Turnover enzymes

pp value is less than the true n because throughout most of the velocity curve, the ES and SE complexes contribute to a substantial portion of the observed velocity. (For example, at u = 0.366 ES + SE account for 29.2% of the total enzyme while SES accounts for only 22%.) If a were much smaller (e.g., 0.02), then most of the enzyme will be present as either E or SES and tiapp would approach 2- 

Feedback inhibition, activation, and allosteric phenomena are extremely rapid modes of regulating enzyme activity in all types of cells. Repression 

A particular enzyme of liver is synthesized at a constant rate of 12.5 units per gram tissue per minute. The steady-state level of the enzyme is 250 units per gram tissue, (a) Calculate the first-order rate constant for the degradation phase of the turnover, (b) Administration of a hormone caused the rate of enzyme synthesis to increase sixfold without affecting the first-order rate-constant of degradation. After a lag, the enzyme attained a new steady-state level. What is the new rate of degradation What is the new steady-state level of the enzyme  

That is, 5% of the steady-state enzyme level is turned over each minute. 

a sixfold increase in the rate of enzyme synthesis results in a sixfold higher, steady-state enzyme level if ft remains constant. 

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During oxidation of the MoFe protein the P clusters are the first to be oxidized at about -340 mV. This redox potential was first measured (40) using Mossbauer spectroscopy and exhibited a Nemst curve consistent with a two-electron oxidation process. It is possibly low enough for this redox process to be involved in enzyme turnover (see Section V). No additional EPR signal was observed from this oxidized form at this time. However, later a weak signal near g = 12 was detected and was finally confirmed, using parallel mode EPR... 

These potentials are almost certainly too high for this oxidation state to he involved in enzymic turnover. One electron is removed from each FeMoco center, rendering it EPR inactive and diamagnetic. 

In this cycle, step 3, the dissociation of the two proteins, is rate determining and is rate determining in the overall enzyme turnover. 

These lags in product formation were interpreted in terms of repetitive slow steps in enzyme turnover, each slow step being equated to... 

Figure 1.9 Reaction cycle for an aspartyl protease illustrating the conformational changes within the active site that attend enzyme turnover. Source Model based on experimental data summarized in Northrop (2001). 

The examples above serve to illustrate that the conformational dynamics of enzyme turnover create multiple, specific binding pocket configurations throughout the reaction pathway, each representing a distinct opportunity for drug binding and inhibition. 

Figure 3.1 Equilibrium scheme for enzyme turnover in the presence and absence of reversible inhibitors...

An enzyme s turning over is not like a trick your dog can do. Enzyme turnover refers to the cyclic process by which the enzyme turns the substrate over into product and is regenerated. 

The oxidation of o-diphenols occurs via a 2e process because no semiquinone intermediate appears during the enzyme turnover. 

HRP C contains two different types of metal center (i.e., iron(III) protoporphyrin IX-heme group and two calcium atoms) that are fundamental for the integrity of the enzyme. The heme group is attached to the enzyme at His 170 by a coordinate bond between the histidine side-chain NE2 atom and the heme iron atom. The second axial coordination site is unoccupied in the resting state of the enzyme but available to hydrogen peroxide during enzyme turnover. Small molecules such as carbon monoxide, cyanide, fluoride, and azide bind to the heme iron atom at this distal site, giving six-coordinated PX complexes. 

As in the luminol case, the main role of the enhancer (EnH) seems to be related to turnover of the enzyme, generating enhancer radicals (En rad) in the process that are capable of oxidizing the acridan ester (AcH). The structure of the enhancer obviously is very important. To accelerate HRP turnover, the enhancer must on the one hand be able to rapidly react with the reactive HRP intermediates Cl and especially CII (k2 and k3 large). On the other hand, the oxidized enhancer intermediate (radical or radical cation) must be able to oxidize the acridan ester (light-generating step). This last reaction also depends on the structure of the acridan ester in a very unfavorable case, adding an enhancer for enzyme turnover could actually diminish the light production if k 4 > fct (Fig. 5), i.e., if the enhancer radical would not be able to oxidize the acridan ester. 

An outline mechanism for tyrosine activation has been proposed (Fersht, 1975 Fersht et al., 1975a,b Ward and Fersht, 1988a) on the basis of conventional kinetic and binding studies, and this is shown in (49). For the aminoacylation step, some aspects of the reaction are still not known such as the point at which AMP is displaced, but the currently preferred mechanism (Fersht and Jakes, 1975 Ward and Fersht, 1988b) is that given in (50). This is compatible with the observed kinetics which show that two moles of tyrosine bind in each enzyme turnover during which one molecule of Tyr-tRNA appears. 

Acetylcholinesterase is a remarkably efficient enzyme turnover has been estimated as over 10 000 molecules per second at a single active site. This also makes it a key target for drug action, and acetylcholinesterase inhibitors are of considerable importance. Some natural and synthetic toxins also function by inhibiting this enzyme (see Box 7.26). 

Phillips MA, Coffino P, Wang CC. Cloning and sequencing of the ornithine decarboxylase gene from Trypanosoma brucei. Implications for enzyme turnover and selective difluoromethylornithine inhibition. JBiol Chem 1987 262 8721-8727. 

Blumberg, W. E. et at Is trivalent copper a viable oxidation state in the enzymic turnover of copper proteins In Oxidases and Related Redox Systems (King, T. E., Mason, H. S., Morrison, M., eds.), Oxford-New York, Pergamon Press, 1981... 

Oj had fallen considerably in particles prepared from the anaerobically stimulated cells. Whether a third type of inactivation was the explanation for these observations is not clear. Certainly the very low of the oxidase of Oj of 10 pM might indicate that only the most rigid measures would exclude Oj sufficiently to abolish the enzymic turnover in PMNs. Therefore, the partial effect of anaerobiosis might have been to slow both enzymic turnover and the consequent autoinactivation found by Light et al. in the partially purified enzyme. 

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