Proteins phosphorolysis

Fig. 5 Catalytic mechanism of glycogen phosphorylases. The reaction scheme accounts for the reversibility of phosphorolysis of oligosaccharides (R) in the presence of orthophosphate (upper half) and primer-dependent synthesis in the presence of glucose-l-phosphate (lower half). PL enzyme-bound pyridoxal BH-f a general base contributed by the enzyme protein. Reprinted with permission from [109]. Copyright 1990 American Chemical Society...

The adenylation and phosphorolysis reactions are catalyzed by the same enzyme, adenylyl transferase. Sequence analysis indicates that this adenylyl transferase comprises two homologous halves, suggesting that one half catalyzes the adenylation reaction and the other half the phospholytic de-adenylation reaction. What determines whether an AMP unit is added or removed The specificity of adenylyl transferase is controlled by a regulatory protein (designated P or Pjj), a trimeric protein that can exist in two forms, P and Pq (Figure 24.27). The complex of P and adenylyl transferase catalyzes the attachment of an AMP unit to glutamine synthetase, which reduces its activity. Conversely, the complex of Pj) and adenylyl transferase removes AMP from the adenylylated enzyme. 

The process of digestion and the activity of carbonic anhydrase are major ev cryday concerns in the medical profession. A clearer understanding of the concepts of protein digestion and carbonic anhydrase acti ity is provid by simplified descriptions of the relevant biochemical mechanisms. Protein digestion involves the addition of water to a peptide bond. Carbonic anhydrase activity involves the addition of water to COj. A related process, the phosphorolysis of glycogen, is also described. 

Specific chemical reactions and catalysts are required for digestion of the three major classes of nutrients — proteins, fats (lipids), and carbohydrates. Others are required for the absorption of digesbve products through the wall of the small intestine, Hydrolysis, phosphorolysis, and the condensabon of COj with water are important biochemical mechanisms in digestion and absorption. 

Nucleotide C-N bond hydrolysis and phosphorolysis at the monomer level form part of purine salvage pathways and their mechanisms have been intensively investigated for pharmacological reasons. Humans can biosynthesise purine nucleosides de novo, whereas protozoal parasites such as those causing bilharzia and Chagas disease rely on hydrolysis of preformed nucleosides from the host. Finally, a series of ribosyl transfers from NAD are important in the modification of proteins by pathogens and have been studied extensively. 

Fig. 8. Stereochemistry of reactions catalyzed by maltose phosphorylase with inversion of configuration. (A) a-Maltose synthesis from /3-D-glucopyranosyl phosphate and the reverse phosphorolysis of a-maltose (B) a-maltose synthesis from /3-D-glucopyranosyl fluoride plus a-D-glucose. X represents a protein component whose interaction with the axial 1-OH of a-D-glucopyranose is required to activate all reactions promoted by the enzyme. Reproduced from Tsumuraya et al., Arch. Biochem. Biophys., 281 (1990) 58-65, with permission of Academic Press.

This results in activation of cAMP-dependent protein kinase (protein kinase A), with consequent phosphorylation of target proteins, such as phosphorylase b kinase in cells that activate glycogen phosphorolysis. 

Am. The hormones epinephrine and glucagon cannot penetrate cell membranes. They affect metabolic processes by binding to specific receptors on the membrane, which receptors in turn activate a specific enzyme bound to the inner membrane surface, adenylate cyclase. This enzyme converts ATP to cyclic AMP (cyclic adenosine monophosphate), or c-AMP. The presence of c-AMP activates another enzyme, protein kinase, which phosphorylates and activates phosphorylase kinase. Phosphorylase kinase phosphorylates phosphorylase b (inactive) to form phosphorylase a (active) which in turn cleaves glucose from glycogen by phosphorolysis to yield glucose-I-PO4. 

Cellobiose must be cleaved into its constituent monosaccharides in order to be metabolized by . coli. There are two main ways in which this reaction can occur, hydrolysis and phosphorolysis. P-Glucosidase and cellobiose phosphorylase from Saccharophagus degradans were expressed in E. coli. The results showed that phosphorolysis cells tolerate common inhibitors (sodium acetate) more effectively and produce recombinant proteins more effectively than hydrolysis cells. However, hydrolysis cells utilize xylose more effectively in combination with cellobiose [192]. 

Fig. 3.8. The role of cAMP as an intermediate to environmental stimulus (hormone secretion) and the onset of physiological response. Adrenaline and GTP bind to a specific membrane bound regulatory subunit of adenyl cyclase. This elicits a conformational change such as to activate the catalytic subunit and stimulate cAMP synthesis. cAMP activates a protein kinase by causing its dissociation into an active catalytic subunit (R = regulatory subunit C = catalytic subunit). This in turn catalyzes the phosphorylation of two serine residues of an inactive phosphorylase b enzyme. Two dimers combine to form an active tetramer phosphorylase a. Phosphorylase catalyzes the breakdown of the sugar storage form glycogen (phosphorolysis) to glucose 1-...

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