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S Khalid

Publications and source records attributed to S Khalid.

6 recordsLinked to original sources
PubMed1993

An extended X-ray absorption fine structure investigation of the structure of the active site of lactoperoxidase.

Native lactoperoxidase, compound III, and the reduced forms (at pH 6 and 9) were studied using X-ray absorption spectroscopy (XAS). Native lactoperoxidase has four pyrrole nitrogen ligands at an average distance of 2.04 +/- 0.01 A, a proximal ligand at 1.91 +/- 0.02 A, and a sixth (distal) ligand at 2.16 +/- 0.03 A. Lactoperoxidase native enzyme has a first coordination shell structure that is similar to that of native lignin peroxidase [Sinclair, R., Yamazaki, I., Bumpus, J., Brock, B., Chang, C.-S., Albo, A., & Powers, L. (1992) Biochemistry 31, 4892-4900] and different from that of horseradish peroxidase [Chance, B., Powers, L., Ching, Y., Poulos, T., Schonbaum, G., Yamazaki, I., & Paul, K. (1984) Arch. Biochem. Biophys. 235, 596-611]. Similarly, lactoperoxidase compound III resembles lignin peroxidase compound III. The five-coordinated ferrous form was stable at pH 9, but at pH 6 it was rapidly converted to the six-coordinated form with a distal ligand at 2.18 +/- 0.03 A. No evidence typical of changes in spin state was obtained at the different pH values.

Absorptiometry, Photon↗
PubMed1993

pH dependence of the active site of horseradish peroxidase compound II.

Using X-ray absorption spectroscopy, we investigated the active site of horseradish peroxidase (HRP) compound II at two different pH values. The results indicate that the bond length of the sixth coordinated ligand of the active site was 1.90 +/- 0.02 A at pH 7, decreasing to 1.72 +/- 0.02 A at pH 10. The average iron-to-pyrrole nitrogen and the proximal ligand bond lengths showed no significant changes. The position of higher coordination shells around the iron center changed, implying that some movement or deformation of nearby amino acid residues and/or of the heme occurred. Results of this study suggest that the decrease of the Fe-O bond length of HRP compound II at the higher pH might be attributed to the loss of a hydrogen bond which is present between the oxygen ligand and an amino acid residue in the heme pocket at pH 7.

Absorptiometry, Photon↗
PubMed1989

Extended X-ray absorption fine structure study of Rhodospirillum rubrum and Rhodospirillum molischianum cytochromes c': relationship between heme stereochemistry and spin state.

An EXAFS study on the oxidized and reduced forms of cytochromes c' from Rhodospirillum rubrum and Rhodospirillum molischianum was performed at pH 7. The cytochromes c' have an apparent coordination number of 5 in both oxidation states. Average Fe-ligand bond lengths of 2.02 +/- 0.025 and 2.06 +/- 0.025 A are obtained in their oxidized and reduced forms, respectively. By use of suitable values for the Fe-NHis bond length and Fe out-of-plane displacement, as determined by small molecule crystallographic techniques, the Fe-Npyrrole bond lengths and the porphyrin center-to-Npyrrole distance have been estimated for cytochrome c' in both of its oxidation states. With this model, estimates of the Fe-Npyrrole bond lengths are 2.01 +/- 0.03 and 2.05 +/- 0.03 A, for the oxidized and reduced cytochromes c', respectively. The center-to-Npyrrole distance is estimated to be 1.99 +/- 0.03 A for oxidized cytochrome c' and 2.03 +/- 0.03 A for reduced cytochrome c'.

Cytochrome c Group↗
PubMed1987

Kinetic, structural, and spectroscopic identification of geminate states of myoglobin: a ligand binding site on the reaction pathway.

Elementary steps or geminate states in the reaction of gaseous ligands with transport proteins delineate the trajectory of the ligand and its rebinding to the heme. By use of kinetic studies of the 765-nm optical "conformation" band, three geminate states were identified for temperatures less than approximately 100 K. MbCO, which is accumulated by photolysis between 1.2 and approximately 10 K, was characterized by our previous optical and X-ray absorption studies [Chance, B., Fischetti, R., & Powers, L. (1983) Biochemistry 22, 3820-3829]. Between 10 and approximately 100 K, geminate states that are also identified that have recombination rates of approximately 10(3) s-1 and approximately 10(-5) s-1 (40 K). Thus, it is possible to maintain a steady-state nearly homogeneous population of the slowest recombining geminate state, Mb, by regulated continuous illumination (optical pumping). Both X-ray absorption and resonance Raman studies under similar conditions of optical pumping show that the heme structure around the iron in Mb is similar to that of MbCO. In both geminate states, the iron-proximal histidine distance remains unchanged (+/- 0.02 A) from that of MbCO while the iron to pyrrole nitrogen average distance has not fully relaxed to that of the deoxy state. In MbCO the CO remains close to iron but not bound, and the Fe...CO angle, which is bent in MbCO (127 +/- 4 degrees C), is decreased by approximately 15 degrees [Powers, L., Sessler, J. L., Woolery, G. L., & Chance, B. (1984) Biochemistry 23, 5519-5523]. The CO molecule in Mb, however, has moved approximately 0.7 A further from iron. Computer graphics modeling of the crystal structure of MbCO places the CO in a crevice in the heme pocket that is just large enough for the CO molecule end-on. Above approximately 100 K resonance Raman studies show that this structure relaxes to the deoxy state.

Binding Sites↗
PubMed1984

Antimalarial psychosis revisited.

Drug-induced psychosis resulted from the administration of quinacrine hydrochloride at a dosage of 100 mg twice daily for the treatment of discoid lupus. After two weeks of drug treatment, the patient exhibited symptoms similar to those seen in the manic phase of bipolar affective disorder. Although the incidence of antimalarial psychosis is unclear and perhaps small, we recommend close observation of patients for behavioral changes suggesting the development of antimalarial psychosis. Astute observation, leading to the early detection of behavioral changes, may help avert the development of a costly and disabling complication of drug therapy.

Female↗