Protein Chemistry: Structure, Properties, and Research Methods - Shendryk, A.N. 2022

Proteins of blood and muscle tissues
Proteins of the contractile system
Mechanism of muscle contraction - Energy sources for muscle contraction

In isolated Muscles subjected to repeated contraction-relaxation cycles, lactic acid accumulates and Glycogen content decreases. This fact alone suggests that the energy required to perform work is released via Glycolysis. Studies show, however, that inhibiting The Glycolytic Pathway does not result in the loss of Muscle motor function. Consequently, glycolysis is not the decisive energetic factor in contraction. The contractile capacity of the muscle is also preserved when cellular Respiration is blocked by cyanides. These facts indicate that muscles contain energy-rich compounds capable of sustaining muscle performance—at least for a certain period of time—even when ATP synthesis is inhibited.

Historically, the acquisition of key experimental data regarding the mechanisms of Muscle contraction coincided with the discovery of ATP. Therefore, one of the initial hypotheses concerning the energy source of muscle contraction under conditions where respiration and glycolysis were blocked was that ATP alone served as this source. However, this hypothesis found no experimental confirmation and was rejected for the following main reasons.

1. The ATP content in muscles is extremely low and cannot sustain contraction even for a brief period. For instance, maintaining mammalian muscle activity requires the Hydrolysis of approximately 10 mol of ATP per 1 g of muscle per minute. Yet, the intracellular ATP concentration is roughly 5∙10^-6 mol/g. This amount of ATP is sufficient for no more than 0.5 s of muscle work.

2. Measurements of muscle ATP content before and after contraction revealed neither a decrease in ATP nor an increase in ADP upon the completion of the contraction cycle.

Subsequent research established that muscles contain sufficiently large reserves of phosphagens. The most prominent among these are creatine phosphate (found in most vertebrates) and Arginine phosphate (in invertebrates).

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Creatine phosphate is a guanidine derivative in which a phosphorus atom is directly bonded to nitrogen. The hydrolytic Cleavage of the phosphate group releases a large amount of energy:

Creatine phosphate + ADP Creatine + ATP, ∆G⁰ = -12 kJ/mol

The reaction is catalyzed by the enzyme creatine kinase. At the physiological pH value of pH = 6, the equilibrium is shifted to the left. Therefore, during single muscle contractions, the quasi-equilibrium concentration of ATP remains virtually unchanged because There is a rapid ATP replenishment pathway mediated by creatine phosphate, the content of which exceeds the muscle ATP concentration by a factor of 4–5. The creatine kinase pathway of ATP resynthesis is a rapid and maximally efficient source of ATP. If muscle contraction is stimulated over a longer duration in the absence of glycolysis or respiration, the creatine phosphate content decreases. This, in turn, leads to a drop in ATP concentration and, ultimately, to the loss of contractile capacity.

The vital role of creatine phosphate as an ATP reservoir is further supported by inhibitor experiments (Cain et al., 1962). For instance, the creatine phosphokinase activity of intact muscles can be completely suppressed by 2,4-dinitrofluorobenzene (Sanger's reagent). Muscles poisoned with this inhibitor exhibit a very rapid drop in ATP concentration during contractions. This occurs because the pathway generating ATP from creatine phosphate is blocked by 2,4-dinitrofluorobenzene.

There is evidence that creatine phosphate is utilized in muscles—particularly cardiac muscle—not only as a depot of readily mobilizable high-energy phosphate bonds, but also for transporting the high-energy phosphate products of Oxidative Phosphorylation. A scheme for such Transport from the Cell/35.html">Mitochondria to the Cytoplasm of myocardial Cells has been proposed. According to this model, ATP from the mitochondrial matrix is transported across the inner membrane via a specific ATP-ADP translocase to the Active Site of the mitochondrial creatine kinase isoenzyme. The latter is located on the outer side of the mitochondrial inner membrane. In the intermembrane space, in the presence of magnesium ions and ambient creatine, a ternary enzyme-substrate complex is formed: creatine-creatine kinase-ATP-Mg²⁺. This complex then dissociates to yield creatine phosphate and ADP-Mg²⁺. Creatine phosphate diffuses into the cytoplasm, where it is utilized in the myofibrillar creatine kinase reaction for rephosphorylation. It is hypothesized that a similar pathway of energy transport from mitochondria to myofibrils operates in Skeletal Muscle as well.

The process of energy transport from the mitochondria to the cytoplasm of a myocardial cell (according to Saks et al.) is shown schematically in the figure.

Where a, b denote the outer and inner membranes, respectively; Cr – creatine; CrP – creatine phosphate; CK – creatine kinase; T – translocase.

The processes of ADP rephosphorylation to ATP and creatine phosphate formation are sustained in all muscles by glycolysis and respiratory activity. However, THE CONTRIBUTION OF glycolysis and respiration varies across different muscle types. In highly active, or red muscles (where the color is due to a high content of Myoglobin and Cytochromes), the primary energy source is mitochondrial oxidative phosphorylation. These muscles include avian flight muscles and mammalian limb muscles. In low-activity, or white skeletal muscles with a low content of myoglobin and cytochromes, the main energy source is glycolysis.