Reaction rates substrate concentration

Fig. 5.35. Effect of positive and negative modulators of reaction rate. Substrate concentration - rate curves for...

Fig. 2. Reaction rate-substrate concentration profile for a reaction obeying Michaelis-Menten (or saturation) kinetics. 

Fig. 8. Reaction rate-substrate concentration profiles for a given free energy of activation (fixed cat/ m) tit with a varying degree of binding of the substrate and enzyme. A higher maximal rate is achieved with weak binding of the substrate.

Enzyme Value (E.V.), as defined by R. Weidenhagen (Ergebn. Enzymforschung, 1, 168 (1932)) equals k/(g X log 2), where k = first order rate constant at 30° (minutes), g grams of enzyme in 50 ml. of reaction mixture. Substrate concentration in the experiments shown in this table = 0.052 M. 

Example 11.10 Sensitivity of the rate of the enzymatic reaction to substrate concentration Using the reaction network in Eq. (11.63), the sensitivity of this enzyme to the substrate X0 is defined by the elasticity coefficient s 1... 

R versus T. An allosteric enzyme that follows the concerted mechanism has a T/R ratio of 300 in the absence of substrate. Suppose that a mutation reversed the ratio. How would this mutation affect the relation between the rate of the reaction and substrate concentration ... 

FIGURE 18.19 The dependence of the rate of an enzyme reaction on substrate concentration. The dashed line gives the rate attained for very high [S],... 

This is the Michaelis-Menten equation, which describes the kinetics of many enzyme-catalyzed reactions. Fignre 18.19 shows a typical experimental cnrve for the dependence of the rate of snch a reaction on substrate concentration [S]. For low substrate concentrations the rate is linear in [S], but at high concentrations it levels off. This behavior is easily nnderstood on physical gronnds. When [S] is small, the more substrate is added, the greater is the rate of formation of prodnct. As the amount of substrate becomes large, all the active sites of the enzyme molecules are bound to substrate, so adding more S does not affect the rate of the reaction. Satnration occurs, and the rate levels off. 

Figure 8-4 Michael is-Menten curve relating velocity (rate) of an enzyme-catalyzed reaction to substrate concentration. The value of Km is given by the substrate concentration at which one half of the maximum velocity is obtained.

Michaelis-Menten kinetics in 1913 L. Michaelis and M. Menten realized that the rate of an enzymatic reaction differed from conventional chemical reactions. They postulated a scheme whereby the reaction of a substrate plus enzyme yields enzyme plus substrate and placed it into the form of the equation reaction velocity = (maximal velocity of the reaction x substrate concentration)/(concentration of substrate + a fitting constant Km). The Km (referred to as the Michaelis-Menten constant) is the concentration of the substrate at which the reaction rate is half the maximal value it also characterizes the tightness of the binding between substrate and enzyme. 

Fig. 4.4. Effect of the mole ratio between maltose and fi-CD on the conversion rate of Mal-/3-CD. Reaction conditions substrate concentration of 70%, 100 U puUulanase per gram of /S-CD, the temperature of 60°C, reaction time of 60 h and pH = 4.0 [6].

What happens to the rate of an enzyme-catalyzed reaction as substrate concentration is raised beyond the saturation point ... 

In an enzyme-catalysed reaction the substrate concentration was varied and the resulting initial rates... 

Rate constant With respect to chemical reactions, a constant that relates the reaction rate for a particular reaction to substrate concentrations. 

Figure 14.29 Plot of the initial rate (vq) of an enzyme-catalyzed reaction versus substrate concentration.

Figure 16.11 Graph of the rate of an enzyme-catalysed reaction against substrate concentration... 

Plotting the graph of rate of reaction against substrate concentration, as in Figure 2.8, permits only an approximate determination of the values of and and a number of methods have been developed to convert this hyperbolic relationship into a linear relationship, to permit more precise fitting of a line to the experimental points, and hence more precise estimation of K and V. ... 

Not all enzymes show the simple hyperbolic dependence of rate of reaction on substrate concentration shown in Figure 2.8. Some enzymes consist of several separate protein chains, each with an active site. In many such enzymes, the binding of substrate to one active site causes changes in the conformation not only of that active site, but of the whole multi-subunit array. This change in conformation affects the other active sites, altering the ease with which substrate can bind to the other active sites. This is cooperativity — the different subunits of the complete enzyme cooperate with each other. Because there is a change in the conformation (or shape) of the enzyme molecule, the phenomenon is also called allostericity (from the Greek for different shape ), and such enzymes are called allosteric enzymes. 

Some enzymes are inhibited by high concentrations of their own products, (a) Sketch a plot of reaction rate against concentration of substrate for an enzyme that is prone to product inhibition. 

Enzymologists frequently study how reaction rates, substrate binding etc. vary under a variety of experimental conditions. Parameters which may be changed include pH, ionic concentration and the addition of water-soluble inhibitors. Thus, the membrane protein is only probed in regions where it is exposed to bulk water or at the interfacial region where such water meets the bilayer. This often only represents a small part of the whole membrane protein. 

Michaelis constant An experimentally determined parameter inversely indicative of the affinity of an enzyme for its substrate. For a constant enzyme concentration, the Michaelis constant is that substrate concentration at which the rate of reaction is half its maximum rate. In general, the Michaelis constant is equivalent to the dissociation constant of the enzyme-substrate complex. 

A plot of equation 13.18, shown in figure 13.10, is instructive for defining conditions under which the rate of an enzymatic reaction can be used for the quantitative analysis of enzymes and substrates. Eor high substrate concentrations, where [S] Kjq, equation 13.18 simplifies to... 

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