PubMed Health⌕ Search

Biomedical subjects

G C Chu

Publications and source records attributed to G C Chu.

13 recordsLinked to original sources
PubMed2000

Histidine 20, the crucial proximal axial heme ligand of bacterial heme oxygenase Hmu O from Corynebacterium diphtheriae.

The hemin complex of Hmu O, a 24-kDa soluble heme degradation enzyme in Corynebacterium diphtheriae, is coordinated axially to a neutral imidazole of a proximal histidine residue in Hmu O. To identify which of the eight histidines in Hmu O is the proximal heme ligand, we have constructed and expressed the plasmids for eight His --> Ala Hmu O mutants. Reconstituted with hemin, the active site structures and enzymatic activity of these mutants have been examined by EPR, resonance Raman, and optical absorption spectroscopy. EPR of the NO-bound ferrous heme-Hmu O mutant complexes reveals His(20) as the proximal heme ligand in Hmu O, and this is confirmed by resonance Raman results from the ligand-free ferrous heme-H20A. All eight His --> Ala mutants bind hemin stoichiometrically, proving that none of the histidines is essential for hemin-Hmu O formation. However, His(20) is crucial to Hmu O catalysis. Its absence by point mutation has inhibited the conversion of hemin to biliverdin. The ferric heme-H20A complex is pentacoordinate. Resonance Raman of the CO-bound ferrous heme-H20A corroborates this and reveals an Fe-C-O bending mode, delta(Fe-C-O), the first reported for a pentacoordinate CO-bound hemeprotein. The appearance of delta(Fe-C-O) in C. diphtheriae Hmu O H20A but not mammalian HO-1 mutant H25A indicates that the heme environment between the two heme oxygenases is different.

Alanine↗
PubMed1999

The heme complex of Hmu O, a bacterial heme degradation enzyme from Corynebacterium diphtheriae. Structure of the catalytic site.

Hmu O, a heme degradation enzyme in Corynebacterium diphtheriae, forms a stoichiometric complex with iron protoporphyrin IX and catalyzes the oxygen-dependent conversion of hemin to biliverdin, carbon monoxide, and free iron. Using a multitude of spectroscopic techniques, we have determined the axial ligand coordination of the heme-Hmu O complex. The ferric complex shows a pH-dependent reversible transition between a water-bound hexacoordinate high spin neutral pH form and an alkaline form, having high spin and low spin states, with a pK(a) of 9. (1)H NMR, EPR, and resonance Raman of the heme-Hmu O complex establish that a neutral imidazole of a histidine residue is the proximal ligand of the complex, similar to mammalian heme oxygenase. EPR of the deoxy cobalt porphyrin IX-Hmu O complex confirms this proximal histidine coordination. Oxy cobalt-Hmu O EPR reveals a hydrogen-bonding interaction between the O(2) and an exchangeable proton in the Hmu O distal pocket and two distinct orientations for the bound O(2). Mammalian heme oxygenase has only one O(2) orientation. This difference and the mixed spin states at alkaline pH indicate structural differences in the distal environment between Hmu O and its mammalian counterpart.

Bacterial Proteins↗
PubMed1999

Heme degradation as catalyzed by a recombinant bacterial heme oxygenase (Hmu O) from Corynebacterium diphtheriae.

Hmu O, a heme degradation enzyme in the pathogen Corynebacterium diphtheriae, catalyzes the oxygen-dependent conversion of hemin to biliverdin, carbon monoxide, and free iron. A bacterial expression system using a synthetic gene coding for the 215-amino acid, full-length Hmu O has been constructed. Expressed at very high levels in Escherichia coli BL21, the enzyme binds hemin stoichiometrically to form a hexacoordinate high spin hemin-Hmu O complex. When ascorbic acid is used as the electron donor, Hmu O converts hemin to biliverdin with alpha-hydroxyhemin and verdoheme as intermediates. The overall conversion rate to biliverdin is approximately 4-fold slower than that by rat heme oxygenase (HO) isoform 1. Reaction of the hemin-Hmu O complex with hydrogen peroxide yields a verdoheme species, the recovery of which is much less compared with rat HO-1. Reaction of the hemin complex with meta-chloroperbenzoic acid generates a ferryl oxo species. Thus, the catalytic intermediate species and the nature of the active form in the first oxygenation step of Hmu O appear to be similar to those of the mammalian HO. However, the considerably slow catalytic rate and low level of verdoheme recovery in the hydrogen peroxide reaction suggest that the active-site structure of Hmu O is different from that of its mammalian counterpart.

Amino Acid Sequence↗
PubMed1999

Crystallization and preliminary X-ray diffraction analysis of a recombinant bacterial heme oxygenase (Hmu O) from Corynebacterium diphtheriae.

Hmu O is a 24-kDa soluble bacterial heme degradation enzyme found in the pathogen Corynebacterium diphtheriae, the causative agent of diphtheria. Similar to the mammalian heme oxygenase, it binds hemin stoichiometrically and catalyzes the oxygen-dependent conversion of hemin to biliverdin, carbon monoxide, and free iron. Iron is an essential nutrient for bacteria and especially important for pathogenesis. Here we report the first crystallization and preliminary crystallographic study of the heme-Hmu O complex formed from hemin and a recombinant Hmu O, which was expressed in Escherichia coli from a synthetic gene based on the putative hmu O gene sequence. Crystals of the heme-Hmu O complex were obtained by the sitting drop vapor diffusion method using a precipitant solution containing 18% (w/v) PEG 8000 and 0.2 M calcium acetate in 0.1 M sodium cacodylate (pH 6.5). Using synchrotron radiation, the heme-Hmu O crystal diffracted to 2.8 A resolution. It belongs to the monoclinic space group C2, with unit cell parameters a = 123.18 A, b = 44.51 A, c = 92.10 A, and beta = 123.3 degrees. Assuming one molecule of the heme-Hmu O complex per asymmetric unit, the calculated value of Vm is 2.89 A3/Da.

Bacterial Proteins↗
PubMed1996

Maturation of the acetylcholine receptor in skeletal muscle: regulation of the AChR gamma-to-epsilon switch.

During the development of the mammalian neuromuscular junction, acetylcholine receptors (AChRs) become localized to the postsynaptic muscle membrane. As this process nears completion, the fetal form of the receptor, containing a gamma subunit (composition alpha 2 beta gamma delta) is gradually replaced by an epsilon subunit-containing adult form (alpha 2 beta epsilon delta). To understand how this transition is controlled, we compared the expression and regulation of the AChR gamma and epsilon subunits in developing, adult, and cultured muscles. Immunostaining with subunit-specific antibodies showed that replacement of gamma subunit- by epsilon subunit-containing AChRs occurs largely during the first postnatal week in fast-twitch muscles, and occurs homogeneously throughout individual endplates. In the slow-twitch soleus, however, this transition is delayed, and in the multiply innervated slow fibers of extraocular muscle, gamma subunit expression persists into adulthood. The transcriptional bases of the AChR subunit transition, and of these intermuscular variations, were demonstrated in mice bearing transgenes containing promoter elements from the AChR gamma and epsilon subunit genes, each coupled to a nuclear-localized beta-galactosidase (nlacZ) reporter. We show that transgene expression is stimulated by the nerve-derived inducer of AChR expression, ARIA, in myotubes cultured from gamma-nlacZ as well as epsilon-nlacZ mice. However, the expression of gamma-nlacZ, but not epsilon-nlacZ, is increased by treatment of myotubes with TTX, and the ARIA sensitivity of gamma-nlacZ is dependent on the electrical state of the myotube. Thus, the promoters of the gamma and epsilon subunit genes may integrate ARIA- and activity-dependent signals in different ways to generate their complementary patterns of expression.

Animals↗
PubMed1996

ARIA/HRG regulates AChR epsilon subunit gene expression at the neuromuscular synapse via activation of phosphatidylinositol 3-kinase and Ras/MAPK pathway.

AChR-inducing activity (ARIA)/heregulin, a ligand for erbB receptor tyrosine kinases (RTKs), is likely to be one nerve-supplied signal that induces expression of acetylcholine receptor (AChR) genes at the developing neuromuscular junction. Since some RTKs act through Ras and phosphatidylinositol 3-kinase (PI3K), we investigated the role of these pathways in ARIA signaling. Expression of activated Ras or Raf mimicked ARIA-induction of AChR epsilon subunit genes in muscle cells; whereas dominant negative Ras or Raf blocked the effect of ARIA. ARIA rapidly activated erk1 and erk2 and inhibition of both erks also abolished the effect of ARIA. ARIA stimulated association of PI3K with erbB3, expression of an activated PI3K led to ARIA-independent AChR epsilon subunit expression, and inhibition of PI3K abolished the action of ARIA. Thus, synaptic induction of AChR genes requires activation of both Ras/MAPK and PI3K signal transduction pathways.

Calcium-Calmodulin-Dependent Protein Kinases↗
PubMed1995

Failure of postsynaptic specialization to develop at neuromuscular junctions of rapsyn-deficient mice.

Of numerous synaptic components that have been identified, perhaps the best-studied are the nicotinic acetylcholine receptors (AChRs) of the vertebrate neuromuscular junction. AChRs are diffusely distributed on embryonic myotubes, but become highly concentrated (approximately 10,000 microns-2) in the postsynaptic membrane as development proceeds. At least two distinct processes contribute to this accumulation. One is local synthesis: subsynaptic muscle nuclei transcribe AChR subunit genes at higher rates than extra-synaptic nuclei, so AChR messenger RNA is concentrated near synaptic sites. Second, once AChRs have been inserted in the membrane, they form high-density clusters by tethering to a subsynaptic cytoskeletal complex. A key component of this complex is rapsyn, a peripheral membrane protein of relative molecular mass 43K (refs 4, 5), which is precisely colocalized with AChRs at synaptic sites from the earliest stages of neuromuscular synaptogenesis. In heterologous systems, expression of recombinant rapsyn leads to clustering of diffusely distributed AChRs, suggesting that rapsyn may control formation of clusters. To assess the role of rapsyn in vivo, we generated and characterized mutant mice with a targeted disruption of the Rapsyn gene. We report that rapsyn is essential for the formation of AChR clusters, but that synapse-specific transcription of AChR subunit genes can proceed in its absence.

Animals↗
PubMed1995

Synapse-associated expression of an acetylcholine receptor-inducing protein, ARIA/heregulin, and its putative receptors, ErbB2 and ErbB3, in developing mammalian muscle.

Developing motor axons induce synaptic specializations in muscle fibers, including preferential transcription of acetylcholine receptor (AChR) subunit genes by subsynaptic nuclei. One candidate nerve-derived signaling molecule is AChR-inducing activity (ARIA)/heregulin, a ligand of the erbB family of receptor tyrosine kinases. Here, we asked whether ARIA and erbB kinases are expressed in patterns compatible with their proposed signaling roles. In developing muscle, ARIA was present not only at synaptic sites, but also in extrasynaptic regions of the muscle fiber. ARIA was synthesized, rather than merely taken up, by muscle cells, as indicated by the presence of ARIA mRNA in muscle and of ARIA protein in a clonal muscle cell line. ARIA-responsive myotubes expressed both erbB2 and erbB3, but little EGFR/erbB1 or erbB4. In adults, erbB2 and erbB3 were localized to the postsynaptic membrane. ErbB3 was restricted to the postsynaptic membrane perinatally, at a time when ARIA was still broadly distributed. Thus, our data are consistent with a model in which ARIA interacts with erbB kinases on the muscle cell surface to provide a local signal that induces synaptic expression of AChR genes. However, much of the ARIA is produced by muscle, not nerve, and the spatially restricted response may result from the localization of erbB kinases as well as of ARIA. Finally, we show that erbB3 is not concentrated at synaptic sites in mutant mice that lack rapsyn, a cytoskeletal protein required for AChR clustering, suggesting that pathways for synaptic AChR expression and clustering interact.

Aging↗
PubMed1995

Regulation of the acetylcholine receptor epsilon subunit gene by recombinant ARIA: an in vitro model for transynaptic gene regulation.

Structural specialization of the postsynaptic skeletal muscle membrane is in part mediated by the motor neuron-induced transcriptional regulation of synaptic muscle nuclei. ARIA, a factor that stimulates production of acetylcholine receptors (AChRs), is a candidate signaling molecule for such regulation. Here we examine the transynaptic inducing potential of this polypeptide factor. ARIA immunoreactivity is detectable at synaptic sites in vivo. In vitro, recombinant heregulin beta 1 (rHRG beta 1), the human homolog of ARIA, induces expression of the AChR epsilon gene, the subunit most sensitive to synaptic input. The inducing property of rHRG beta 1 is demonstrated most dramatically in primary muscle cultures from transgenic mice bearing an epsilon promoter-nuclear lacZ reporter transgene. Transient transfection experiments using the Sol 8 muscle cell line indicate that sequences that confer responsiveness to ARIA are located within a 150 bp epsilon subunit promoter region and are E box-independent. These results suggest that ARIA performs a vital role by directing spatially restricted gene expression at the neuromuscular junction.

Animals↗
PubMed1975

Consumer perceptions of health care services: implications for academic medicine.

The factor analytic development of various measures of consumer perceptions regarding characteristics of doctors and health care services is described. Index scores meeting factor analytic and reliability criteria were used to study the importance of consumer perceptions in relation to behavioral outcomes. Numerous dimensions of consumer perceptions were identified and described, including beliefs about doctor conduct in terms of quality of care and humaneness of health care delivery as well as satisfaction with such enabling components as the continuity of care, availability and convenience of services, and various access mechanisms (cost, payment mechanisms, and ease of emergency care facilities). Measures of these perceptions were shown to be related to differences in several estimates of health services utilization. The use of the index scores which have met empirical criteria is in contrast to the common practice of using individual questionnaire items as the unit of analysis in health care research. Findings are discussed in relation to program planning and evaluation in medical education, and suggestions for future research are noted.

Attitude to Health↗