46 The Hidden Truth About Gender Bias in Surgery. A Systematic Review

Introduction

Gender discrimination remains pervasive in surgery, significantly impacting current and future surgeons and population health. This study analyses the gender barriers and critical retention factors for female surgeons and trainees in surgery.

Method

Five electronic databases were searched till May 2020. Titles and abstracts were sieved, followed by a full text review. Data synthesis and inductive thematic analysis were conducted using the Thomas and Harden methodology.

Result

14 articles were included, involving 528 participants. Four themes were generated–unfavourable working environment, male-dominated culture, societal pressures and progress towards gender equality. Females in surgery often faced harassment, disrespect and perceptions of incompetence, resulting in hostile work conditions, which were aggravated by the inadequate support and mentorship. The persistence of male-dominated cultures was observed, with females facing prejudice and exclusion from professional and social circles. Differential treatment and higher expectations of female surgeons also arose from entrenched societal pressures. Despite these, increased acceptance of motherhood and greater recognition of contributions by female surgeons were reported, indicating some progress in gendered culture.

Conclusions

There is a need to increase female surgical leadership and allocate resources to address the deep-rooted causes of biased surgical culture and ingrained perceptions, to achieve greater gender equality in surgery.

Pseudogout - a rare manifestation of hungry bone syndrome after focused parathyroidectomy

Pseudogout, also known as calcium pyrophosphate deposition disease, is a rheumatological condition arising from accumulation of calcium pyrophosphate dihydrate crystals in connective tissues. We present a case of a 56-year-old Bangladeshi woman who underwent focused right inferior parathyroidectomy for primary hyperparathyroidism from a right inferior parathyroid adenoma. On the first post-operative day, she complained of left elbow painful swelling with redness and warmth. Arthrocentesis of left elbow was done due to suspicion of septic arthritis. Two weeks prior to this surgery, she had sudden bilateral knee swelling was diagnosed in her home country of bilateral knee osteoarthritis with effusion and arthrocentesis showed no crystals. Aspiration of left elbow showed calcium pyrophosphate crystals, associated with post parathyroidectomy hypocalcemia, hypomagnesemia confirming pseudogout. Her uric acid level was normal. Bilateral wrist x-rays showed triangular fibrocartilage complex chondrocalcinosis. The patient's condition improved with colchicine and naproxen, as well as calcium and magnesium replacement. Her left elbow swelling and pain resolved. Pseudogout flare is a rare but known sequelae after parathyroidectomy. Early recognition and expeditious treatment is essential.

Intrapallidal injection of 6-hydroxydopamine induced changes in dopamine innervation and neuronal activity of globus pallidus

The globus pallidus (GP) plays an important role in basal ganglia circuitry. In contrast to the well-characterized actions of dopamine on striatal neurons, the functional role of the dopamine innervation of GP is still not clearly determined. The present study aimed to investigate the effects of intrapallidal injection of 6-hydroxydopamine (6-OHDA) on rotational behavior induced by apomorphine, on the loss of dopamine cell bodies in the substantia nigra pars compacta (SNc) and fibers in the GP and striatum and on in vivo extracellularly-recorded GP neurons in the rat. Injection of 6-OHDA into GP induced severe loss of tyrosine hydroxylase-immunoreactive (TH-IR) fibers in GP (-85%) with a reduction in the number of TH-IR cell bodies in the SNc (-52%) and fibers in the striatum (-50%). S.c. injection of apomorphine in these rats induced a moderate number of turns (26+/-6 turns/5 min). Electrophysiological recordings show that 6-OHDA injection in GP induced a significant decrease of the firing rate of GP neurons (16.02+/-1.11 versus 24.14+/-1.58 spikes/sec in control animals and 22.83+/-1.28 in sham animals, one-way ANOVA, P<0.0001) without any change in the firing pattern (chi(2)=1.03, df=4, P=0.90). Our results support the premise of the existence of collaterals of SNc dopaminergic axons projecting to the striatum and GP and that dopamine plays a role in the modulation of the firing rate but not the firing pattern of GP neurons. Our data provide important insights into the functional role of the SNc-GP dopaminergic pathway suggesting that dopamine depletion in GP may participate in the development of motor disabilities.

Expression of aminopeptidase N in bile canaliculi: a predictor of clinical outcome in biliary atresia and a potential tool to implicate the mechanism of biliary atresia

BACKGROUND

Only a few studies on extrahepatic biliary atresia (BA) have reported that the morphological changes of bile canaliculi could predict the clinical outcome after portoenterostomy and provide differential diagnosis of neonatal jaundice. Aminopeptidase N (APN) is an ectoenzyme of bile canaliculi that is involved in bile secretion. In this study, we tried to see whether APN of bile canaliculi had a significant role in BA.

PATIENTS AND METHODS

We used monoclonal antibody 9B2 to compare the expression of APN in livers with BA, neonatal hepatitis, and choledochal cysts, as well as in nontumorous portions of pediatric hepatic livers with tumors. The expression of APN in fetuses, preterm babies, and term neonates was also studied.

RESULTS

A high degree of 9B2 expression in BA was closely related to poor outcome. Cholestasis in choledochal cysts, rather than neonatal hepatitis, made 9B2 expression stronger. Increasing expression of 9B2 from fetuses to neonates was noted and the degree of 9B2 expression was similar between term neonates and nontumorous portions of pediatric livers with tumors. Interestingly, some cases of BA had 9B2 expression like that of preterm babies.

CONCLUSIONS

APN of bile canaliculi progressively develops from fetuses to neonates and is well developed in neonates. APN can be induced to stronger expression by obstructive jaundice. The amount of expression of APN of bile canaliculi in BA is a predictor of clinical outcome and may be a tool for implicating the mechanism of BA.

Characterization of the allosteric anion-binding site of O-acetylserine sulfhydrylase

A new crystal structure of the A-isozyme of O-acetylserine sulfhydrylase-A (OASS) with chloride bound to an allosteric site located at the dimer interface has recently been determined [Burkhard, P., Tai, C.-H., Jansonius, J. N., and Cook, P. F. (2000) J. Mol. Biol. 303, 279-286]. Data have been obtained from steady state and presteady-state kinetic studies and from UV-visible spectral studies to characterize the allosteric anion-binding site. Data obtained with chloride and sulfate as inhibitors indicate the following: (i) chloride and sulfate prevent the formation of the external aldimines with L-cysteine or L-serine; (ii) chloride and sulfate increase the external aldimine dissociation constants for O-acetyl-L-serine, L-methionine, and 5-oxo-L-norleucine; (iii) chloride and sulfate bind to the allosteric site in the internal aldimine and alpha-aminoacrylate external aldimine forms of OASS; (iv) sulfate also binds to the active site. Sulfide behaves in a manner identical to chloride and sulfate in preventing the formation of the L-serine external aldimine. The binding of chloride to the allosteric site is pH independent over the pH range 7-9, suggesting no ionizable enzyme side chains ionize over this pH range. Inhibition by sulfide is potent (K(d) is 25 microM at pH 8) suggesting that SH(-) is the physiologic inhibitory species.

Electroacupuncture-induced neural activation detected by use of manganese-enhanced functional magnetic resonance imaging in rabbits

OBJECTIVE

To investigate the effects of acupuncture on neural activity detected by use of manganese-enhanced functional magnetic resonance imaging (fMRI) and elucidate the relationship between somatic acupoint stimulation and brain activation.

ANIMALS

40 New Zealand White rabbits.

PROCEDURE

Manganese-enhanced fMRI was performed in anesthetized rabbits manipulated with electroacupuncture (EA) on Zusanli (ST-36) and Yanglingquan (GB-34) acupoints. Image acquisition was performed on a 1.5T superconductive clinical scanner with a circular polarized extremity coil. T1-weighted images were acquired sequentially as follows: baseline, after mannitol injection, after manganese infusion, and 5 and 20 minutes after initiation of EA.

RESULTS

Changes in focal neural activity were detected by use of manganese-enhanced fMRI. Stimulation on Zusanli (ST-36) for 5 minutes resulted in activation of the hippocampus, whereas stimulation on Yanglingquan (GB-34) resulted in activation of the hypothalamus, insula, and motor cortex. Activation became less specific after 20 minutes of EA. Furthermore, stimulation on ipsilateral acupoints led to bilateral brain activation.

CONCLUSIONS AND CLINICAL RELEVANCE

Each acupoint has a corresponding cerebral linkage, and stimulation on these points resulted in time-dependent neural activation. Understanding the linkage between peripheral acupoint stimulation and central neural pathways may provide a useful guide for clinical applications of acupuncture.

Pyridoxal 5'-phosphate-dependent alpha,beta-elimination reactions: mechanism of O-acetylserine sulfhydrylase

O-Acetylserine sulfhydrylase catalyzes the replacement of the beta-acetoxy group of O-acetyl-L-serine with sulfide to generate L-cysteine. The reaction represents the final step in the biosynthesis of L-cysteine in enteric bacteria and plants. A quinonoid intermediate has not been detected using a variety of kinetic and spectroscopic probes for the wild-type or mutant enzymes, ruling out an E1 mechanism. The structure of the external Schiff base intermediate indicates an anti elimination. O-Acetylserine sulfhydrylase is the only known pyridoxal 5'-phosphate-dependent enzyme that catalyzes a beta-elimination reaction to have an anti E2 mechanism.

Identification of an allosteric anion-binding site on O-acetylserine sulfhydrylase: structure of the enzyme with chloride bound

A new crystal structure of O-acetylserine sulfhydrylase (OASS) has been solved with chloride bound at an allosteric site and sulfate bound at the active site. The bound anions result in a new "inhibited" conformation, that differs from the "open" native or "closed" external aldimine conformations. The allosteric site is located at the OASS dimer interface. The new inhibited structure involves a change in the position of the "moveable domain" (residues 87-131) to a location that differs from that in the open or closed forms. Formation of the external aldimine with substrate is stabilized by interaction of the alpha-carboxyl group of the substrate with a substrate-binding loop that is part of the moveable domain. The inhibited conformation prevents the substrate-binding loop from interacting with the alpha-carboxyl group, and hinders formation of the external Schiff base and thus subsequent chemistry. Chloride may be an analog of sulfide, the physiological inhibitor. Finally, these results suggest that OASS represents a new class of PLP-dependent enzymes that is regulated by small anions.

O-acetylserine sulfhydrylase

The 31P NMR data suggest slight differences in the structures around the 5'-P for the internal Schiff base and the lanthionine external Schiff base (both largely ketoeneamine) and a large difference for enolimine portion of the serine external Schiff base. Addition of cysteine or serine increase delayed fluorescence and triplet to singlet energy transfer. Addition of OAS exhibits a splitting of the 0,0 vibronic, the result of two distinct conformations, likely enolimine and ketoeneamine tautomers. Nonetheless, the alpha-amino-acrylate Schiff base conformation differs from either the internal or external Schiff base conformations. All of the time-resolved fluorescence data are consistent with conformation changes reflecting redistribution of ketoeneamine and enolimine tautomers as catalysis occurs. It is important to remember that the structural changes are substantial. The native structure (internal Schiff base) is active site open, while the K41A mutant enzyme (ketoeneamine external Schiff base) is active site closed. The trigger for the conformational change from open to closed as one goes from the internal to external Schiff base is the occupancy of the alpha-carboxyl subsite of the active site (Burkhard et al., 1999). Associated with this, as observed in pH-rate profiles, pH-dependent changes in phosphorescence, and pH-dependent changes in fluorescence enhancement upon binding acetate or cysteine is an enzyme group with a pK in the range 7-8. Dependent on the protonation state of the enzyme group, structural changes likely occur that also reflect a redistribution of the tautomeric equilibrium. Finally, the minimal catalytic cycle can likely be pictured as shown in Fig. 20. The changes may be pH dependent, and the open conformations for the internal Schiff base and the alpha-aminoacrylate Schiff base are not identical structurally, as expected because of the increased stability of the latter.

pH-induced intramolecular electron transfer between the iron-sulfur protein and cytochrome c(1) in bovine cytochrome bc(1) complex

Structural analysis of the bc(1) complex suggests that the extra membrane domain of iron-sulfur protein (ISP) undergoes substantial movement during the catalytic cycle. Binding of Qo site inhibitors to this complex affects the mobility of ISP. Taking advantage of the difference in the pH dependence of the redox midpoint potentials of cytochrome c(1) and ISP, we have measured electron transfer between the [2Fe-2S] cluster and heme c(1) in native and inhibitor-treated partially reduced cytochrome bc(1) complexes. The rate of the pH-induced cytochrome c(1) reduction can be estimated by conventional stopped-flow techniques (t1/2, 1-2 ms), whereas the rate of cytochrome c(1) oxidation is too high for stopped-flow measurement. These results suggest that oxidized ISP has a higher mobility than reduced ISP and that the movement of reduced ISP may require an energy input from another component. In the 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole (UHDBT)-inhibited complex, the rate of cytochrome c(1) reduction is greatly decreased to a t1/2 of approximately 2.8 s. An even lower rate is observed with the stigmatellin-treated complex. These results support the idea that UHDBT and stigmatellin arrest the [2Fe-2S] cluster at a fixed position, 31 A from heme c(1), making electron transfer very slow.

Monitoring of the levodopa concentration-response relationship in Parkinson's disease

Motor fluctuations and abnormal involuntary movements are common complications encountered in advanced Parkinson's disease (PD) patients with long-term levodopa therapy. Monitoring of plasma levodopa concentrations and clinical effects has been reported to benefit the management of these complications. However, to our knowledge, there is no data available in Taiwan concerning the correlation between the plasma levodopa levels and motor fluctuations. In this study, we developed the laboratory methodology for plasma levodopa determination by using the aluminum extraction procedure and HPLC-ED. Serial blood samples and motor scores were obtained from 7 PD patients, and the correlation between plasma levodopa levels and motor responses were studied individually. In three patients with wearing-off phenomenon, plasma levodopa concentrations are compatible with the clinical "on" and "off" states. In the other four patients with complex fluctuating responses, their levodopa dosages were adjusted by the results of monitoring. Better motor responses were achieved by optimization of the levodopa pharmacokinetics in these patients. Our preliminary data suggest that simultaneous monitoring of plasma levodopa concentrations and clinical effects might be helpful to improve the therapeutic strategy in some of the parkinsonian patients with fluctuating responses to levodopa.

Substitution of pyridoxal 5'-phosphate in D-serine dehydratase from Escherichia coli by cofactor analogues provides information on cofactor binding and catalysis

D-Serine dehydratase (DSD) is a pyridoxal 5'-phosphate-dependent enzyme that catalyzes the conversion of D-serine to pyruvate and ammonia. Spectral studies of enzyme species where the natural cofactor was substituted by pyridoxal 5'-sulfate (PLS), pyridoxal 5-deoxymethylene phosphonate (PDMP), and pyridoxal 5'-phosphate monomethyl ester (PLPMe) were used to gain insight into the structural basis for binding of cofactor and substrate analogues. PDMP-DSD exhibits 35% of the activity of the native enzyme, whereas PLS-DSD and PLPMe-DSD are catalytically inactive. The emission spectrum of native DSD when excited at 280 nm shows maxima at 335 and 530 nm. The energy transfer band at 530 nm is very likely generated as a result of the proximity of Trp-197 to the protonated internal Schiff base. The cofactor analogue-reconstituted DSD species exhibit emission intensities decreasing from PLS-DSD, to PLPMe-DSD, and PDMP-DSD, when excited at 415 nm. Large increases in fluorescence intensity at 530 (540) nm can be observed for cofactor analogue-reconstituted DSD in the presence of substrate analogues when excited at 415 nm. In the absence and presence of substrate analogues, virtually identical far UV CD spectra were obtained for all DSD species. The visible CD spectra of native DSD, PDMP-DSD, and PLS-DSD exhibit a band centered on the visible absorption maximum with nearly identical intensity. Addition of substrate analogues to native and cofactor analogue-reconstituted DSD species results in most cases in a decrease or elimination of ellipticity. The results are interpreted in terms of local conformational changes and/or changes in the orientation of the bound cofactor (analogue).

Ligand binding induces a large conformational change in O-acetylserine sulfhydrylase from Salmonella typhimurium

Covalent binding of L-methionine as an external aldimine to the pyridoxal 5'-phosphate-cofactor in the K41A mutant of O-acetylserine sulfhydrylase from Salmonella typhimurium induces a large conformational change in the protein. Methionine mimics the action of the substrate O-acetyl-L-serine during catalysis. The alpha-carboxylate moiety of L-methionine in external aldimine linkage with the active site pyridoxal 5'-phosphate forms a hydrogen bonding network to the "asparagine-loop" P67-T68-N69-G70 which adopts a different conformation than in the native protein. The side-chain nitrogen of Asn69 moves more than 7 A to make a hydrogen bond to the alpha-carboxylate group of the inhibitor. As the external aldimine is formed, the PLP tilts by 13 degrees along its longitudinal axis such that C4' moves toward the entrance to the active site and the side-chain of the methionine is directed toward the active site entrance. The local rearrangement acts as a trigger to induce a large global conformational change in the protein. A subdomain comprised of beta-strand 4, alpha-helix 3, beta-strand 5 and alpha-helix 4 moves towards the active site by a rotation of 7 degrees. This subdomain movement results in a reduction of the severe twist of its central beta-sheet and reduces the active site entrance to a small hole, giving access only to small molecules like sulfide, the second substrate, or acetate, the first product.

Reaction mechanism of fructose-2,6-bisphosphatase. A mutation of nucleophilic catalyst, histidine 256, induces an alteration in the reaction pathway

A bifunctional enzyme, fructose-6-phosphate,2-kinase/fructose 2, 6-bisphosphatase (Fru-6-P,2-kinase/Fru-2,6-Pase), catalyzes synthesis and degradation of fructose 2,6-bisphosphate (Fru-2,6-P2). Previously, the rat liver Fru-2,6-Pase reaction (Fru-2,6-P2 --> Fru-6-P + Pi) has been shown to proceed via a phosphoenzyme intermediate with His258 phosphorylated, and mutation of the histidine to alanine resulted in complete loss of activity (Tauler, A., Lin, K., and Pilkis, S. J. (1990) J. Biol. Chem. 265, 15617-15622). In the present study, it is shown that mutation of the corresponding histidine (His256) of the rat testis enzyme decreases activity by less than a factor of 10 with a kcat of 17% compared with the wild type enzyme. Mutation of His390 (in close proximity to His256) to Ala results in a kcat of 12.5% compared with the wild type enzyme. Attempts to detect a phosphohistidine intermediate with the H256A mutant enzyme were unsuccessful, but the phosphoenzyme is detected in the wild type, H390A, R255A, R305S, and E325A mutant enzymes. Data demonstrate that the mutation of His256 induces a change in the phosphatase hydrolytic reaction mechanism. Elimination of the nucleophilic catalyst, H256A, results in a change in mechanism. In the H256A mutant enzyme, His390 likely acts as a general base to activate water for direct hydrolysis of the 2-phosphate of Fru-2,6-P2. Mutation of Arg255 and Arg305 suggests that the arginines probably have a role in neutralizing excess charge on the 2-phosphate and polarizing the phosphoryl for subsequent transfer to either His256 or water. The role of Glu325 is less certain, but it may serve as a general acid, protonating the leaving 2-hydroxyl of Fru-2,6-P2.

Cysteine 42 is important for maintaining an integral active site for O-acetylserine sulfhydrylase resulting in the stabilization of the alpha-aminoacrylate intermediate

O-Acetylserine sulfhydrylase-A (OASS-A) is a pyridoxal 5'-phosphate (PLP) dependent enzyme from Salmonella typhimurium that catalyzes the beta-replacement of acetate in O-acetyl-L-serine (OAS) by sulfide to give L-cysteine. The reaction occurs via a ping-pong kinetic mechanism in which alpha-aminoacrylate in Schiff base with the active site PLP is an intermediate [Cook, P. F., Hara, S., Nalabolu, S. R., and Schnackerz, K. D. (1992) Biochemistry 31, 2298-2303]. The sequence around the Schiff base lysine (K41) has been determined [Rege, V. D., Kredich, N. M., Tai, C.-H., Karsten, W. E., Schnackerz, K. D., & Cook, P. F. (1996) Biochemistry 35, 13485-13493], and the sole cysteine in the primary structure is immediately C-terminal to the lysine. In an effort to assess the role of C42, it has been changed to serine and alanine by site-directed mutagenesis. The mutant proteins are structurally nearly identical to the wild-type enzyme on the basis of UV-visible, fluorescence, far-UV and cofactor-induced CD, and 31P NMR studies, but subtle structural differences are noted. Kinetic properties of both mutant proteins differ significantly from those of the wild-type enzyme. The C42S mutant exhibits a > 50-fold increase in the OAS:acetate lyase activity and a 17-fold decrease in V for the cysteine synthesis compared to the wild-type enzyme, while decreases of > 200-fold in the OAS: acetate lyase activity and a 30-fold decrease in V for the cysteine synthesis are found for the C42A mutant enzyme. In both cases, however, the pH dependence of kinetic parameters for cysteine synthesis and OAS: acetate lyase activity yield, within error, identical pK values. In the three-dimensional structure of OASS-A, cysteine 42 is located behind the cofactor, pointing away from the active site, toward the interior of the protein. The dramatic change in the OAS:acetate lyase activity of OASS-A in the C42S and C42A mutant proteins likely results from a localized movement of the serine hydroxyl (compared to the cysteine thiol) toward additional hydrophilic, hydrogen-bonding groups in C42S, or away from hydrophilic groups for C42A, repositioning structure around and including K41. Subtle movement of the epsilon-amino group of K41 may change the geometry for nucleophilic displacement of the amino acid from PLP, leading to changes in overall activity and stability of the alpha-aminoacrylate intermediate. Data indicate that single amino acid substitutions that yield only subtle changes in structure can produce large differences in reaction rates and overall mechanism.

Substitution of pyridoxal 5'-phosphate in the O-acetylserine sulfhydrylase from Salmonella typhimurium by cofactor analogs provides a test of the mechanism proposed for formation of the alpha-aminoacrylate intermediate

O-Acetylserine sulfhydrylase (OASS) is a pyridoxal 5'-phosphate (PLP)-dependent enzyme that catalyzes the final step in the de novo synthesis of L-cysteine in Salmonella typhimurium. Complementary cofactor mutagenesis in which the active site PLP is substituted with cofactor analogs is used to test the mechanism proposed for the OASS. Data obtained with the pyridoxal 5'-deoxymethylenephosphonate-substituted enzyme suggest that the binding of OAS as it forms the external Schiff base is such that the acetate side chain is properly positioned for elimination (orthogonal to the developing alpha,beta-double bond) only about 1% of the time. Data support the assignment of an enzyme group with a pK of 6.7 that interacts with the acetyl side chain, maintaining it orthogonal to the developing alpha,beta-double bond. Similar studies of the 2'-methylpyridoxal 5'-phosphate-substituted enzyme suggest that, although the mechanism is identical to that catalyzed by native OASS, the reaction coordinate for alpha-proton abstraction may be decreased compared with that observed for the native enzyme.

A change in the internal aldimine lysine (K42) in O-acetylserine sulfhydrylase to alanine indicates its importance in transimination and as a general base catalyst

O-Acetylserine sulfhydrylase (OASS) is a pyridoxal 5'-phosphate dependent enzyme that catalyzes a beta-replacement reaction forming L-cysteine and acetate from O-acetyl-L-serine (OAS) and sulfide. The pyridoxal 5'-phosphate (PLP) is bound at the active site in Schiff base linkage with a lysine. In the present study, the Schiff base lysine was identified as lysine 42, and its role in the OASS reaction was determined by changing it to alanine using site-directed mutagenesis. K42A-OASS is isolated as an external aldimine with methionine or leucine and shows no reaction with the natural substrates. Apo-K42A-OASS can be reconstituted with PLP, suggesting that K42 is not necessary for cofactor binding and formation of the external Schiff base. The apo-K42A-OASS, reconstituted with PLP, shows slow formation of the external aldimine but does not form the alpha-aminoacrylate intermediate on addition of OAS, suggesting that K42 is involved in the abstraction of the alpha-proton in the beta-elimination reaction. The external aldimines formed upon addition of L-Ala or L-Ser are stable and represent a tautomer that absorbs maximally at 420 nm, while L-Cys gives a tautomeric form of the external aldimine that absorbs at 330 nm, and is also seen in the overall reaction after addition of primary amines to the assay system. The use of a small primary amine such as ethylamine or bromoethylamine in the assay system leads to the initial formation of an internal (gamma-thialysine) or external (ethylamine) aldimine followed by the slow formation of the alpha-aminoacrylate intermediate on addition of OAS. Activity could not be fully recovered, and only a single turnover is observed. Data suggest a significant rate enhancement resulting from the presence of K42 for transimination and general base catalysis.

Formation of the alpha-aminoacrylate immediate limits the overall reaction catalyzed by O-acetylserine sulfhydrylase

O-Acetylserine sulfhydrylase-A (OASS-A) catalyzes the final step in the synthesis of L-cysteine, viz., the beta-substitution of acetate in O-acetyl-L-serine (OAS) by sulfide via a ping-pong kinetic mechanism . Rapid-scanning stopped-flow and single-wavelength absorbance and fluorescence stopped-flow experiments were carried out to obtain information on the location and amount of limitation of rate-determining steps for the overall reaction and the individual half-reactions of OASS-A. The first half-reaction, conversion of OAS to the alpha-aminoacrylate intermediate and acetate, is rate-limiting for the overall reaction catalyzed by OASS-A. No intermeidates are detected within the second half-reaction, and thus rate constants for all steps must be > or = 1000s-1 at the lowest sulfide concentration used. Within the first half reaction, formation of the extrernal Schiff base (Kassociation = 0.2 mM-1) is observed in the first milliseconds, followed by its slower conversion to the alpha-aminocacrylate intermediate with a rate constant of 300 s-1, close to the value of 130 s-1 obtained for V/Et [Tai, C.H., Nalabolu, S.R., Jacobson, T.M., Minter D.E., & Cook, P.F. (1993) Biochemistry 32, 6433-6442]. Addition of L-cysterine ot OASS-A results in a rapid formation of the external Schiff base (Kassociation = 6.7 mM-1), followed by transient formation of the alpha-aminoacylate intermediate with a slightly lower rate (70-100 s-1) compared to OAS. The alpha-aminoacrylate intermediate decays to generate a species absorbing maximally at 418 nm, resulting from attack of the cysteine thiol to give ether in external Schiff base linkage with the active site PLP.

Identification and spectral characterization of the external aldimine of the O-acetylserine sulfhydrylase reaction

The O-acetylserine sulfhydrylase (OASS) reaction has been studied using a number of spectral probes including UV--visible, fluorescence, circular dichroism, and 31P NMR spectroscopy. The addition of L-cysteine, L-alanine, and glycine to OASS results in a shift in lambda max of 412 nm for the internal Schiff base to 418 nm resulting from the formation of the external Schiff base. The addition of L-serine or O-methyl-D,L-serine gives decreases of the absorbance of unliganded enzyme at 412 nm of about 50% and 20%, respectively, concomitant with an increase in the absorbance at 320 nm and a shift in the lambda max of the remaining visible absorbance to 418 nm. The spectral shifts observed in the presence of L-serine are suggestive of establishing an equilibrium between different forms of external Schiff base. The concentration dependence of the changes at 440 (L-cysteine) and 320 nm (L-serine) provides an estimate of the dissociation constant for the external aldimine. The pH dependence of the dissociation constant suggests the alpha-amine of the amino acid must be unprotonated for nucleophilic attack at C4' of PLP, and an enzyme side chain must be unprotonated to hydrogen-bond the thiol or hydroxyl side chain of the amino acid. When L-cysteine is the amino acid, the thiol side chain must be protonated to hydrogen-bond to the unprotonated enzyme side chain. The 31P NMR chemical shift is increased from 5.2 ppm for unliganded enzyme to 5.3 ppm in the presence of L-cysteine, signaling a tighter interaction at the 5'-phosphate upon formation of the external Schiff base.(ABSTRACT TRUNCATED AT 250 WORDS)

Acid-base chemical mechanism of O-acetylserine sulfhydrylases-A and -B from pH studies

The pH dependence of kinetic parameters using natural and alternative reactants was determined in order to obtain information on the chemical mechanisms of the A and B isozymes of O-acetylserine sulfhydrylase (OASS) from Salmonella typhimurium. A general mechanism is proposed for OASS in which OAS binds with its alpha-amine unprotonated to carry out a nucleophilic attack on C4' of the protonated Schiff base and with the acetyl carbonyl hydrogen-bonded to a protonated enzyme group (or a water molecule), which aids in the beta-elimination of acetate. The enzyme lysine that was in Schiff base linkage with the active site pyridoxal 5'-phosphate deprotonates the alpha-carbon in the beta-elimination reaction, and a proton is likely released with the acetate product. Sulfide likely binds as HS- to undergo nucleophilic attack on the alpha-aminoacrylate intermediate, followed by protonation of the alpha-carbon by the enzyme lysine. In OASS-A, HS- is hydrogen-bonded to the enzyme group that assists in the beta-elimination of acetate, but this is not the case for OASS-B. The pH independent equilibrium constant for the first half-reaction of OASS-A is 1.6 x 10(-3), while the second half-reaction is practically irreversible.

Kinetic mechanisms of the A and B isozymes of O-acetylserine sulfhydrylase from Salmonella typhimurium LT-2 using the natural and alternative reactants

The resonance-stabilized quinonoid 5-mercapto-2-nitrobenzoate (TNB) is a substrate for O-acetylserine sulfhydrylase-A (OASS-A) and -B (OASS-B), giving rise to the product S-(3-carboxy-4-nitrophenyl)-L-cysteine (S-CNP-cysteine) as confirmed by ultraviolet-visible and 1H NMR spectroscopies. A comparison of the kinetics of OASS-A and OASS-B indicates that the mechanism proceeds predominantly via a bi-bi ping pong kinetic mechanism as suggested by an initial velocity pattern consisting of parallel lines at low concentrations of reactants, but competitive inhibition by both substrates as the reactant concentrations are increased. Thus, in the first half-reaction, O-acetyl-L-serine (OAS) or beta-chloro-L-alanine (BCA) is converted to alpha-aminoacrylate in Schiff base with the active site pyridoxal 5'-phosphate, while in the second half-reaction cysteine (with sulfide as the reactant) or S-CNP-cysteine (with TNB as the reactant) is formed. The ping pong mechanism is corroborated by a qualitative and quantitative analysis of product and dead-end inhibition. Product inhibition by acetate is S-parabolic noncompetitive. These data are consistent with acetate reversing the first half-reaction and producing more free enzyme to which acetate may also bind. Thus, there may be some randomness to the mechanism at high concentrations of the nucleophilic substrate.