Creatine kinase reaction, phosphoryl

The lower panel shows the decreasing concentration of ATP, to about 60% of resting levels, and the simultaneous equimolar increase in IMP. The fall in ATP started when most of the PCr store was utilized, resulting in a decreased rate of ADP phosphorylation via the creatine kinase reaction. The resultant accumulation of ADP stimulates adenylate kinase activity and subsequently IMP is formed via the AMP deaminase reaction ... 

When a sudden demand for energy depletes ATP, the PCr reservoir is used to replenish ATP at a rate considerably faster than ATP can be synthesized by catabolic pathways. When the demand for energy slackens, ATP produced by catabolism is used to replenish the PCr reservoir by reversal of the creatine kinase reaction. Organisms in the lower phyla employ other PCr-like molecules (collectively called phosphagens) as phosphoryl reservoirs. 

In the course of studying the mechanism of action of creatine kinase from rabbit skeletal muscle (M.M isoenzyme), Kenyon and coworkers (4,90) have been involved in the design of specific irreversible inhibitors that are active-site-directed (affinity labels). Creatine kinase catalyzes the reversible transfer of a phosphoryl group ( the elements of "POi") from ATP to creatine, as shown in the following reaction ... 

Phosphocreatine (Fig. 13-5), also called creatine phosphate, serves as a ready source of phosphoryl groups for the quick synthesis of ATP from ADP. The phosphocreatine (PCr) concentration in skeletal muscle is approximately 30 nra, nearly ten times the concentration of ATP, and in other tissues such as smooth muscle, brain, and kidney [PCr] is 5 to 10 mM. The enzyme creatine kinase catalyzes the reversible reaction... 

The creatine synthesized in the liver is transported through the bloodstream to skeletal and heart muscle. It enters the mitochondria, where it is phosphorylated to crealine-P Creatine kinase catalyzes this reversible addition of a phosphate group, as shown in Figure 4.34. Creatine-P is unique in that its only known function is as an energy buffer. The creatine P formed in the mitochondria travels to the contractile proteins in the cytoplasm of the muscle fiber. The polymer, or complex, of contractile proteins is called a myofibril. Contraction of a myofibril is coupled to the hydrolysis of ATP to ADP. The immediate replenishment of ATP is catalyzed by a second creatine kinase, residing on the myofibril, that catalyzes the conversion of creatine-P to creatine. This reversal of the reaction takes place in the... 

ATP phosphorylates creatine to form creatine phosphate in a reaction catalyzed by creatine kinase (also known as creatine phosphokinase). 

The amount of ATP in muscle suffices to sustain contractile activity for less than a second. Creatine phosphate in vertebrate muscle serves as a reservoir of high-potential phosphoryl groups that can be readily transferred to ATP. Indeed, we use creatine phosphate to regenerate ATP from ADP every time that we exercise strenuously. This reaction is catalyzed by creatine kinase. 

As implied above, a dissociative or Sul mechanism for phosphoryl transfer is more difficult to establish than an Su2 mechanism. An SnI mechanism may be operative in the reaction catalyzed by creatine kinase as suggested by the observation that planar trigonal monoanions such as nitrate and formate (54) which are analogs of metaphosphate, (Fig. 5)... 

Since metal coordination or immobilization of the transferred phosphoryl group by multiple hydrogen bonds would inhibit the formation of a metaphosphate intermediate in an S l mechanism and would facilitate nucleophilic attack in an Sy2 mechanism, the latter process seems likely for the reactions catalyzed by staphylococcal nuclease, DNA polymerase, pyruvate kinase, fructose diphosphatase, phosphoglucomutase, (Na + K) ATPase and possibly PEP carboxylase. In creatine kinase where an S l mechanism is possible, the enzyme would have to prevent access of nucleophiles other than ADP and creatine to the reactive metaphosphate intermediate. 

As its high phosphate transfer potential suggests (see Figure 3.7), this compound is capable of phosphorylating ADP very efficiently. The reaction is catalyzed by the enzyme creatine kinase as follows ... 

The use of paramagnetic probes in magnetic resonance studies on phosphoryl transfer enzymes, e.g. creatine kinase, has been reviewed, and model reactions with phosphoroguanidates have led to new ideas on the mechanism of action of this enzyme. The pH-rate profile for the... 

Phosphocreatine can be used as a phosphoryl donor for the synthesis of ATP in a reaction catalyzed by creatine kinase. Refer to Section 14.1.5 of the text for free energies of hydrolysis of ATP and creatine phosphate. 

During periods of rest when ATP is abundant, creatine is phos-phorylated by creatine kinase to form phosphocreatine. This reaction is especially important in muscles. When a sudden explosive burst of muscle activity occurs, phosphocreatine phosphorylates adenosine diphosphate (ADP) to generate the ATP needed for muscle contraction (Fig. 10.4). For this reason, phosphocreatine is known as a phosphagen . 

Biosynthesis of ATP. ATP is the irrunediate product of all cellular processes leading to the chemical storage of energy. It is biosynthesized by phosphorylation of ADP in the course of Substrate phosphorylation (see). Oxidative phosphorylation (see) and non-cyclic Photophosphorylation (see) in plants. Energy in the form of a third phosphate may also be transferred to ADP from other high-energy phosphates, such as creatine phosphate (see Creatine) or other nucleoside triphosphates, or in the adenylate kinase reaction. 

Magnesium is closely associated with calciiun and phosphorus. About 70 per cent of the total magnesium is found in the skeleton but the remainder, which is distributed in the soft tissues and fluids, is of crucial importance to the well-being of the animal. Magnesium is the commonest enzyme activator, for example in systems with thiamin pyrophosphate as a cofactor, and oxidative phosphorylation is reduced in magnesium deficiency. Magnesium is an essential activator of phosphate transferases (e.g. creatine kinase) and it activates pyruvate carboxylase, pyruvate oxidase and reactions of the tricarboxylic acid cycle therefore, it is essential for the... 

Metal cofactors do not always bind to the enzyme but rather bind to the primary substrate. The resulting substrate-metal complex binds to the enzyme and facilitates its activity. Creatine kinase catalyses the transfer of phosphoryl groups from adenosine triphosphate (ATP), which is broken down to adenosine diphosphate (ADP). The reaction requires the presence of magnesium ions. These, however, do not bind to the enzyme but bind to ATP, forming an ATP Mg complex. It is this complex that binds to the enzyme and allows transfer of the phosphoryl group ... 

Creatine is readily phosphorylated by ATP in a reversible reaction catalysed by creatine kinase. The standard free energy of hydrolysis of phosphocreatine is —43 kJ (—10-3 kcal) so the equilibrium favours ATP formation. 

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