Myosin catalysis

Figure 4 (a) Schematic of experimental design of detecting single-head myosin catalysis using a fluorescent substrate. The laser excitation is in TIR geometry. The Cy5-label on the enzyme helps to identify the location of individual enzymes. 

S-1 (molecular mass approximately 115 kDa) does exhibit ATPase activity, binds L chains, and in the absence of ATP will bind to and decorate actin with arrowheads (Figure 49-5). Both S-1 and HMM exhibit ATPase activity, which is accelerated 100- to 200-fold by complexing with F-actin. As discussed below, F-actin greatly enhances the rate at which myosin ATPase releases its products, ADP and Pj. Thus, although F-actin does not affect the hydrolysis step per se, its ability to promote release of the products produced by the ATPase activity greatly accelerates the overall rate of catalysis. 

Most of the machinery of living cells is made of enzymes. Thousands of them have been extracted from cells and have been purified and crystallized. Many others are recognized only by their catalytic action and have not yet been isolated in pure form. Most enzymes are soluble globular proteins but an increasing number of RNA molecules are also being recognized as enzymes. Many structural proteins of the cell also act as catalysts. For example, the muscle proteins actin and myosin together catalyze the hydrolysis of ATP and link the hydrolysis to movement (Chapter 19). Catalysis is one of the most fundamental characteristics of life. 

Proteins constitute a universally essential class of macromolecules which perform a wide range of specialized functions in living systems. Examples of these functions include the enzymatic catalysis of metabolic pathways, hormonal signaling in the endocrine system, and antibody mediated defense in the immune system. Proteins also perform critical structural roles, for example as the muscle proteins actin and myosin. The study of protein structure and function is therefore essential to our understanding of life and the advancement of medicine [1]. 

Nucleotide binding to a second site increases the rate of catalysis of ATP hydrolysis as well as dissociation [24, 25]. An alternative rule for avoiding hydrolysis is simply that binding of the proton and nucleotide bring about dissociation of ATP faster than it reverts to ADP and Pj it is possible that the dissociation of ATP is brought about specifically by ADP and P [11]. There is precedent for such a mechanism in the actin-driven dissociation of ATP in good yield from the equilibrium mixture of M ATP M ADP Pi, which is the analogous reaction in the myosin system [26]. 

Here we describe how a simulation approach using combined QM/classical MM methodology enabled us to identify the energetically most favorable mechanism. We found that in both EcoRV and myosin the energetically most favorable pathway clearly proceeds through a dissociative mechanism. Based on the similarities between the EcoRV and myosin mechanisms for catalysis of phosphate hydrolysis, we can answer the following questions (i) Why is the dissociative mechanism... 

Three types of reactions were identified. These are rapid equilibrium binding (1,5 and 7), chemical catalysis (3) and isomerizations (2,4 and 6). There is evidence that steps 4 and S are preceded by an isomerization of the complex. M designates myosin and the stars indicate increase in fluorescence of the protein. 

The phenomena of cytoplasm streaming and muscle contraction are generally related to enzymatic catalysis of ATP hydrolysis by the actin-myosin (A-M) system [1]. Therefore, by assuming a common driving mechanism, a simple question is raised What comes first -streaming or contraction ... 

Furthermore, rotational catalysis was proposed for the FO-ys-(aP)3-Fl complex [45]. This rotation might be electrically driven by the reversible ballistic proton mechanism, as follows. In ATP synthesis, each ADP-Pi loaded aP-site of the water exposed Fl head is bound in turn to the hydrophobic, topically bent, ys axis. This internal axis is inserted into the FO membrane component so as to form an effective channel for ballistic protons. The aP catalytic unit is comparable to a myosin head, while the biochemical role of the ys axis is quite similar to that of the actin filament in the enzymatic cycle (Scheme 2). Thus, in the direction of synthesis, the impact of a trans-membrane ballistic Fi+ within the hydrophobic catalytic region is proposed to drive three concomitant effects First, dehydration of the terminal phosphate bond results in ATP synthesis. Second, molecular recoil upon the FI+ impact dissociates the ys-aP complex. Third, the abrupt increase of electric charge at the hydrophobic site drives fast relative rotation at 120° towards hydrophobic ys interaction with the next, ADP-Pi loaded, aP-site. Simultaneous exchange of products and substrates, carried out at the other two, water exposed, aP sites, might electrically dictate ongoing rotation in the appropriate direction. 

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