Conformation of myosin. Effects of substrate and modifiers

beta-Site covalent reactions trigger transitions between open and closed conformations of the tryptophan synthase bienzyme complex

The tryptophan synthase bienzyme complex (alpha2beta2) from Salmonella tryphimurium catalyzes the final steps in the biosynthesis of L-Trp. To investigate the roles played by conformational change in tryptopthan synthase catalysis, the fluorophore 8-anilino-1-naphthalensulfonate (ANS) is used to identify conformational states. The binding of ANS to the alpha2beta2 bienzyme complex is accompanied by a dramatic enhancement of ANS fluorescence and a shift of the emission maximum from 520 to 482 nm. The ANS binding isotherm is biphasic and consists of a class of moderately high-affinity, noninteracting sites with a stoichiometry of 1 site/alpha beta dimeric unit (Kd' = 62 + or - 15 micrometer) and a much weaker set of non-specific interactions with K'd>1mM. Our findings show that the affinity of the enzyme for ANS is strongly decreased (> 10-fold) by interactions at two loci 30 angstroms apart: (i) the binding of the alpha-site ligands, 3-indole-D-glycerol 3'-phosphate or alpha-glycerol phosphate (GP) or (ii) reaction at the beta-subunit to form either the alpha-aminoacrylate Schiff base, E(A-A), or quinonoid species, E(Q). In contrast, formation of the L-Ser and L-Trp external aldimines E(Aex1) and E(Aex2) at the beta-site causes a 2-3 fold decrease in the affinity of the enzyme for ANS. The combination of E(A-A)or E(Q) with GP brings about almost complete displacement of ANS, indicating that these interactions drive a conformation change in alphabeta subunit pairs which prevents the binding of ANS. These results are consistent with a model which postulates that alphabeta subunit pairs undergo ligand-mediated transitions between open and closed conformations during the catalytic cycle. Consistent with the kinetic data showing that binding of alpha-site ligands increases the affinity of the beta site for L-Ser and that formation of E(A-A) activates the alpha reaction [Brzović, P. S., Ngo, K., & Dunn, M. F. (1992) Biochemistry 31, 3831-3839], while mutations in alpha subunit loops 2 and 6 prevent the ligand- mediated transition to a closed structure [Brzović, P.S., Hyde, C.C., Miles, E.W., & Dunn, M.F. (1993) Biochemistry 32, 10404-10413], we conclude that reciprocal ligand-mediated allosteric interactions between the heterologous subunits promote conformational transitions between open and closed structures in alphabeta subunit pairs which function to coordinate catalytic activities and facilitate the channeling of indole between the two catalytic sites.

Effect of lysine methylation and other ATPase modulators on the active site of myosin subfragment 1

Many and diverse modifications of the myosin subfragment 1 (S-1) increase (modulate) its ATPase activity, including interaction of this particle with actin; a recent addition to these modifications is the extensive lysine modification of S-1 that seems prerequisite to crystallizing it for structure analysis. In this study we first established kinetically the ATPase modulations induced by various treatments of the myosin S-1 enzyme, and we also measured two properties of the S-1 active site--the affinity with which the site binds (a fluorescent analog of) the enzymatic nucleotide product and the access that a fluorescence quencher has to the bound ADP product--in an effort to get at the mechanism of modulation. Modulations achieved by substituting Ca2+ for the normal Mg2+ cocatalyst or by substituting Cl- for the normal carboxylate anion seem due to the product being held more loosely by the modulated enzyme. In other illustrative modulations (lysine methylation, or alkylation of Cys-707, or transition from neutral pH to pH 9.2) nucleotide product affinity and access to quencher do change, but not in a pattern explained simply by a lifting of product inhibition. Lysine methylation results in weaker binding of nucleotide product.

Mechanisms of meat batter stabilization: a review

Comminuted meat products are a complex mixture of muscle tissue, solubilized proteins, fat, salt, and water. The two theories that have been presented to explain meat batters stabilization are reviewed. The emulsion theory explains stabilization by the formation of a protein film around fat globules, whereas the physical entrapment theory emphasizes the role of the protein matrix in holding the fat in place during chopping and subsequent heating. However, some aspects of stabilization cannot be explained adequately by either one of these theories. In this article the role of meat proteins, aqueous phase, and lipid phase are examined in light of past and recent research findings.

The binding of 8-anilino-1-naphthalene sulfonate (ANS) to fish myosin and the effect of salts on the thermal transitions of fish myosin-ANS complex

Myosin prepared from tilapia (Serotherodon aureus) was complexed with 8-anilino-1-naphthalene sulfonate (ANS) and continuously heated at 1 degree C/min. A large increase in fluorescence was observed with a transition temperature of 34 degrees C. The effect of several salts on the transition temperature was tested. A plot based on the equation of E. E. Schrier and E. B. Schrier [(1967) J. Phys. Chem. 71, 1851-1860] gave a value of less than or equal to 500 cal/mol-deg for the change in enthalpy per residue due to exposure to solvent. The ratio of hydrophobic group to amide group exposure to solvent was intermediate compared with the ratio of RNase and gelatin. Fluorescence titrations yielded one high affinity site with a Kb of 2 X 10(6) M-1 and at least 200 low affinity sites with an average value of 1 X 10(5) M-1. The parameters did not change significantly with temperature. We propose that the increase in ANS fluorescence reflects changes in conformation of myosin as monitored by these low affinity sites, resulting in an increase in surface hydrophobicity and representing a small enthalpic change in the conformation of the myosin molecule. As a consequence, the change in conformation accelerates polymerization of myosin oligomers.

Tryptic digestion as a probe of myosin S-1 conformation

One of the products of the limited tryptic hydrolysis of chymotryptic myosin subfragment 1 is the 27,000-dalton NH2-terminal fragment. This fragment is generated by two parallel routes from either the 75,000- or 95,000-dalton peptide of the heavy chain: (i) through a 20,500-dalton precursor or (ii) directly without participation of a precursor. Lowering of pH and temperature and increasing of ionic strength inhibited route i digestion in comparison to route ii. MgATP and its derivatives in millimolar concentration substantially suppressed route i digestion. Suppression of route i digestion depended on the concentration of MgATP. It occurred after a lag phase when the ratio of MgATP to subfragment 1 concentrations was greater than 0.5. In contrast, the MgATP-induced increase in tryptophan fluorescence of myosin subfragment 1 appeared without a lag phase. The generation of the 27,000-dalton fragment by either route was not affected by F-actin however, the suppression of route i digestion induced by MgADP was abolished when myosin subfragment 1 was in ternary complex with actin and MgADP. We conclude that the 27,000/50,000-dalton hinge region is a flexible domain of the myosin head and that conformation of this region is sensitive to the presence of nucleotides and actin and to variations in ambient factors.

Does myosin-substrate interaction in vitro result in a delocalized conformation change?

The effect of substrate on the far UV (185-250 nm) and near UV (250-325 nm) circular dichroism (CD) of myosin and heavy meromyosin (HMM) was studied. The following results were obtained with the addition of ATP (during various conditions of hydrolysis), ADP, and pyrophosphate: (1) no changes were observed in the far UV CD, (2) ATP and ADP perturbed the near UV CD only at spectral regions below 280 nm coinciding with the regions of their optical activity, (3) the optically inactive pyrophosphate caused no change in the near UV CD, and (4) myosin and HMM gave exactly the same results. These results suggest that myosin-substrate interaction in vitro does not result in a delocalized conformational change.

Heavy meromyosin Mg-ATPase: presteady-state and steady-state Hplus release

The presteady state and steady state release of protons during hydrolysis of ATP by myosin has been studied by using a proteolytic subfragment of myosin, heavy meromyosin, isolated from rabbit skeletal muscle. Under the conditions of 0.5 M KCl, 9 mM MgCl(2), pH 8, and 25 degrees , the rate of the H(+) initial burst is proportional to the concentration of ATP added, and shows no plateau even when the concentration of ATP added is increased up to 750 muM. The apparent second order rate constant is approximately 7 x 10(5) M(-1) sec(-1). When the concentration of ATP added is equal to or less than the concentration of heavy meromyosin catalytic sites, the H(+) initial burst is followed by a slow exponential release of H(+) with a rate constant of 0.017 sec(-1). This rate constant is equal to the steady state rate constant calculated for each heavy meromyosin catalytic site, suggesting that both sites split ATP independently. The magnitude of the H(+) released per ATP bound is always 0.4 in the initial burst, and 0.6 in the slow exponential phase, which equals to a total of 1 H(+) released per ATP hydrolyzed. On the basis of these observations, it is concluded that the H(+) initial burst may well represent a fast ionization of H(+) due to a conformational change as a result of nucleotide binding.