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This paper illustrates a method to calculate the sensitivities of control coefficients to the elasticities which determined their values and it is shown that these sensitivities are systemic properties. We show, both theoretically and with a practical example, how they can be used to investigate: (a) the relative importance of a particular elasticity in the determination of the value of a control coefficient; (b) the effect of the experimental error on the values of the control coefficients and (c) the construction limits around the values of the control coefficients. [ABSTRACT FROM AUTHOR]

The matrix algebra procedure for determining the flux control coefficients of enzymes in metabolic pathways has been extended to allow determination of the concentration control coefficients. Although it is shown that the procedure is essentially unchanged in most cases, the presence of moiety-conserved cycles in a pathway places additional limitations on the form of the equations that can be used in the matrix formulation for concentration control coefficients. In the case of branched pathways, a new coefficient has been defined, the branch distribution control coefficient, which can be obtained via the matrix procedure. Thus a single matrix equation permits calculation or algebraic evaluation of the control coefficients for flux, concentration and distribution of flux at branches, so that the complete response of a pathway to alteration of enzyme content, or to modulation by an effector, can be determined. The relationships have been determined between flux control coefficients in isolated sections of metabolic pathways and the coefficients for the same enzymes when part of a larger metabolic system. It is shown that the control analysis of the isolated system provides useful information towards determining the control properties of the extended system. [ABSTRACT FROM AUTHOR]

Smooth and non-muscle tropomyosins were found to produce a 2–3-fold Ca-insensitive stimulation of the ATPase activity of reconstituted skeletal muscle actomyosin at normal MgATP concentrations and physiological ratios of myosin to actin. Under the same conditions skeletal muscle tropomyosin had no effect. Similar effects of these three tropomyosins were observed for the low myosin/F-actin ratios necessary for kinetic measurements. Since it could be established that this actomyosin system, with or without tropomyosin, obeyed Michaelian kinetics, the tropomyosin effects could be interpreted in terms of their influence on maximal turnover (V) or on the affinity of myosin for actin (Kapp). Accordingly, gizzard tropomyosin produced an approximately twofold increase in both V and the affinity, while skeletal muscle tropomyosin had practically no effect on the affinity and reduced only slightly the value of V, compared to pure actin. In contrast to gizzard tropomyosin, brain tropomyosin produced an approximately twofold increase in both Kapp and V; i.e. it increased the turnover rate but decreased the affinity. It is apparent from the data that brain tropomyosin acts as an uncompetitive activator with respect to pure actin, while having the same V as the actin plus gizzard tropomyosin complex. Further studies on these tropomyosins show that only skeletal and smooth muscle tropomyosin have similar functional properties with respect to troponin inhibition and the activation of the ATPase at low ATP concentrations. It is suggested that the noted increases in V by tropomyosin are caused by the acceleration of the dissociation of the myosin head from actin at the end point of the cross bridge movement. [ABSTRACT FROM AUTHOR]

Chymotryptic cleavage patterns of myosin rods from pig stomach, chicken gizzard, and rabbit skeletal muscle indicate that short (≈ 45 nm) heavy meromyosin subfragment 2 (SF2) is a consistent product of all three rods, whereas long (≈ 60 nm) SF2 is derived only from skeletal muscle myosin. Differential scanning calorimetry was used to follow the thermally induced melting transition of the rods and certain of their subfragments. In 0.12 M KCl, sodium phosphate buffer, pH 6.2–7.6, the light meromyosin (LMM) and SF2 domains of each rod had essentially identical conformational stabilities. Temperature midpoints for the melting transitions were 54-56°C for the two smooth muscle myosin rods and 50-53°C for the skeletal muscle myosin rod. In 0.6 M KCl buffer, melting transitions for the smooth muscle myosin rods were essentially unchanged, but skeletal muscle myosin rods showed multiphase melting, with major transitions at 43°C and 52°C. The first of these was tentatively attributed to LMM, and the second to SF2. In 0.12 M KCl buffer, the LMM transition was stabilised so that it superimposed on that of SF2. No melting was observed in any of the rods at physiological temperature. These results indicate that, excluding a possible but only narrow hinge region, the entire myosin rod has essentially uniform conformational stability at physiological pH and ionic strength, and thus that the contractile and elastic properties of the cross-bridge exist in the heavy meromyosin subfragment I (SF1) domains of the molecule. [ABSTRACT FROM AUTHOR]