Adenosine triphosphate dephosphorylation

In the presence of calcium, the primary contractile protein, myosin, is phosphorylated by the myosin light-chain kinase initiating the subsequent actin-activation of the myosin adenosine triphosphate activity and resulting in muscle contraction. Removal of calcium inactivates the kinase and allows the myosin light chain to dephosphorylate myosin which results in muscle relaxation. Therefore the general biochemical mechanism for the muscle contractile process is dependent on the avaUabUity of a sufficient intraceUular calcium concentration. 

Phosphorylation and dephosphorylation Phosphorylation reactions are catalyzed by a family of enzymes called protein kinases that use adenosine triphosphate (ATP) as a phosphate donor. Phosphate groups are cleaved from phosphorylated enzymes by the action of phosphoprotein phosphatases (Figure 5.18). 

The kinases are inhibited by pyruvate and adenosine disphosphate (ADP), and the phosphatases are activated by calcium ions. There is normally a constant process of phosphorylation and dephosphorylation of the enzyme, so that it is very sensitive to changes in intraceUular free calcium and the adenosine triphosphate (ATP) ADP ratio. 

Approximately 80% of red blood cell purines are in the form of adenosine triphosphate (ATP) with an intracellular concentration estimated to be 2-3 mM. In glucose-deprived or aged red cells there is a progressive decline in the ATP content of the erythrocyte leading to the formation of adenosine diphosphate (ADP) and adenosine monophosphate (AMP) AMP is then dephosphorylated (via 5 -nucleotidase) to... 

Many of these effects can be explained by an interference with a central signal cascade (Fig. 5.195). Adrenaline activates adenylylcyclase, which converts adenosine triphosphate (ATP) into cyclic 3 ,5 -adenosine monophosphate (cAMP). This is a secondary messenger, which is deactivated by a phosphodiesterase to adenosine monophosphate (AMP). Dephosphorylation with a 5 -nucleotidase releases the neuromodulator adenosine, which is enriched extracellularly in the waking state and is degraded during sleep. When adenosine binds in the presynaptic cleft to adenosine-Aj-receptors of the nerve cells, the release of most neurotransmitters, like glutamate, y-aminobutyric acid, norephedrine, serotonin and acetylcholine is inhibited. In addition, adenosine inhibits adenylylcyclase. 

The main sources of ribose in meat are inosine 5 -monophosphate (IMP) and smaller quantities of ribose 5-phosphate and free ribose. The IMP is formed in muscle post slaughter from the enzymatic dephosphorylation and deamination of adenosine triphosphate (ATP), the ribonucleotide that is essential to muscle function in the live animal [5]. Further enzymatic breakdown of IMP may lead to hypoxanthine, ribose, and ribose 5-phosphate (Fig. 1), although most of the ribose in meat remains bound within IMP. 

---