Energy metabolism creatine phosphate

FIGURE 14.21 The structures of creatine and creatine phosphate, guanidiniutn compounds that are important in muscle energy metabolism. 

In the standard state, the equilibrium for this reaction lies far to the left in other words, the reaction is unfavored. However, in the standard state, all the reactants and products are at one molar concentration. In other words, the ratio of ATP to ADP concentrations would be 1. In an actively metabolizing state, the ratio of ATP to ADP is as much as 50 or 100 to 1—this means that the formation of Cr P will occur to a reasonable level. Creatine phosphate forms a reservoir for high-energy phosphate in the same way that water can be pumped upstream to a reservoir and released for use later on. 

Muscle has an additional energy reserve in creatine phosphate, which generates ATP without the need for metabolizing fuels (see here). This reserve is exhausted early in a period of exertion and must be replenished, along with glycogen stores, as muscle rests after prolonged exertion. 

Fig. 6.16. Electrophoretic separation of serum creatine kinase enzymes from a normal healthy adult and from a patient who had a myocardial infarction 24 hours previously. Creatine kinase catalyzes the reversible transfer of a phosphate from ATP to creatine to form phos-phocreatine and ADP. The reaction is an important part of energy metabolism in heart muscle, skeletal muscle, and brain. Three different forms of the dimer exist BB (or CK-1) found in brain, MB (or CK-2) found only in heart, and MM (or CK-3), found only in skeletal and heart muscle (cathode, — ve anode, +ve).

Initially, all four energy sources are available to the muscle. When the creatine phosphate runs out, only the other sources are left. When muscle glycogen runs out, the anaerobic boost it provided slows down, and when the liver glycogen is gone, only aerobic metabolism to carbon dioxide and water is left. 

Even though AMP-induced activation of glycolysis and glycogenolysis is rapid, cellular reserves of ATP would only allow vigorous contraction of skeletal muscle for 1 s. An instantly available store of high-energy phosphate is provided by creatine phosphate. Creatine kinase is exceptionally active and maintains the reaction between creatine and ATP, and creatine phosphate and ADP, in rapid dynamic equilibriiun. This pool of creatine phosphate serves as a buffer for ATP that occurs without any activation of metabolic pathways. 

Several nonprotein amino acids have special functions in the physiology of the producer organisms, cf. arginine phosphate or creatine phosphate, which are used as energy-rich substances in metabolism (E 2.2), or betaine, which is accumulated in plants as an osmotic (E 2.2). 

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