Enzymes behavior

The Km is a landmark to help you find your way around a rectangular hyperbola and your way around enzyme behavior. When [S] < Km (this means [S] + Km = Km), the Michaelis-Menten equation says that the velocity will be given by v = CVnvdepends linearly on [S], Doubling [S] doubles the rate. 

Circumstances Under Which Enzyme Behavior Is Not Fully Described by... 

If unireactant enzyme behavior is not described adequately by existing equations, a reaction scheme should be constructed which seems to describe the behavior based on available experimental data. In doing so, Occam s razor should be applied and the simplest possible explanations should be first discounted before more complex explanations are proposed. 

Thereafter, and V ax values for substrate turnover are determined in the absence (controls) and presence of several concentrations of the inhibitor of interest. It is recommended that substrate turnover in the presence of at least four concentrations of inhibitor are examined, at concentrations between 1/3 x IC50 and 4 x IC50. Velocity data are then plotted versus substrate concentration, yielding a control plot and plots at each of the concentrations of inhibitor assessed. Hyperbolic curves are then fitted to data with the Michaelis-Menten equation, or with whichever variation of the Michaelis-Menten equation was found to describe control enzyme behavior most appropriately (see Section 4.1.4 etseq.). In this way, a pattern of changes in Km and Vmax> or both, should become apparent with changing inhibitor concentration. 

The quotient of rate constants obtained in steady-state treatments of enzyme behavior to define a substrate s interaction with an enzyme. While the Michaelis constant (with overall units of molarity) is a rate parameter, it is not itself a rate constant. Likewise, the Michaelis constant often is only a rough gauge of an enzyme s affinity for a substrate. 2. Historically, the term Michaelis constant referred to the true dissociation constant for the enzyme-substrate binary complex, and this parameter was obtained in the Michaelis-Menten rapid-equilibrium treatment of a one-substrate enzyme-catalyzed reaction. In this case, the Michaelis constant is usually symbolized by Ks. 3. The value equal to the concentration of substrate at which the initial rate, v, is one-half the maximum velocity (Lmax) of the enzyme-catalyzed reaction under steady state conditions. 

Is there evidence for the conditioning of enzyme behavior by the membrane ... 

There are two kinds of effects of the membrane on the enzyme behavior a specific interaction between the enzyme and the lipid membrane and a nonspecific interaction of the membrane structure by itself on the enzyme kinetics. In the case of ATPase, the enzyme in solution is working in homogeneous and isotropical conditions. At the opposite extreme, in the membrane the enzyme is working under asymmetrical boundary conditions. In the last case there is a coupling between a scalar process and the vectorial transport effect. In conclusion, the effect of the membrane on the enzyme behavior is not only a chemical effect, but also a geometrical one. 

The first reported preparation of cross-linked enzyme crystals was by Quiocho and Richards in 1964 [1], They prepared crystals of carboxypeptidase-A and cross-linked them with glutaraldehyde. The material they prepared retained only about 5% of the activity of the soluble enzyme and showed a measurable increase in mechanical stability. The authors quite correctly predicted that cross-linked enzyme crystals, particularly ones of small size where the diffusion problem is not serious, may be useful as reagents which can be removed by sedimentation and filtration. Two years later the same authors reported a more detailed study of the enzymic behavior of CLCs of carboxypeptidase-A [2], In this study they reported that only the lysine residues in the protein were modified by the glutaraldehyde cross-linking. The CLCs were packed in a column for a flow-through assay and maintained activity after many uses over a period of 3 months. 

FA Quiocho, FM Richards. The enzymic behavior of carboxypeptidase-A in the solid state. Biochemistry 5 4062-4075, 1966. 

Neutral mutations are neutral with respect to fitness. This does not mean they are neutral with respect to all enzyme behaviors. In fact, many neutral mutations will be deleterious to stability, catalytic ability, or any other property that does not contribute directly to fitness. Properties not protected by the purifying effects of natural selection can change as mutations accumulate, but the process is random and contains litde information that can be used to elucidate mechanisms (Benner and Ellington, 1990 Benner, 1989). 

The different enzyme behavior observed in the case of ionic liquids can be attributed to the lower solubility of long chain acyl donors in these media, compared to the less polar organic solvents used for the enzymatic modification of natural polyhydroxylated compounds. Due to the low solubUily of long chain acyl substrates in ionic liquids, a two-phase system was formed, which is expected to decrease the availability of substrates to the enzyme and therefore the biocata-lytic acylation of phenohc compounds [5j. 

Speculation about the precise roles of active-site sulfur is tempered by an appreciation of the redox versatihty and interplay of sulfur and molybdenum. This is evident from synthetic systems, where the catenation of sulfur (with attendant redox and/or ligand elaboration) and induced internal electron-transfer reactions are frequently observed. The redox interplay of Mo and S, reflected in undesirable synthetic outcomes, may prove crucial to a fifll description of enzyme behavior. see also Sulfur Inorganic Chemistry)... 

Enzyme behavior has been particularly effectively mimicked by those receptors discussed in Section V.B with the metal ions beginning to perform functional catalytic roles. The selectivity of binuclear hosts for a variety of anionic substrates has been investigated and, in particular, by combination of coordination interactions with intermolecular forces, novel anion selectivities have been observed. 

Much of the work in this area concerns modeling zinc-based enzymes. Scorpionates can be considered as the most popular ligands able to mimic the enzyme behavior. 

Water levels also have important general effects on enzyme behavior. If too little water is present, the catalytic activity of most enzymes falls dramatically. On the other hand, reduction in water levels often leads to an increase in enzyme stability. A decline in catalytic activity at high water levels is also commonly observed, with several possible explanations ... 

Even in terms of water activity, hydration effects are not quite so simple however. As well as the current value, enzyme behavior depends on the history of hydration to which the catalyst has been exposed. In other words, there can be strong hysteresis effects. Nevertheless, water activity values are usually the best basis to define the previous history reproducibly. 

One of the equilibria most commonly of interest is esterification. It may be desired to hydrolyze an ester, or reverse this in condensation of an alcohol and acid. Alternatively the hydrolytic equilibrium may be an undesirable side-reaction during transesterification. In this case, at a given water activity, the equilibrium position is quite strongly solvent dependent. The fraction of ester will increase dramatically on going from a polar solvent to non-polar solvent (Fig. 8-7). Hence alkanes are preferred solvents for esterification, while acetonitrile, a ketone or tertiary alcohol would be best for ester hydrolysis. If the equilibrium constant is expressed in terms of concentrations (including that of water), it is relatively solvent independent. However, optimal enzyme behavior in the different solvents usually requires maintaining the same water activity. At fixed water activity, the ratio of ester to acid and alcohol concentrations will be maximized in the least polar solvents. 

About 2500 enzymes have been identified and characterized since urease was first crystallized in 1926 by J B Summer. An excellent review of enzyme behavior and kinetics is given in [1]. Because of the difficulties with uniquely naming so many materials for publications, a standard system of enzyme classification was developed. This compendium of enzymes and their properties is available from the International Union of Biochemistry [2]. 

In these circumstances of membrane permeability limitations, shown as curves 1 and 2 in figure 7.11 at external substrate concentrations below 0.01 M, the response of the sensor will be completely independent of the enzyme prop>erties. A sensor operated in this regime would be independent of factors affecting enzyme behavior, such as denaturation, temperature, and pH. This is the preferred regime for operating an enzyme biosensor. 

The mass transfer mechanisms operative in substrate conversion are essentially those described by Waterland et al47 in their model of the compartmentalized enzyme membrane reactor. Since kinetic parameters cannot be assumed equal to those of native enzymes, a kinetic analysis has to be performed in order to characterize enzyme behavior after the immobilization procedure. 

The enzymes are the biological catalysts. Their action shows some resemblance to the catalytic action of adds and bases, but is considerably more complicated. The details of the mechanisms of enzyme action are still being worked out, and much research remains to be done. The present chapter can give only a brief introduction to the subject, with emphasis on the kinetic effects of concentration, pH and temperature, and on some special aspects of enzyme behavior. 

The two principal models for allosteric enzyme behavior are called the concerted model and the sequential model. 

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