ATP-Dependence

The kinetic mechanism of kinase inhibitors often has been investigated by varying the concentration of ATP. The dependence upon concentration of phosphoacceptor substrate is rarely reported. ATP-dependence allows comparison across all kinases as it is the universal phosphate donor, and it gives useful information, because the most inhibitors use the purine site. 

The mechanism of inhibition with respect to ATP is assigned on the basis of the potency of the compound after extrapolating to both insignificant and saturating concentrations of the phosphate donor.29 These concentrations are defined relative to that giving the half-maximal rate, which is the Am or Michaelis constant for ATP. The value of Am for ATP may be dependent on several aspects of the assay conditions (See Section 4.4.1.2.). The substrate-dependence of inhibition usually can be characterised using similar rate equations, where [Substrate] replaces [ATP] and the Km value refers to that for substrate. 

The mechanism of inhibition is defined by the relative values of Ais and Ay, which are respectively the inhibition constants at [ATP] Km and Km.2S 29 Inhibition constants are measures of potency, because they equal the free inhibitor concentration when the rate is reduced by 50%. The mechanism is competitive if inhibition tends to zero when ATP is saturating dATP] Km). This mechanism is seen if Ais Ay. Conversely, the mechanism is uncompetitive if inhibition tends to zero when [ATP] Km, because As Ay. Inhibition is noncompetitive when it occurs both at [ATP] Km and [ATP] Am. Pure noncompetitive inhibition (Ais = Ay) arises when potency is independent of ATP-concentration. Mixed noncompetitive inhibition (Als A Ay) occurs if there is a tendency towards competitive or uncompetitive. 

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The dopamine is then concentrated in storage vesicles via an ATP-dependent process. Here the rate-limiting step appears not to be precursor uptake, under normal conditions, but tyrosine hydroxylase activity. This is regulated by protein phosphorylation and by de novo enzyme synthesis. The enzyme requites oxygen, ferrous iron, and tetrahydrobiopterin (BH. The enzymatic conversion of the precursor to the active agent and its subsequent storage in a vesicle are energy-dependent processes. 

The GroEL-GroES complex binds and releases newly synthesized polypeptides in an ATP-dependent cycle... 

Proteins that can flip phospholipids from one side of a bilayer to the other have also been identified in several tissues (Figure 9.11). Called flippases, these proteins reduce the half-time for phospholipid movement across a membrane from 10 days or more to a few minutes or less. Some of these systems may operate passively, with no required input of energy, but passive transport alone cannot establish or maintain asymmetric transverse lipid distributions. However, rapid phospholipid movement from one monolayer to the other occurs in an ATP-dependent manner in erythrocytes. Energy-dependent lipid flippase activity may be responsible for the creation and maintenance of transverse lipid asymmetries. 

FIGURE 13.4 Blunt-end ligation using phage T4 DNA ligase, which catalyzes the ATP-dependent ligation of DNA molecules. AMP and PP are by-products. 

T"he extraordinary ability of an enzyme to catalyze only one particular reaction is a quality known as specificity (Chapter 14). Specificity means an enzyme acts only on a specific substance, its substrate, invariably transforming it into a specific product. That is, an enzyme binds only certain compounds, and then, only a specific reaction ensues. Some enzymes show absolute specificity, catalyzing the transformation of only one specific substrate to yield a unique product. Other enzymes carry out a particular reaction but act on a class of compounds. For example, hexokinase (ATP hexose-6-phosphotransferase) will carry out the ATP-dependent phosphorylation of a number of hexoses at the 6-posi-tion, including glucose. 

Substrate RuBP binds much more tightly to the inactive E form of rubisco (An = 20 nM) than to the active ECM form (A, for RuBP = 20 ixM). Thus, RuBP is also a potent inhibitor of rubisco activity. Release of RuBP from the active site of rubisco is mediated by rubisco activase. Rubisco activase is a regulatory protein it binds to A-form rubisco and, in an ATP-dependent reaction, promotes the release of RuBP. Rubisco then becomes activated by carbamylation and Mg binding. Rubisco activase itself is activated in an indirect manner by light. Thus, light is the ultimate activator of rubisco. 

Even when the latter choice has been made, however, the cell must still be cognizant of the relative needs for ribose-5-phosphate and N/VDPH (as well as ATP). Depending on these relative needs, the reactions of glycolysis and the pentose phosphate pathway can be combined in novel ways to emphasize the synthesis of needed metabolites. There are four principal possibilities. 

Tsuji, F. I. (1985). ATP-dependent bioluminescence in the firefly squid, Watasenia scintillans. Proc. Natl. Acad. Sci. USA 82 4629-4632. 

The ABC-transporter superfamily represents a large group of transmembrane proteins. Members of this family are mainly involved in ATP-dependent transport processes across cellular membranes. These proteins are of special interest from a pharmacological point of... 

Diabetes Mellitus Insulin Receptor Glucose Transporters ATP-dependent K+Channel PPARs... 

Biotin is involved in carboxylation and decarboxylation reactions. It is covalently bound to its enzyme. In the carboxylase reaction, C02 is first attached to biotin at the ureido nitrogen, opposite the side chain in an ATP-dependent reaction. The activated C02 is then transferred from carboxybiotin to the substrate. The four enzymes of the intermediary metabolism requiring biotin as a prosthetic group are pyruvate carboxylase (pyruvate oxaloacetate), propionyl-CoA-carboxylase (propionyl-CoA methylmalonyl-CoA), 3-methylcroto-nyl-CoA-carboxylase (metabolism of leucine), and actyl-CoA-carboxylase (acetyl-CoA malonyl-CoA) [1]. 

Inwardly Rectifying K+ Channels ATP-dependent K+ Channels Voltage-dependent Ca2+ Channels Ryanodine Receptor Voltage-dependent Na2+ Channels... 

Chaperones bind to exposed hydrophobic surfaces of polypeptide substrates, and through either ATP-dependent or ATP-independent mechanisms facilitate the folding/assembly, intracellular transport, degradation, and activity of polypeptides. 

Motor proteins move along MTs in an ATP-dependent manner. Members of the superfamily of kinesin motors move only to the plus ends and dynein motors only to the minus ends. The respective motor domains are linked via adaptor proteins to their cargoes. The binding activity of the motors to MTs is regulated by kinases and phosphatases. When motors are immobilized at their cargo-binding area, they can move MTs. 

Insulin Receptor ATP-dependent K+ Channels Diabetes Mellitus... 

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