Half-saturation constant

Half-saturation constant (concentration for 50% saturation of the transport protein). 

They used a value of 4.1 x 10-11 molal s-1 for rmax, the maximum reaction rate, and 1.4 x 10-5 molal for KA, the half saturation constant. We consider application of this kinetic law in detail in Chapter 28. 

This relation is the broadly known Michaelis-Menten equation. The effect of substrate concentration ni on the rate predicted by this equation follows a characteristic pattern. Where substrate concentration is considerably smaller than the half saturation constant (ni <reactive intermediate EA depends on the availability of the substrate A. In this case, (mA + K A ) and reaction rate r+ given by 17.18 is proportional to mA. For the opposite case, (mA K ), little free enzyme E is available to complex with A. Now, (mA + mA and reaction... 

If the concentrations of only the electron donor and acceptor are considered to vary, each mD+ is invariant and the term ] [ n/ 1 in Equation 18.23 reverts to a half-saturation constant K[y Similarly, the corresponding term in Equation 18.24 may be represented by K A. Now, we see the dual Monod equation (Eqn. 18.16) is a specific simplification of the general rate law (Eqn. 18.22). 

Since the enzyme concentration was not observed separately from the rate constant, we carry the product k+ m. in this equation as rmax, the maximum reaction rate. Bekins el al. (1998) fitted their results using values of 1.4 mg kg-1 day-1 (or 1.7 x 10-10 molal s-1) for rmax, and 1.7 mg kg-1 (1.8 x 10-5 molal) for Ka, the half-saturation constant. In a field application lasting many years, of course, the assumption that enzyme concentration remains constant might not be valid,... 

In the calculation results, shown in Figure 28.4, phenol concentration decreases with time at a constant rate for about the first 30 days of reaction. Over this interval, the concentration is greater than the value of K, the half-saturation constant, so the ratio m/(m + K ) in Equation 28.9 remains approximately constant, giving a zero-order reaction rate. Past this point, however, concentration falls below K and the reaction rate becomes first order. Now, phenol concentration does not decrease linearly, but asymptotically approaches zero. 

Following the calculations in Section 18.5, we take a rate constant k+ for sulfate reduction of 10-9 mol mg-1 s-1, a half-saturation constant for acetate of 70 p, molal, and a growth yield of 4300 mg mol-1 from a study of the kinetics of Desulfobacter postgatei by Ingvorsen el al. (1984). We set a value for KA, the half-saturation constant for sulfate, of 200 p molal, as suggested by Ingvorsen el al. (1984) and Pallud and Van Cappellen (2006). 

The relationship between i. and S as depicted in Figure 2.7 is relevant because it quantifies the importance of a substrate in terms of its concentration on the growth rate. As seen from Equation (2.16), X= 1/2 imax for S=Ks. For this reason, Ks is also named the half saturation constant. Equation (2.16) and the corresponding curves shown in Figure 2.7 are called the Monod expression and Monod curve, respectively. 

You are concerned about the longevity of the herbicide, linuron, leaching into a river from some neighboring farmland. Given the structure of this urea derivative, you expect it will be biodegraded via a hydrolysis mechanism. You recall a report of a hydrolase enzyme from a common bacterium that exhibits a half-saturation constant, KMM, for linuron of 2 /tM and a maximum degradation rate, Vmax, for linuron of 2500 /nmol kg-1 protein s l. 

Using one liter of subsurface site material (containing 0.33 L of water and 0.67 L of solids Altoluene(1 = 0.1 L-kg-1) in an enclosed column in the laboratory, you flush it with water containing 100 /tM toluene and 02 (added as H202) in stoichiometric excess. You find the steady-state dry biomass is 10 mg biomass. L- (i.e., 30 mg biomass - L 1 of water). By varying the influent toluene concentration, you find the pmax on this substrate is 1 d l, the die-off coefficient is 0.15 d l, the half-saturation constant with respect to dissolved toluene is 10 fjM, and the dry biomass yield from toluene is 8 x 104 mg biomass.mor1 toluene. 

Much confusion exists over the effects of sulfate concentration and carbon availability on rates of sulfate reduction (cf. 1, 4, 5, 72, 85, 106). Sulfate reducers in lake sediments exhibit low half-saturation constants for sulfate (10-70 xmol/L 4, 72, 78, 85) as well as for acetate and hydrogen (4, 13, 87). The low half-saturation constants allow them to outcompete methano-gens for these substrates until sulfate is largely consumed within pore waters (4, 90). Low concentrations of sulfate in lakes confine the zone of sulfate reduction to within a few centimeters of the sediment surface (e.g., 4, 90, 98). The comparability of rates of sulfate reduction in freshwater and marine... 

Eppley, R.W., Rogers, 3.N. and McCarthy, 3.3., 1969. Half-saturation constants for uptake of nitrate and ammonium by marine phytoplankton. Limnol. Oceanogr., 14 912-920. 

Opportunists are defined as consumers of labile DOC (LDOC). The designation refers to their hypothesized capacity to assimilate substrates at low concentration and to the observation that LDOC sources tends to be ephemeral relative to the background of RDOC that buffers the metabolism of many freshwater ecosystems (Tranvik, 1998 see Chapter 19). Other proposed characteristics of opportunists include small cell size, low half-saturation constants for substrate uptake (Ks) and ectoenzyme activity (Km), and high growth efficiencies, because they do not express extracellular enzymes in large quantities and there is less DNA to replicate. 

Packard, T.T. (1979) Half-saturation constants for nitrate reductase and nitrate translocation in marine phytoplankton. Deep Sea Res. 26, 321-326. 

Fig. 4 Nitrate-specific growth rates (/iN) of conlonial Phaeocystis in the incubation bottles between days 25 and 31, versus initial dissolved iron concentrations. The Monod hyperbola (dashed line) was fitted using an Eadie-Hofstee linear transformation (r2 = 0.85), and excludes the +1.8 nM Fe datum (in parentheses). The half-saturation constant for growth (K ) and maximum nitrate-specific growth rate (/ Nmax) are indicated...

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