Biochemical Foundations of Human Vital Activity - Volkov, N.I., Nesen, E.N. 2000

Biochemistry of Sports
Bioenergetics of Muscular Activity
Creatine Phosphokinase Mechanism of ATP Resynthesis

The alactic anaerobic Mechanism of ATP resynthesis involves utilizing the ATP already present in the Muscles and its rapid resynthesis via a high-energy phosphagen substance, creatine phosphate, the concentration of which in muscles is 3–4 times higher than that of ATP (see Chapter 3).

Creatine phosphate is localized directly on the contractile filaments of myofibrils and is capable of rapidly entering into a transphosphorylation reaction involving the enzyme creatine phosphokinase (CPK) According to the equation

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In human skeletal muscles, CPK exhibits high activity, while CrP and ADP show a high chemical affinity for each other, which leads to an acceleration of this reaction at the very beginning of muscular activity when ATP begins to break down and ADP accumulates.

The maximum power of the creatine phosphokinase reaction develops as early as the 0.5–0.7th second of intense work, indicating a high rate of activation, and is maintained for 10–15 seconds in untrained individuals, whereas in highly trained sprinters it can be sustained for 25–30 seconds.

The creatine phosphokinase mechanism is the first to engage in the ATP resynthesis process at the onset of intense muscular work and proceeds at maximum speed until the CrP reserves in the muscles are depleted. This reaction acts as a unique "energy buffer" since it ensures the maintenance of a constant ATP content in muscles during its rapid utilization or excessive ATP accumulation during periods of rest (see Fig. 122). In the latter case, the transphosphorylation reaction between ATP and free creatine proceeds intensively, i.e., The rate of the reverse creatine phosphokinase reaction increases.

The maximum power of the creatine phosphokinase reaction is 3.8 kJ ⋅ kg^-1 ⋅ min^-1, which is significantly higher than the power of anaerobic Glycolysis (by 1.5–2 times) and the aerobic process (by 3–4 times). The total phosphagen reserves in untrained subjects provide energy generation of about 420 kJ ⋅ kg^-1 of Muscle tissue, and in trained subjects—2 times more. The rate of CrP breakdown in working muscles is directly dependent on the intensity of the exercise performed or the magnitude of muscle tension, as well as The activity of the CPK enzyme.

The metabolic capacity of this mechanism is small, since the CrP reserves in muscles exceed the ATP content by only 3 times (Table 23). Consequently, maintaining the ATP level via creatine phosphate is limited by its reserves, which are already reduced by 1/3 by the 5th second, and by half by the 15th second (Fig. 123). After this, the rate of the creatine phosphokinase reaction decreases, and glycolytic and oxidative mechanisms are recruited into ATP resynthesis.

The efficiency of the creatine phosphokinase reaction is very high (76%), as the reaction occurs directly between two substances on the myofibrils. CrP reserves depend on the body's creatine content. Supplementing with creatine leads to an increase in creatine phosphate reserves in the muscles (from 84 to 91 mmol ⋅ kg^-1 of dry muscle tissue) as well as an improvement in physical performance.

TABLE 23. ATP and CrP reserves in human skeletal muscles (calculated for an average body weight of 70 kg)

Fig. 123 Changes in ATP, CrP, and lactate concentrations in skeletal muscles during exercise

The creatine phosphate content in skeletal muscles increases by 1.5–2 times during the body's adaptation to speed and strength physical loads, which affects the capacity of the creatine phosphokinase energy supply mechanism for muscular activity.

The creatine phosphokinase pathway of ATP resynthesis plays a decisive role in energy supply for short-term work of maximum intensity lasting 15–30 seconds, such as 100-meter sprints, short-distance swimming, jumping, throwing, weightlifting exercises, etc. It provides The ability to transition rapidly from rest to work, make sudden changes in pace during performance, and deliver a finishing kick. The creatine phosphokinase system Functions predominantly in fast-twitch muscle fibers, thus forming the BIOCHEMICAL BASIS OF speed and local muscular strength (endurance).