Principles of Protein Structure - H. Schulz 1982

Structural Basis of Protein Mechanism, Action, and Function
Skeletal muscle: a system where protein action can be linked to the overall activity of the organ
Response to nerve impulses and hormonal action in the muscle cell

Response of Muscle Cells to nerve impulses and hormonal action

When a muscle Cell is at rest, the attachment sites for cross-bridges on the thin filaments are blocked by Tropomyosin molecules [767–769], which are rigid two-stranded structures [214]. Tropomyosin is a prototype of a supercoiled $\alpha$-helix (Section 5.2) with a molecular length of 400 Å, covering seven globular Actin subunits [768, 769]. This arrangement of tropomyosin is presumably determined by troponin, or more precisely, by the overall conformation of the troponin complex in the absence of Ca²⁺ (Fig. 11.6).

The chain of events leading to the displacement of tropomyosin originates at The Cell membrane. When nerve impulses activate a muscle cell with a volume of 1 µL, Ca²⁺ ions are released from the sarcoplasmic reticulum [770] into the Cytoplasm, where the concentration of free Ca²⁺ ions rises by two orders of magnitude to 1 µM (Fig. 11.7). This leads to the saturation of troponin C, the calcium-sensitive component of the thin filament [771]; 90% of the total 10¹⁴ ions bind to the troponin C molecules. The binding of Ca²⁺ induces conformational changes throughout the entire troponin complex [772]. With this altered troponin Structure, tropomyosin can no longer be held in the "off" state. The tropomyosin helix slides sideways to a new position closer to the center of the groove. Thus, a single tropomyosin molecule exposes seven actin monomers capable of interacting with Myosin [767, 769, 785].

Activation of the contractile apparatus by Ca²⁺ ions as a cooperative process model [772, 773]. The activation of the contractile apparatus by Ca²⁺ ions (Fig. 11.7) clearly illustrates cooperative behavior [92, 678, 682, 774] and allosteric control [92, 681, 775, 776] in Proteins. In Cooperative processes, a component of a protein or supramolecular structure—such as thin filaments—undergoes a transition from one stable state ("off") to another ("on"). A typical "switching" process within structural units, such as troponin components, is a conformational transition. This structural isomerization, in turn, is triggered by a specific allosteric Ligand, in our case the Ca²⁺ ion. The behavior of tropomyosin demonstrates that not all protein–protein dynamic interactions rely on structural isomerization within individual proteins; tropomyosin exerts its effect by shifting as an entire molecule (Fig. 11.6).

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Fig. 11.7. Effect of Ca²⁺ ion concentration on muscle tension and myosin ATPase activity [785].

pCa is a concept analogous to pH. The observed cooperativity (solid lines) apparently reflects the fact that structural and enzymatic transitions in the contractile apparatus are "switched on" upon the binding of the fourth Ca²⁺ ion to troponin C [615, 773, 785]. The dashed curves show the hypothetical activity of the contractile apparatus in the absence of structural cooperativity.

Metabolic, neural, and hormonal stimuli, often amplified by enzymatic cascades, correlate Energy Expenditure and energy reserves. What mechanisms supply chemical energy to the active contractile system? An important role in directly correlating energy reserves with energy demand is played by ADP, the primary product of myosin ATPase activity. As a metabolic signal, ADP triggers ATP production by activated proteins at key regulatory points in The Glycolytic Pathway and Oxidative Phosphorylation [19]. Long-term energy storage is regulated by Ca²⁺ ions, which stimulate Glycogen breakdown [777]. However, in this case, the calcium-activation mechanism involves multiple steps, unlike the single-step activation of the contractile apparatus by the same ion (Fig. 11.1).

Glycogen mobilization can be influenced by the hormone epinephrine, which acts independently or in parallel with Nerve Impulse stimulation. The cascade of membrane and cytoplasmic events leading from epinephrine binding to the phosphorylation of glucose residues in glycogen Functions as a highly efficient kinetic Amplification mechanism (Table 11.1). The hormone concentration in the extracellular space is approximately 10^-11 M; cyclic AMP, formed from ATP as the primary effector of hormonal action, activates protein kinase at concentrations as low as 10^-8 to 10^-7 M. This is followed by a 10-fold amplification upon stimulation of phosphorylase b kinase, and a final 20- to 50-fold amplification is achieved during The conversion of phosphorylase b to phosphorylase a.

By analogy with the events occurring in muscle, it can be assumed that in other physiological processes as well, the action of cyclic AMP—the second messenger for many Hormones—is associated with the modulation of protein phosphorylation–dephosphorylation reaction systems [777, 778].

The rapid dissociation of calcium–Protein Complexes serves as a mechanism for switching protein activity on and off. Let us now examine the intracellular processes that take place after the stimulation (nerve impulses or hormone binding) ceases. Metabolic processes will terminate with a time lag on the order of seconds after the concentration of cyclic AMP or Ca²⁺ ions drops below a critical threshold (Table 11.1). In contrast, the contractile apparatus is switched off within an interval of 10–20 ms through the removal of Calcium Ions from the thin filaments. The speed of this process highlights the efficiency of the sarcoplasmic reticulum as a calcium pump [701]. On the other hand, the presence of this complex system, dedicated exclusively to the uptake and release of Ca²⁺ ions, suggests that Ca²⁺ ions possess unique properties regarding switching functions. This is also indicated by the fact that Ca²⁺ ions participate in many other physiological processes as mediators between incoming stimuli and cellular responses [615], such as in photoreceptor cells [779] or in the ovum, where a Ca²⁺-dependent mechanism apparently ensures Fertilization by only a single sperm cell [780]. All these processes are characterized by the rapid turning on and off of protein activity. The same applies to the contractile apparatus: rapid contraction and rapid relaxation are essential for A wide variety of movements.

Apparently, calcium was evolutionarily selected for the control of rapid biological processes due to The properties of its coordination compounds [615], which under physiological conditions are capable of dissociating at acceptable rates [781]. In a triggering system based on a signaling molecule S and a target molecule T, association and dissociation occur According to the equation

where k₊₁ and k_-1 are the rate constants for

the formation and dissociation of the complex, respectively. The stability constant (association constant) is given by K = k₊₁/k_-1 = [ST]/[S] ∙ [T]. The optimal value for a triggering system operating within a physiologically relevant concentration range varies from 10⁵ to 10⁷ M^-1 [782]. The value of k₊₁ for the substitution of Ca²⁺ in Water is 10⁸ M^-1 ∙ s^-1, and for Mg²⁺ it is 10⁵ M^-1 ∙ s^-1 [762]. Hence, the value of k_-1 = k₊₁/K is on the order of 1000 s^-1 for Ca²⁺ complexes and 1 s^-1 for Mg²⁺ complexes. Under these conditions, the shortest half-lives of the calcium and magnesium complexes are 0.7 ms and 0.7 s, respectively.

Thus, unlike Mg²⁺ ions and other potential biological activators, Ca²⁺ ions are uniquely suited to perform the on–off switching function regardless of whether the functional timescale of the protein is on the order of seconds or milliseconds.

Finally, it should be emphasized that the biochemical processes within calcium coordination compounds appear to be the exact factors limiting The rate of control in vertebrate Muscles. The fast-acting extrinsic Muscles of the human eye perform up to 50 contraction–relaxation cycles per second. On the other hand, the flight muscles of winged insects can oscillate at frequencies in the sonic range exceeding 1000 Hz. Such rapid oscillations are based on protein processes in which Ca²⁺ ions do not participate, notably the contraction-coupled myosin ATPase reaction [783, 784].