Biomedical subjects

Brian P Callahan

Publications and source records attributed to Brian P Callahan.

8 recordsLinked to original sources

PubMed2006

Nanomolar inhibition of the enterobactin biosynthesis enzyme, EntE: synthesis, substituent effects, and additivity.

2,3-Dihydroxybenzohydroxamoyl adenylate (I) was prepared as a potential product analog inhibitor of EntE (EC# 2.7.7.58), a 2,3-dihydroxybenzoate AMP ligase from Escherichia coli that is required for the biosynthesis of enterobactin. This compound, obtained by the aqueous reaction of imidazole-activated adenosine 5'-phosphate and 2,3-dihydroxybenzohydroxamic acid, is a competitive inhibitor with a Ki value of 4.5 x 10(-9)M. Deletion of the catecholic 3-OH group of (I), in compound (II), reduced inhibitory activity by a factor of 3.5, whereas, removal of both the 3-OH and 2-OH groups, in (III), reduced inhibitory activity by a factor of approximately 2000. Acetohydroxamoyl adenylate (IV), in which the entire catechol moiety of (I) is replaced by a hydrogen atom, gave <or= 10% inhibition at 6 x 10(-4)M, indicating a reduction in affinity by more than 10(5). The binding free energy of (I) is nearly equivalent to the sum of the corresponding values for adenosine 5'-phosphate and 2,3-dihydroxybenzoate.

Adenosine Monophosphate↗

PubMed2006

A Raman-active competitive inhibitor of OMP decarboxylase.

6-Cyanouridine 5'-phosphate was shown to act as a competitive inhibitor of yeast OMP decarboxylase, with a K(i) value of 1.1 x 10(-5)M. Upon binding by the active site of yeast OMP decarboxylase (EC 4.1.1.23), the Raman stretching frequency of the nitrile group of 6-cyanouridine 5'-phosphate decreases from 2240 to 2225 cm(-1). Based on the behavior of a model compound, 6-cyano-1,3-dimethyluracil, and on vibrational calculations, the observed change in stretching frequency is attributed to desolvation of the ligand, and distortion of the ligand in which the nitrile group moves out of the plane of the pyrimidine ring. Similar distortions may play a role in substrate activation by OMP decarboxylase, contributing to the catalytic process.

Catalysis↗

PubMed2005

The burden borne by urease.

At the active site of urease, urea undergoes nucleophilic attack by water, whereas urea decomposes in solution by elimination of ammonia so that its rate of spontaneous hydrolysis is unknown. Quantum mechanical simulations have been interpreted as indicating that urea hydrolysis is extremely slow, compared with other biological reactions proceeding spontaneously, and that urease surpasses all other enzymes in its power to enhance the rate of a reaction. We tested that possibility experimentally by examining the hydrolysis of 1,1,3,3-tetramethylurea, from which elimination cannot occur. In neutral solution at 25 degrees C, the rate constant for the uncatalyzed hydrolysis of tetramethylurea is 4.2 x 10-12 s-1, which does not differ greatly from the rate constants observed for the uncatalyzed hydrolysis of acetamide (5.1 x 10-11 s-1) or N,N-dimethylacetamide (1.8 x 10-11 s-1) under the same conditions. We estimate that the proficiency of urease as a catalyst, (kcat/Km)/knon, is 8 x 1017 M-1, slightly higher than the values for other metalloenzymes (carboxypeptidase b and cytidine deaminase) that catalyze the hydrolysis of similar bonds.

Catalysis↗

PubMed2004

OMP decarboxylase: an experimental test of electrostatic destabilization of the enzyme-substrate complex.

6-Methylaminouridine 5'-phosphate (MAUMP) inhibits OMP decarboxylase (Ki = 3 x 10-6 M) maximally at pH values where its amino group is uncharged. Comparison of the chemical shift of free [7-13C]-MAUMP in solutions of varying pH, with that of the enzyme-bound species confirms that this inhibitor is bound with its amino group uncharged. This enzyme's apparent lack of affinity for a cationic substituent, located near the position that would ordinarily be occupied by the scissile carboxylate group of the substrate, does not appear to support the view that the E-S complex is destabilized by electrostatic repulsion in the ground state.

Enzyme Stability↗

PubMed2004

Charge development in the transition state for decarboxylations in water: spontaneous and acetone-catalyzed decarboxylation of aminomalonate.

A covalent imminium adduct, formed by condensation of aminomalonate with acetone, undergoes decarboxylation (k = 0.03 s-1 at 25 degrees C) in water 30 000 times more rapidly than does aminomalonate in the absence of acetone. A Brønsted plot of the observed rates of decarboxylation of these and other ionized carboxylic acids, as a function of the pKC-H values of the carbon acids generated by their decarboxylation, exhibits a betalg value of 0.7, indicating that the structures of the transition states for decarboxylation of the carboxylate forms of these acids approaches the structures of the carbanions generated by their decarboxylation. On the basis of an estimated pKC-H value for benzene in water ( approximately 43), extrapolation of that Brønsted plot leads to the prediction that benzoate decarboxylation should proceed at detectable rates in water at temperatures below the critical point. That prediction was confirmed experimentally. Using this same relationship, and extrapolating to the observed rate constant for enzymatic decarboxylation of orotidine 5'-monophosphate, we estimate that the "effective" pKa value of the 6-CH group of uridine 5'-monophosphate, the product of decarboxylation, is 9.5 at the active site of yeast OMP decarboxylase.

Acetone↗

PubMed2003

Migration of methyl groups between aliphatic amines in water.

Glycine undergoes spontaneous decarboxylation in dilute aqueous solution at elevated temperatures to form methylamine. During that process, we noticed the appearance of dimethylamine and trimethylamine in smaller amounts that increased gradually with time. These observations suggested the existence of disproportionation reactions of methylamines in water, for which there appears to be no direct precedent in the literature. Every member of the methylamine series is found to yield other members of the methylamine series. When the total concentration of amine was held constant and the rate of reaction was examined as a function of changing pH using the amine itself as the buffer, the initial rate of appearance of the products was found to reach a maximum when the conjugate acid and the conjugate base were present at equivalent concentrations. Near this equivalence point, the rate of reaction varied with pH as expected for a second-order reaction between the protonated and the unprotonated species. Under similar conditions, methyl groups were also found to migrate between the nitrogen atoms of N,N-dimethyl-1,3-propanediamine in a first-order process. With dimethylamine as a common acceptor, trimethylsulfonium ion was found to be approximately 104-fold more reactive than the tetramethylammonium ion at ambient temperature.

Amines↗

PubMedDate not supplied

The posterior superior alveolar injection technique: a report on technique variations and complications.

The posterior superior alveolar (PSA) injection technique has varied over time with respect to the depth and angle of penetration, the location for deposition of anesthetic agent, and the number of injections necessary to assure adequate anesthesia to the maxillary molars. Of the standard intraoral injections, the PSA carries with it the second highest risk for anesthesia complications. With changes in armamentarium and technique, the complication rates have declined and more often are associated with anatomical considerations with respect to neurovascular compromise and/or anesthetic solution. In this study, the PSA injection technique and complication histories were investigated. Seventeen variations to the technique are reported along with 12 injection complications. A historic injection complication classification system is presented along with a management strategy based upon a review of reported provider experiences and treatment suggestions.

Alveolar Process↗

PubMedDate not supplied

The triangle of safety: a modified posterior superior alveolar injection technique based on the anatomy of the PSA artery.

The posterior superior alveolar (PSA) injection technique is commonly used to anesthetize soft and hard tissues of the posterior maxilla. As with all injections, complications arise, including hematoma formation secondary to needle-induced vascular trauma. In an attempt to develop a hemorrhage-free PSA injection technique, 361 infratemporal dissections were completed on human cadaver specimens. Three distribution patterns were identified for the external branch of the PSA artery. Regardless of distribution pattern, an anatomical Triangle of Safety was found superior to the maxillary second molar that was free of neurovascular tissues in more than 99% of individuals. Injection into this area appears to meet anesthetic needs while reducing the risk of hematoma formation. The combination of this anatomical triangle with newer anesthetic agents and computerized delivery systems holds promise for continued improvement of the PSA injection technique.

Adult↗