Low substrate concentration

Equation 1-106 predicts that the initial rate will be proportional to the initial enzyme concentration, if the initial substrate concentration is held constant. If the initial enzyme concentration is held constant, then the initial rate will be proportional to the substrate concentration at low substrate concentrations and independent of the substrate concentration at high substrate levels. The maximum reaction rate for a given total enzyme concentration is... 

Figure 11-1a. Simple Michaelis-Menten kinetics. At low substrate concentration...

FIGURE 13.41 A plot of the rate of an enzyme-catalyzed reaction (relative to its maximum value, k2[E]0, when S is in very high concentration) as a function of concentration of substrate for various values of (CM. At low substrate concentrations, the rate of reaction is directly proportional to the substrate concentration (as indicated by the black line for KM = 10). At high substrate concentrations, the rate becomes constant at k2[E]0 once the enzyme molecules are "saturated" with substrate. The units of S are the same as those of KM. 

Another important point to consider is that of control. As Fig. 2.17 shows, when the enzymes are almost saturated the rate hardly changes with the concentration of the substrate, implying that the rate of product formation cannot be controlled by [S]. Of course, control is optimally possible in the low substrate concentration regime. Hence, in cases where substrate control of the rate is important, the reaction should ideally proceed in the region of [S] between 5 and IOKm. 

Co(III) mandelic acid k [Cotlll)] [substrate] (at low substrate concentration only) - - 188... 

It was found out that reaction of the hydrolysis of highlymetoxilated beet pectin (catalyzed by P. fellutanum pectinesterase) obeyed Michaelis—Menten equation only under low substrate concentrations (up to 1.2%), when graph of the dependence of reaction speed was hyperbolic in form. 

The pyrethroid insecticides fenvalerate and cypermethrin are hydrolyzed under alkaline conditions at low substrate concentrations, but at higher concentrations the initially formed 3-phenoxybenzaldehyde reacts further with the substrate to form dimeric compounds (Figure 1.22) (Camilleri 1984). 

There is an indeterminacy in the term oligotroph, and the dilemma is exacerbated by the fact that it may be impossible to isolate obligate oligotrophs by established procedures. The application of DNA probes should, however, contribute to an understanding of the role of these noncultivable organisms. Oligotrophic bacteria in the marine environment are able to utilize low substrate concentrations, and they may be important in pristine environments. 

There may be a limiting substrate concentration required for induction of the appropriate catabolic enzyme. At low substrate concentrations the necessary enzymes wonld simply not be synthesized, and this could be the determining factor in some circumstances (Janke 1987). 

These results clearly show the advantage and desirability of using indigenous organisms under appropriate conditions, and that they may effectively degrade relatively low substrate concentrations. The latter is consistent with the ability of bacteria in natural aquatic systems to utilize low substrate concentrations, which has been noted in Chapter 4. 

A high level of poly(3HB) accumulation is also obtained if the cells are grown under carbon substrate limitation, and the cultivation in the second fermenter is also carried out under carbon limitation. In this case, a substrate flow rate (F2) below that corresponding to the maximum specific poly(3HB) formation rate should be chosen [114]. This cultivation strategy is especially convenient when using toxic substrates like acetic acid. Low substrate concentrations are more conveniently maintained in continuous cultivation than in fed-batch cultivation. The only additional equipment needed is a system to ensure constant working volumes and flow rates. 

Both active and passive transport occur simultaneously, and their quantitative roles differ at different concentration gradients. At low substrate concentrations, active transport plays a major role, whilst above the concentration of saturation passive diffusion is the major transport process. This very simple rule can be studied in an experimental system using cell culture-based models, and the concentration dependency of the transport of a compound as well as asymmetric transport over the membrane are two factors used to evaluate the presence and influence of transporters. Previous data have indicated that the permeability of actively absorbed compounds may be underestimated in the Caco-2 model due to a lack of (or low) expression of some uptake transporters. However, many data which show a lack of influence of transporters are usually derived from experiments... 

At low substrate concentrations [S] Km and the reaction rate is linear with respect to the substrate concentration. The reaction in this region is first... 

When there is no substrate present ([S] = 0), there is no velocity— so far, so good. As the substrate concentration [S] is increased, the reaction goes faster as the enzyme finds it easier and easier to locate the substrate in solution. At low substrate concentrations ([S] < Km), doubling the concentration of substrate causes the velocity to double. 

The actual velocity of the reaction depends on how much of the total amount of enzyme is present in the enzyme-substrate (ES) complex. At low substrate concentrations, very little of the enzyme is present as the ES complex—most of it is free enzyme that does not have substrate bound. At very high substrate concentrations, virtually all the enzyme is... 

The term kcJKm describes the reaction of any enzyme and substrate at low substrate concentration. At low substrate concentration, the velocity of an enzyme-catalyzed reaction is proportional to the substrate concentration and the enzyme concentration. The proportionality constant is kcJKm and v = (/cc u/A ,)l l [E]x- If you re real astute, you ll have noticed that this is just a second-order rate equation and that the second-order rate constant is kcJKm. 

The term kcJKm is also the second-order rate constant for the reaction of the free enzyme (E) with the substrate (S) to give product. The kc.JKm is a collection of rate constants, even for the simple reaction mechanism shown earlier. Formally, kcJKm is given by the pile of rate constants kik3/(k2 + k3). If k3 > k2, this reduces to ku the rate of encounter between E and S. Otherwise, kcJKm is a complex collection of rate constants, but it is still the second-order rate constant that is observed for the reaction at low substrate concentration. 

An inhibitor can have different effects on the velocity when the substrate concentration is varied. If the inhibitor and substrate compete for the same form of the enzyme, the inhibition is COMPETITIVE. If not, the inhibition is either NONCOMPETITIVE or UNCOMPETITIVE depending on whether or not the inhibitor can affect the velocity at low substrate concentrations. 

At low concentrations of substrate ([S] < Km), the enzyme is predominantly in the E form. The competitive inhibitor can combine with E, so the presense of the inhibitor decreases the velocity when the substrate concentration is low. At low substrate concentration ([S] < Km), the velocity is just Vmay IKm. Since the inhibitor decreases the velocity and the velocity at low substrate concentration is proportional to Vmax/Km, the presence of the inhibitor affects the slopes of the Lineweaver-Burk plots the slope is just the reciprocal of Vmax/Km. Increasing the inhibitor concentration causes Km/Vmax to increase. The characteristic pattern of competitive inhibition can then be rationalized if you simply remember that a competitive inhibitor combines only with E. 

At very low substrate concentration ([S] approaches zero), the enzyme is mostly present as E. Since an uncompetitive inhibitor does not combine with E, the inhibitor has no effect on the velocity and no effect on Vmsa/Km (the slope of the double-reciprocal plot). In this case, termed uncompetitive, the slopes of the double-reciprocal plots are independent of inhibitor concentration and only the intercepts are affected. A series of parallel lines results when different inhibitor concentrations are used. This type of inhibition is often observed for enzymes that catalyze the reaction between two substrates. Often an inhibitor that is competitive against one of the substrates is found to give uncompetitive inhibition when the other substrate is varied. The inhibitor does combine at the active site but does not prevent the binding of one of the substrates (and vice versa). 

There are two limiting cases of Michaelis-Menten kinetics. Beginning from Eq. (1) at high substrate excesses (or very small Michaelis constants) Eq. (4 a) results. This corresponds to a zero-order reaction with respect to the substrate, the rate of product formation being independent of the substrate concentration. In contrast, very low substrate concentrations [26] (or large Michaelis constants) give the limiting case of first-order reactions with respect to the substrate, Eq. (4b) ... 

Fig. 10.2 Comparison of Eq. (1) (upper line) with the limiting case of a first-order reaction Eq. (4b) (lower line) for very low substrate concentrations.

For flow rate measurements the volume or, more conveniently, the mass flow is suitable. In the first case a pressure- and temperature-dependent calibration is necessary if the gas does not show ideal behavior. This also applies for heat conductivity as the measured quantity often used in flow meters. Currently, real pressure- and temperature-independent measurement of a hydrogen mass flow of a hydrogenation remains problematic on the laboratory scale, at least for low substrate concentrations. 

Substrate and product inhibition. Few academic researchers are familiar with this phenomenon as they usually mn their hydrogenations at low substrate concentrations and low SCR. However, for industrial applications the space-time yield of a reaction - the amount of product per unit reactor volume per time unit - is quite important. Clearly, the higher the substrate concentration the higher the space-time yield and the more economic the process. More often than not, either substrate or product inhibition becomes a problem when the substrate concentration is increased to 10 wt% or more. 

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