[Show abstract][Hide abstract] ABSTRACT: IsdG and IsdI are paralogous heme degrading enzymes from the bacterium Staphylococcus aureus. Heme bound by these enzymes is extensively ruffled such that the meso-carbons at the sites of oxidation are distorted toward bound oxygen. In contrast, the canonical heme oxygenase family degrades
heme that is bound with minimal distortion. Trp-66 is a conserved heme pocket residue in IsdI implicated in heme ruffling.
IsdI variants with Trp-66 replaced with residues having less bulky aromatic and alkyl side chains were characterized with
respect to catalytic activity, heme ruffling, and electrochemical properties. The heme degradation activity of the W66Y and
W66F variants was approximately half that of the wild-type enzyme, whereas the W66L and W66A variants were inactive. A crystal
structure and NMR spectroscopic analysis of the W66Y variant reveals that heme binds to this enzyme with less heme ruffling
than observed for wild-type IsdI. The reduction potential of this variant (−96 ± 7 mV versus standard hydrogen electrode) is similar to that of wild-type IsdI (−89 ± 7 mV), so we attribute the diminished activity of
this variant to the diminished heme ruffling observed for heme bound to this enzyme and conclude that Trp-66 is required for
optimal catalytic activity.
[Show abstract][Hide abstract] ABSTRACT: IsdI, a heme-degrading protein from Staphylococcus aureus, binds heme in a manner that distorts the normally planar heme prosthetic group to an extent greater than that observed so far for any other heme-binding protein. To understand better the relationship between this distinct structural characteristic and the functional properties of IsdI, spectroscopic, electrochemical, and crystallographic results are reported that provide evidence that this heme ruffling is essential to the catalytic activity of the protein and eliminates the need for the water cluster in the distal heme pocket that is essential for the activity of classical heme oxygenases. The lack of heme orientational disorder in (1)H-NMR spectra of the protein argues that the catalytic formation of β- and δ-biliverdin in nearly equal yield results from the ability of the protein to attack opposite sides of the heme ring rather than from binding of the heme substrate in two alternative orientations.
Proceedings of the National Academy of Sciences 08/2011; 108(32):13071-6. DOI:10.1073/pnas.1101459108 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Enzymatic haem catabolism by haem oxygenases is conserved from bacteria to humans and proceeds through a common mechanism leading to the formation of iron, carbon monoxide and biliverdin. The first members of a novel class of haem oxygenases were recently identified in Staphylococcus aureus (IsdG and IsdI) and were termed the IsdG-family of haem oxygenases. Enzymes of the IsdG-family form tertiary structures distinct from those of the canonical haem oxygenase family, suggesting that IsdG-family members degrade haem via a unique reaction mechanism. Herein we report that the IsdG-family of haem oxygenases degrade haem to the oxo-bilirubin chromophore staphylobilin. We also present the crystal structure of haem-bound IsdI in which haem ruffling and constrained binding of oxygen is consistent with cleavage of the porphyrin ring at the beta- or delta-meso carbons. Combined, these data establish that the IsdG-family of haem oxygenases degrades haem to a novel chromophore distinct from biliverdin.
[Show abstract][Hide abstract] ABSTRACT: Iron is an absolute requirement for nearly all organisms, but most bacterial pathogens are faced with extreme iron-restriction within their host environments. To overcome iron limitation pathogens have evolved precise mechanisms to steal iron from host supplies. Staphylococcus aureus employs the iron-responsive surface determinant (Isd) system as its primary heme-iron uptake pathway. Hemoglobin or hemoglobin-haptoglobin complexes are bound by Near iron-Transport (NEAT) domains within cell surface anchored proteins IsdB or IsdH. Heme is stripped from the host proteins and transferred between NEAT domains through IsdA and IsdC to the membrane transporter IsdEF for internalization. Once internalized, heme can be degraded by IsdG or IsdI, thereby liberating iron for the organism. Most components of the Isd system have been structurally characterized to provide insight into the mechanisms of heme binding and transport. This review summarizes recent research on the Isd system with a focus on the structural biology of heme recognition.
[Show abstract][Hide abstract] ABSTRACT: A major contributor to disease in developing countries is the lack of clean drinking water. Solar water disinfection is a popular method used in such countries and involves elimination of pathogenic bacteria from contaminated water. SODIS requires the storage of contaminated water in clear plastic bottles and exposing the bottles to sunlight for several days. Sunlight contains mainly ultraviolet-A radiation, which has long been known to damage deoxyribonucleic acid and generate formation of reactive oxygen species, making it a popular tool for killing bacteria. Storage in clear plastic bottles ensures the contaminated water is not continuously exposed to bacteria and that sunlight can pass through and kill the bacteria contained within. It has been observed that trace amounts of aromatic compounds, polyethlylene terephthalate, are released from plastic bottles into the water. This suggests that aromatic compounds could play a role in either assisting or interfering with the efficiency of SODIS. In our study, we performed SODIS on contaminated water within PET bottles, with and without the addition of aromatic compounds, tryptophan and phthalate. Due to their ability to react with ROS, aromatic compounds have been speculated to induce quenching of these ROS, leading to a decrease in SODIS efficiency. Surprisingly, the addition of aromatic compounds resulted in a more rapid decline in bacterial survival. Our results suggested that aromatic compounds could enhance the germicidal effects of ROS. _____________________________________________________________ Diarrhea is the second most frequent illness in the world (19). Acute infectious diarrhea and resulting diseases account for an estimated 12,600 deaths each day in children in developing countries, and the primary diarrhea inducers in these areas are enterotoxigenic Escherichia coli (ETEC) and rotaviruses (19). As diarrhea caused by ETEC generally transmits via the fecal-oral route and requires a relatively high infectious dose, environmental contamination in endemic areas is exceptionally prevalent (20). The most common vehicles of infection for these environmental ETEC are food and drinking water (20); therefore, food and water sterilization is essential in controlling the transmittance of infectious diarrhea. Of the various mechanisms of sterilization being utilized to de-contaminate drinking water in developing countries, solar water disinfection (SODIS) is one of the most accessible and affordable. Research on solar water disinfection was initiated in 1970s and matured into a reliable and effective water treatment method in the beginning of 1980s (1). Since high-intensity solar radiation is available in most developing countries, especially those around the equator, this method aims to utilize this solar energy to inactivate fecal coliform bacteria present in water (3). Of the arrays of solar radiation reaching the earth, the ultraviolet (UV)-A spectrum is the most crucial for SODIS (3). Due to its practicality and low cost, SODIS has been increasingly accepted as a feasible water disinfection method in Africa, Asia, and South America (3). Polyethlylene terephthalate (PET) plastic bottles have been recommended as the best water containers for performing SODIS (3). However, it has been reported that small amounts of aromatic compounds, such as phthalates and adipates, are released into the water from PET bottles that have undergone prolonged SODIS usage (1). Since the amount released measured was similar to that present in regular tap water, it has been speculated that water treated by SODIS presents approximately the same level of safety as tap water with respect to phthalates and adipates (3). Interestingly, previous studies have shown a decrease of bacterial killing when the microorganisms being UV-irradiated were in spread plates instead of PET bottles (2). Furthermore, it has been suggested that aromatic compounds found in laboratory media used to culture bacteria, like tryptophan and tyrosine, were quenching the effects of reactive oxygen species (ROS) mediated killing. Quenching of ROS by aromatic compounds is possible because singlet oxygen is known to react with electron-rich double bonds without the formation of free radical intermediates (11). It has been shown that amino acids such as histidine and tyrosine eliminate ROS in the same manner as the quenching of singlet oxygen (11, 12). Moreover, Matysik et al. also demonstrated the ability of proline to eliminate singlet oxygen and hydroxy radicals (14).