Substrate transport rate

Two limit situations can be envisaged. One (Case I) in which r is solely determined by substrate transport rate (diffusion limited) and another (case II) in which r is solely determined by the catalytic potential of the enzyme (kinetically limited). In Case I, reaction rate is so fast with respect to substrate transport rate that substrate profile is steep, ss being negligible with respect to so, while in Case II substrate transport rate across the stagnant layer is fast enough with respect to reaction rate so that no substrate profile develops and ss is equal to so. Eq. 4.10 and 4.11 become Eqs. 4.12 and 4.13 respectively ... 

The rate of side-chain cleavage of sterols is limited by the low solubiUty of substrates and products and thek low transport rates to and from cells. Cyclodextrins have been used to increase the solubiUties of these compounds and to assist in thek cellular transport. Cyclodextrins increase the rate and selectivity of side-chain cleavage of both cholesterol and P-sitosterol with no effect on cell growth. Optimal conditions have resulted in enhancement of molar yields of androsta-l,4-diene-3,17-dione (92) from 35—40% to >80% in the presence of cyclodextrins (120,145,146,155). 

These relationships are identical to Haldane relationships, but unlike the latter, their validity does not derive from a proposed reaction scheme, but merely from the observed hyperbolic dependence of transport rates upon substrate concentration. Krupka showed that these relationships were not obeyed by the set of data previously used by Lieb [64] to reject the simple asymmetric carrier model for glucose transport. Such data therefore cannot be used either to confirm or refute the model. 

The activity of Dsz enzymes in vitro was reported to be almost equal for substituted DBTs from CO to C6-DBTs [71]. This implies that the variation in activity found with whole cells is probably due to the different rates of substrate transport into the cells. 

Recent work at Diversa has provided some insights into the host effects on desulfurization. It was reported that in addition to the obvious substrate transport and growth differences between various strains of same genus, the intracellular redox potential can potentially influence the rate as well as extent of desulfurization [219], Additionally,... 

Mercury point sources and rates of particle scavenging are key factors in atmospheric transport rates to sites of methylation and subsequent entry into the marine food chain (Rolfhus and Fitzgerald 1995). Airborne soot particles transport mercury into the marine environment either as nuclei for raindrop formation or by direct deposition on water (Rawson etal. 1995). In early 1990, both dimethylmercury and monomethylmercury were found in the subthermocline waters of the equatorial Pacific Ocean the formation of these alkylmercury species in the low oxygen zone suggests that Hg2+ is the most likely substrate (Mason and Fitzgerald 1991 Figure 5.1). 

Bosma et al. [1] have proposed including the details of extracellular substrate transport in the calculation of whole-cell Michaelis-Menten kinetics. For the situation of a quasi-steady-state (i.e. when the transport flux and the rate of degradation of the substrate are equal) the consumption of substrate by a microorganism is represented as a function of the distant, and effectively unavailable, substrate concentration ca ... 

Additional experiments in a loop reactor where a significant mass transport limitation was observed allowed us to investigate the interplay between hydrodynamics and mass transport rates as a function of mixer geometry, the ratio of the volume hold-up of the phases and the flow rate of the catalyst phase. From further kinetic studies on the influence of substrate and catalyst concentrations on the overall reaction rate, the Hatta number was estimated to be 0.3-3, based on film theory. 

Reactions carried in aqueous multiphase catalysis are accompanied by mass transport steps at the L/L- as well as at the G/L-interface followed by chemical reaction, presumably within the bulk of the catalyst phase. Therefore an evaluation of mass transport rates in relation to the reaction rate is an essential task in order to gain a realistic mathematic expression for the overall reaction rate. Since the volume hold-ups of the liquid phases are the same and water exhibits a higher surface tension, it is obvious that the organic and gas phases are dispersed in the aqueous phase. In terms of the film model there are laminar boundary layers on both sides of an interphase where transport of the substrates takes place due to concentration gradients by diffusion. The overall transport coefficient /cl can then be calculated based on the resistances on both sides of the interphase (Eq. 1) ... 

With the knowledge of the equilibrium concentrations of hydrogen and aldehyde in water at reaction conditions, the maximum mass transport rates can be determined, assuming that the concentration of the substrate in the aqueous phase is zero (Eqs. 7 and 8) ... 

As already shown by Wiese et al. [17] mass transport rates in biphasic catalysis can be dramatically influenced by hydrodynamics in a tube reactor with Sulzer packings. Above all, the volume rate of the catalyst phase in which the substrates are transported by diffusion plays a decisive role in accelerating the mass transport rate. This effect was also investigated for citral hydrogenation in the loop reactor. Overall reaction rates and conversions as a function of the catalyst volume rate can be seen in Fig. 15. 

The reaction order of one is also in good accordance with the film theory, where the rate of mass transport linearly correlates with the equilibrium concentration of citral in the aqueous phase. As a matter of fact, the mass transport rate is of first order regarding the substrate concentration in the organic phase. Therefore, what is measured is in fact the rate of mass transport and not the rate of chemical reaction. This result is in our opinion a good example of how kinetic parameters could be falsified when the reaction is limited by mass transport and not kinetics. 

Carrier-mediated passage of a molecular entity across a membrane (or other barrier). Facilitated transport follows saturation kinetics ie, the rate of transport at elevated concentrations of the transportable substrate reaches a maximum that reflects the concentration of carriers/transporters. In this respect, the kinetics resemble the Michaelis-Menten behavior of enzyme-catalyzed reactions. Facilitated diffusion systems are often stereo-specific, and they are subject to competitive inhibition. Facilitated transport systems are also distinguished from active transport systems which work against a concentration barrier and require a source of free energy. Simple diffusion often occurs in parallel to facilitated diffusion, and one must correct facilitated transport for the basal rate. This is usually evident when a plot of transport rate versus substrate concentration reaches a limiting nonzero rate at saturating substrate While the term passive transport has been used synonymously with facilitated transport, others have suggested that this term may be confused with or mistaken for simple diffusion. See Membrane Transport Kinetics... 

Net carrier transport can be examined whenever a transportable substrate is present on both sides of a membrane. Again for the case of a single class of carriers, the inward transport rate and outward transport rates show rate-saturation behavior ... 

UNCOMPETITIVE INHIBITION UNCONSUMED SUBSTRATE OR COFACTOR UNDERSHOOT Unfacilitated transport rate,... 

Figure 3.27. Schematic representation of the DPN technique. A water meniscus forms between the AFM tip coated with alkanethiols and the gold substrate. The size of the meniscus, which is controlled by relative humidity, affects the molecular transport rate, the effective tip-substrate contact area, and DPN resolution.

Such multicomponent enzyme complexes have been created through incorporation of oxidase and electron transport protein subunits from both the TDO (todC 1C2B A) of FI and the BPDO (3/>/>AlA2A3A4) of KF707. The hybrid proteins exhibit substrate specificities similar to those of both parents, but some exhibit altered substrate oxidation rates (Hirose et al., 1994). One hybrid, todC 1 bphA2A3A4,... 

Transport is a three-phase process, whereas homogeneous chemical and phase-transfer [2.87, 2.88] catalyses are single phase and two-phase respectively. Carrier design is the major feature of the organic chemistry of membrane transport since the carrier determines the nature of the substrate, the physico-chemical features (rate, selectivity) and the type of process (facilitated diffusion, coupling to gradients and flows of other species, active transport). Since they may in principle be modified at will, synthetic carriers offer the possibility to monitor the transport process via the structure of the ligand and to analyse the effect of various structural units on the thermodynamic and kinetic parameters that determine transport rates and selectivity. 

The external factors comprise the nature of the membrane, the substrate concentration in the aqueous phase and any other external species that may participate in the process. They may strongly influence the transport rates via the phase distribution equilibria and diffusion rates. When a neutral ligand is employed to carry an ion pair by complexing either the cation or the anion, the coextracted uncomplexed counterion will affect the rate by modifying the phase distribution of the substrate. The case of a cationic complex and a counteranion is illustrated schematically in Figure 10 (centre). 

Further demonstrations of this sort of counterflow phenomenon for many different substrates in virtually every type of cell have been used as functional hallmarks of carrier-mediated transport. Experimental demonstration of this effect precludes transport being mediated either by simple diffusion or by fixed pores in the membrane. In reviewing 20 years of experimental work related to the carrier hypothesis, LeFevre (1975) lists a number of key functional properties of carrier mediated transport, all of which have stood the test of the subsequent 20 years. These include saturation of transport with increased substrate concentration and associated phenomena such as competition between similar substrates, high rates of unidirectional transport, and countertransport. Also covered are flux coupling (including trans effects and cotransport), chemical specificity, inhibition by protein-specific reagents, hormonal regulation, and a steep dependence of the rate of transport on temperature (included only to bemoan its common inclusion in textbooks ). 

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