospital stay (mean length of stay 10.4 and 10.1 days in the N-acetyl cysteine and control cohorts, respectively) was also similar in both cohorts [WMD 0.17 (-0.02 to 0.37) days; P = .81]. CONCLUSION: Prophylactic N-acetyl cysteine therapy does not reduce the incidence of renal dysfunction in high-risk patients undergoing cardiac surgery.

Carboxyl group of Glu113 is required for stabilization of the diferrous and bis-Fe(IV) states of MauG.

The diheme enzyme MauG catalyzes a six-electron oxidation required for post-translational modification of a precursor of methylamine dehydrogenase (preMADH) to complete the biosynthesis of its protein-derived tryptophan tryptophylquinone (TTQ) cofactor. Crystallographic studies have implicated Glu113 in the formation of the bis-Fe(IV) state of MauG, in which one heme is Fe(IV)═O and the other is Fe(IV) with His-Tyr axial ligation. An E113Q mutation had no effect on the structure of MauG but significantly altered its redox properties. E113Q MauG could not be converted to the diferrous state by reduction with dithionite but was only reduced to a mixed valence Fe(II)/Fe(III) state, which is never observed in wild-type (WT) MauG. Addition of H2O2 to E113Q MauG generated a high valence state that formed more slowly and was less stable than the bis-Fe(IV) state of WT MauG. E113Q MauG exhibited no detectable TTQ biosynthesis activity in a steady-state assay with preMADH as the substrate. It did catalyze the steady-state oxidation of quinol MADH to the quinone, but 1000-fold less efficiently than WT MauG. Addition of H2O2 to a crystal of the E113Q MauG-preMADH complex resulted in partial synthesis of TTQ. Extended exposure of these crystals to H2O2 resulted in hydroxylation of Pro107 in the distal pocket of the high-spin heme. It is concluded that the loss of the carboxylic group of Glu113 disrupts the redox cooperativity between hemes that allows rapid formation of the diferrous state and alters the distribution of high-valence species that participate in charge-resonance stabilization of the bis-Fe(IV) redox state.

A Trp199Glu MauG variant reveals a role for Trp199 interactions with pre-methylamine dehydrogenase during tryptophan tryptophylquinone biosynthesis.

MauG catalyzes posttranslational modifications of a methylamine dehydrogenase precursor (preMADH) to complete the biosynthesis of its protein-derived tryptophan tryptophylquinone (TTQ) cofactor. Trp199 is present at the site of interaction between MauG and preMADH and is critical to this process as it mediates hole hopping during the inter-protein electron transfer that is required for catalysis. Trp199 was converted to Glu and the structure and reactivity of the W199E/preMADH complex were characterized. The results reveal that the nature of residue 199 is also important for productive complex formation between preMADH and MauG.

Effects of the loss of the axial tyrosine ligand of the low-spin heme of MauG on its physical properties and reactivity.

MauG catalyzes posttranslational modifications of methylamine dehydrogenase to complete the biosynthesis of its protein-derived tryptophan tryptophylquinone (TTQ) cofactor. MauG possesses a five-coordinate high-spin and a six-coordinate low-spin ferric heme, the latter with His-Tyr ligation. Replacement of this tyrosine with lysine generates a MauG variant with only high-spin ferric heme and altered spectroscopic and redox properties. Y294K MauG cannot stabilize the bis-Fe(IV) redox state required for TTQ biosynthesis but instead forms a compound I-like species on reaction with peroxide. The results clarify the role of Tyr ligation of the five-coordinate heme in determining the physical and redox properties and reactivity of MauG.

Mutagenesis of tryptophan199 suggests that hopping is required for MauG-dependent tryptophan tryptophylquinone biosynthesis.

The diheme enzyme MauG catalyzes the posttranslational modification of the precursor protein of methylamine dehydrogenase (preMADH) to complete biosynthesis of its protein-derived tryptophan tryptophylquinone (TTQ) cofactor. Catalysis proceeds through a high valent bis-Fe(IV) redox state and requires long-range electron transfer (ET), as the distance between the modified residues of preMADH and the nearest heme iron of MauG is 19.4 Å. Trp199 of MauG resides at the MauG-preMADH interface, positioned midway between the residues that are modified and the nearest heme. W199F and W199K mutations did not affect the spectroscopic and redox properties of MauG, or its ability to stabilize the bis-Fe(IV) state. Crystal structures of complexes of W199F/K MauG with preMADH showed no significant perturbation of the MauG-preMADH structure or protein interface. However, neither MauG variant was able to synthesize TTQ from preMADH. In contrast, an ET reaction from diferrous MauG to quinone MADH, which does not require the bis-Fe(IV) intermediate, was minimally affected by the W199F/K mutations. W199F/K MauGs were able to oxidize quinol MADH to form TTQ, the putative final two-electron oxidation of the biosynthetic process, but with k(cat)/K(m) values approximately 10% that of wild-type MauG. The differential effects of the W199F/K mutations on these three different reactions are explained by a critical role for Trp199 in mediating multistep hopping from preMADH to bis-Fe(IV) MauG during the long-range ET that is required for TTQ biosynthesis.

Functional importance of tyrosine 294 and the catalytic selectivity for the bis-Fe(IV) state of MauG revealed by replacement of this axial heme ligand with histidine .

The diheme enzyme MauG catalyzes the posttranslational modification of a precursor protein of methylamine dehydrogenase (preMADH) to complete the biosynthesis of its protein-derived tryptophan tryptophylquinone (TTQ) cofactor. It catalyzes three sequential two-electron oxidation reactions which proceed through a high-valent bis-Fe(IV) redox state. Tyr294, the unusual distal axial ligand of one c-type heme, was mutated to His, and the crystal structure of Y294H MauG in complex with preMADH reveals that this heme now has His-His axial ligation. Y294H MauG is able to interact with preMADH and participate in interprotein electron transfer, but it is unable to catalyze the TTQ biosynthesis reactions that require the bis-Fe(IV) state. This mutation affects not only the redox properties of the six-coordinate heme but also the redox and CO-binding properties of the five-coordinate heme, despite the 21 Å separation of the heme iron centers. This highlights the communication between the hemes which in wild-type MauG behave as a single diheme unit. Spectroscopic data suggest that Y294H MauG can stabilize a high-valent redox state equivalent to Fe(V), but it appears to be an Fe(IV)═O/π radical at the five-coordinate heme rather than the bis-Fe(IV) state. This compound I-like intermediate does not catalyze TTQ biosynthesis, demonstrating that the bis-Fe(IV) state, which is stabilized by Tyr294, is specifically required for this reaction. The TTQ biosynthetic reactions catalyzed by wild-type MauG do not occur via direct contact with the Fe(IV)═O heme but via long-range electron transfer through the six-coordinate heme. Thus, a critical feature of the bis-Fe(IV) species may be that it shortens the electron transfer distance from preMADH to a high-valent heme iron.

Long-range electron transfer reactions between hemes of MauG and different forms of tryptophan tryptophylquinone of methylamine dehydrogenase.

The diheme enzyme MauG catalyzes the post-translational modification of a precursor protein of methylamine dehydrogenase (preMADH) to complete the biosynthesis of its protein-derived tryptophan tryptophylquinone (TTQ) cofactor. This six-electron oxidation of preMADH requires long-range electron transfer (ET) as the structure of the MauG-preMADH complex reveals that the shortest distance between the modified residues of preMADH and the nearest heme of MauG is 14.0 A [Jensen, L. M. R., Sanishvili, R., Davidson, V. L., and Wilmot, C. M. (2010) Science 327, 1392-1394]. The kinetics of two ET reactions between MADH and MauG have been analyzed. Interprotein ET from quinol MADH to the high-valent bis-Fe(IV) form of MauG exhibits a K(d) of 11.2 microM and a rate constant of 20 s(-1). ET from diferrous MauG to oxidized TTQ of MADH exhibits a K(d) of 10.1 microM and a rate constant of 0.07 s(-1). These similar K(d) values are much greater than that for the MauG-preMADH complex, indicating that the extent of TTQ maturity rather than its redox state influences complex formation. The difference in rate constants is consistent with a larger driving force for the faster reaction. Analysis of the structure of the MauG-preMADH complex in the context of ET theory and these results suggests that direct electron tunneling between the residues that form TTQ and the five-coordinate oxygen-binding heme is not possible, and that ET requires electron hopping via the six-coordinate heme.