Apparent first-order behavior under second-order kinetic conditions: a general concept illustrated by the reversible binding of hydrogen peroxide to cytochrome c oxidase

The theory of the second-order reversible reaction, A + B<-->A.B, has been extensively discussed. Apparent first-order behavior is observed when, for example, [B] >> [A]. If the reaction exhibits second-order behavior then it is presumed that the concentrations of A and B were initially equal and that they remain equal during the reaction. However, in the case of hydrogen peroxide binding to cytochrome c oxidase, Weng & Baker (1991, Biochemistry 30, 5727-5733) showed that the observed rate was rigorously first order over a broad concentration range of ligand, including the stoichiometric case. It was further shown that kobs increased linearly with [H2O2], precluding the possibility of a rate-limiting, unimolecular pre-step. The current work examines the theoretical rate equation for the bimolecular, reversible reaction when [A] = [B]. Simulations show that this equimolar condition resulted in rigorous exponential binding as kd, the equilibrium dissociation constant for the A.B complex, approached the initial concentration of A (or B). In particular, the second-order simulation was rigorously exponential when [A]o/Kd = 0.5, and showed only minor deviations when the ratio was increased to 25. These results demonstrate that a reversible, bimolecular reaction can appear first order even under second order conditions, without the need for more complicated mechanisms.

Nonuniform phase distribution in ultrasound speckle analysis. I. Background and experimental demonstration

In some cases, the statistical properties of the phase of ultrasound speckle in B-scan images differ from the uniform distribution characteristic exhibited by the fully developed speckle. This phenomenon has been noted when examining scattering structures with a somewhat regular spacing using wideband pulse excitation. It is shown by computer simulation and experiments on phantoms that when the mean scatterer spacing is equal to multiples of a half wavelength at the reference frequency of the receiver quadrature demodulator, the center of the echo phase distribution, plotted on the complex plane, will shift away from the origin. When the spacing is equal to an odd multiple of a quarter wavelength, the phase distribution will have a figure ;8' shape. By noticing those noncircular phase distributions while changing the demodulation frequency, the mean scatterer spacing can be estimated.

Nonuniform phase distribution in ultrasound speckle analysis. II. Parametric expression and a frequency sweeping technique to measure mean scatterer spacing

For pt.I see ibid., vol.39, no.3, p.352-9 (1992). The existence of regularly spaced scatterers in the range cell of an ultrasonic imaging system results in speckle with a nonuniform phase distribution. Characteristic phase distributions occur at those particular demodulator frequencies at which the spacing is a multiple of one-quarter wavelength. A demodulator frequency sweeping technique for scatterer spacing estimation has been developed that uses parametric expressions of the phase statistics to detect and to identify scatterers with regular spacings. Changes in these parameters separately identify the mean scatterer spacing when it is an even and an odd multiple of the quarter wavelength of the demodulator frequency. This technique has good tolerance for variations in mean scatterer spacing, and has real-time implementation capability.

Ultrasound-accelerated immunoassay, as exemplified by enzyme immunoassay of choriogonadotropin

The rate-limiting step in many solid-phase immunoassays is associated with the slow kinetics of binding of macro-molecular antigen and conjugate to the immobilized phase. We demonstrate that the use of ultrasonic energy to enhance mass transport across liquid/solid interfaces can dramatically accelerate antigen binding to immobilized antibodies. We use an ultrasound-accelerated procedure with an enzyme-channelling test strip containing glucose oxidase and specific antibody to the alpha-subunit of human choriogonadotropin (HCG) co-immobilized onto a cellulose support. A horseradish peroxidase conjugate of monospecific antibody to the beta-subunit of HCG is used in the liquid phase to complete the immune "sandwich." Use of ultrasound to accelerate binding and of enzyme channelling to eliminate wash steps result in a simple two-incubation protocol by which 25 int. units of urinary HCG per liter can be detected visually in less than 20 min of assay time.

Ultrasound-accelerated immunoassay, as exemplified by enzyme immunoassay of choriogonadotropin

The rate-limiting step in many solid-phase immunoassays is associated with the slow kinetics of binding of macro-molecular antigen and conjugate to the immobilized phase. We demonstrate that the use of ultrasonic energy to enhance mass transport across liquid/solid interfaces can dramatically accelerate antigen binding to immobilized antibodies. We use an ultrasound-accelerated procedure with an enzyme-channelling test strip containing glucose oxidase and specific antibody to the alpha-subunit of human choriogonadotropin (HCG) co-immobilized onto a cellulose support. A horseradish peroxidase conjugate of monospecific antibody to the beta-subunit of HCG is used in the liquid phase to complete the immune "sandwich." Use of ultrasound to accelerate binding and of enzyme channelling to eliminate wash steps result in a simple two-incubation protocol by which 25 int. units of urinary HCG per liter can be detected visually in less than 20 min of assay time.

Amino acid sequence at the allosteric site of sheep heart phosphofructokinase

Covalent modification of sheep heart phosphofructokinase with the affinity labeling reagent p-fluorosulfonyl[14C]benzoyl-5'-adenosine caused a loss of allosteric properties. This modification appears to occur at the binding site that is specific for the allosteric activators AMP, cAMP, and ADP (Mansour, T.E., and Colman, R.F. (1978) Biochem. Biophys. Res. Commun. 81, 1370-1376). In the current study, the site of modification has been demonstrated to be a lysine residue. A nonapeptide containing a covalently bound [14C]carboxybenzenesulfonyl group attached to alysine residue has been isolated following tryptic digestion. The amino acid sequence of the peptide is Asn-Phe-Ala-Thr-Lys-Met-Gly-Ala-Lys. The fifth residue in this sequence, lysine, contained the covalently bonded reagent.

The covalent structure of bovine liver rhodanese. Isolation and partial structural analysis of cyanogen bromide fragements and the complete sequence of the enzyme

Cyanogen bromide fragments from reduced and carboxymethylated rhodanese have been isolated by gel filtration and ion exchange chromatography on columns of Sephadex G-50 and sulfoethyl-Sephadex C-25, respectively. Partial or complete structural analysis of these fragments has permitted the ordering in sequence of all eleven of the nonaligned tryptic peptides from citraconylated, S-carboxymethylcysteinyl-rhodanese and has thus provided the complete covalent structure of the enzyme. Rhodanese is a single polypeptide of 293 residues and the molecule weight calculated from the covalent structural analysis is about 32,900. The cysteinyl residue implicated in the catalytic function of rhodanese is at position 247. In some preparations of the enzyme the NH2-terminal dipeptide Val-His is missing and the sequence begins with the glutamine at position 3. The rhodanese thus obtained contains 291 amino acid residues and possesses full enzymic activity. X-ray crystallographic analysis of rhodanese has shown that the halves of the molecule (Domains I and II) are nearly identical in conformation. Comparative analysis of the sequences in Domains I and II containing residues with conformationally equivalent alpha C atoms has revealed some degree of homology between the halves of the rhodanese polypeptide. Nethertheless, the structural equivalence of the rhodanese domains is reflected much more by their similarity in tertiary structural than by their sequence homology, even when the sequence comparisons are optimized with reference to the crystallographic results.

The covalent structure of bovine liver rhodanese. NH2-terminal sequence and partial structural analysis of tryptic peptides from the citraconylated protein

Nineteen tryptic peptides produced by cleavage at 18 of the 20 arginyl residues in citraconylated S-carboxymethylcysteinyl-rhodanese have been isolated by a combination of gel filtration and high voltage paper electrophoresis. These Tc fragments account for all of the 293 residues in the parent polypeptide and their partial or complete sequences have been determined by automated and manual Edman degradation. In some cases, sequence analyses were completed by degradation of peptides derived by secondary cleavages of the decitraconylated Tc fragments with trypsin, chymotrypsin, or the protease from Staphylococcus aureus. Automated Edman degradation of intact S-carboxymethylcysteinyl-rhodanese was performed for 60 cycles; the information thus obtained permitted the alignment of seven of the Tc fragments and gave the sequence of the first 79 residues in the polypeptide chain. The Tc peptide at the COOH terminus of rhodanese was placed by virtue of the fact that it contained no arginine. Structural analysis of the Tc peptides provided the sequences surrounding all five of the methionyl residues in the enzyme. One of the methionines was found in a 19-residue Tc fragment which also contained the cysteinyl residue essential for catalysis.

Active site cysteinyl and arginyl residues of rhodanese. A novel formation of disulfide bonds in the active site promoted by phenylglyoxal

Chemical modification studies of bovine liver rhodanese have underscored important distinctions between free rhodanese and the catalytic intermediate in which the sulfane atom of the sulfur donor is bound covalently to the enzyme (sulfur-rhodanese). Treatment of free rhodanese with near-stoichiometric quantities of either iodoacetate or phenylglyoxal results in the rapid modification of the essential sulfhydryl group of Cys-247 and the consequent inactivation of the enzyme. Analysis of rate data for the iodoacetate reaction showed that the apparent pK of this group is 7.8 in free rhodanese and 6.7 to 7.0 in complexes of the enzyme with analogs of sulfur donor substrates, in agreement with the previous inference from steady state kinetic observations. Inactivation of free rhodanese by phenylglyoxal in the presence of cyanide was shown to be caused by a novel reaction in which disulfide bonds are formed between Cys-247 and either Cys-254 or Cys-263. In contrast to these results with free rhodanese, the sulfur-substituted enzyme is not inactivated by iodoacetate and is only relatively slowly inactivted by treatment with substantial excesses of phenylglyoxal. The loss of enzyme activity in sulfur-rhodanese does not involve cysteinyl residues but can be correlated with the modification of guanidino groups, notably that of Arg-186, the side chain of which may play a role in substrate binding. These chemical modification studies have implications with respect to the chemical mechanism of rhodanese catalysis and the interpretation of the x-ray crystallographic analysis of this enzyme.

The covalent and tertiary structure of bovine liver rhodanese

Bovine liver rhodanese is a single polypeptide of 293 amino acids in which the halves of the molecule assume analogous tertiary structures in the absence of substantial sequence homology. The sulphur atom transferred during catalysis is bound in persulphide linkage to Cys-247. Substrate binding seems to involve Arg-186 and Lys-249.