Enzyme enzymatic reaction, schematic

Fig. (1). Schematic view of some branches of phenylpropanoid metabolism. Solid arrows indicate enzymatic reactions with the respective enzyme indicated on the right. PAL, phenylalanine ammonia-lyase C4H, cinnamate 4-hydroxylase 4CL, 4-coumarate CoA ligase CHS, chalcone synthase CF1, chalcone flavavone isomerase F3H, flavanone 3-hydroxylase DFR, dihydroflavonol reductase CHR, chalcone reductase. Broken arrows indicate metabolic branches towards several classes of phenylpropanoids, or several subsequent enzymatic steps. In some cases the enzymes indicated are also involved in other reactions, not shown.

Fig. 10 Schematic representation of the two approaches mainly used in EzILAs. (a) The enzyme conjugate and the analyte compete for the selective binding sites of the polymer finally, a substrate is converted into a product that generates a chemical signal (e.g., fluorescence, absorbance, electrochemical) at a rate which is proportional to the amount of bound enzyme and hence to the concentration of analyte in the sample, (b) Direct assay where the analyte is the enzyme which is quantified by a coupled enzymatic reaction...

The mathematical basis for enzymatic reactions stems from work done by Micha-elis and Menten in 1913 [315]. They proposed a situation in which a substrate reacts with an enzyme to form a complex, one molecule of the enzyme combining with one molecule of the substrate to form one molecule of complex. The complex can dissociate into one molecule of each of the enzyme and substrate, or it can produce a product and a recycled enzyme. Schematically, this can be represented by... 

Figure 4. Schematic representation of the free energy changes in non-enzymatic and enzymatic reactions and in the reaction of a hypothetical transition state analog (TSA) with the enzyme.

Figure 4 A schematic representation of the experimentai approach for time-resoived XAS measurements. XAS provides local structural and electronic information about the nearest coordination environment surrounding the catalytic metal ion within the active site of a metalloprotein in solution. Spectral analysis of the various spectral regions yields complementary electronic and structural information, which allows the determination of the oxidation state of the X-ray absorbing metal atom and precise determination of distances between the absorbing metal atom and the protein atoms that surround it. Time-dependent XAS provides insight into the lifetimes and local atomic structures of metal-protein complexes during enzymatic reactions on millisecond to minute time scales, (a) The drawing describes a conventional stopped-flow machine that is used to rapidly mix the reaction components (e.g., enzyme and substrate) and derive kinetic traces as shown in (b). (b) The enzymatic reaction is studied by pre-steady-state kinetic analysis to dissect out the time frame of individual kinetic phases, (c) The stopped-flow apparatus is equipped with a freeze-quench device. Sample aliquots are collected after mixing and rapidly froze into X-ray sample holders by the freeze-quench device, (d) Frozen samples are subjected to X-ray data collection and analysis.

TTie key factor that sets enzymatic reactions apan from other catalyzed reactions is the formation of an enzyme-substrate complex, E S. Here substrate binds with a specific aaive site of the enzyme to form this complex. Figure 7-5 shows the schematic of the enzyme chymolrypsin (MW 25,000 Daltons), which catalyzes the hydrolytic clearage of polypeptide bonds. In many cases the enzyme s active catalytic sites are found where the various folds or loops interact. For chymoUy psin the catalytic sites are noted by the amino acid numbers 57, 102, and 195 in Figure 7-5. Much of the catalytic power is attributed... 

Polyaniline was formed when the enzyme-catalyzed reactions were carried in dimethylformamide. tetrahydrofuran. dioxane, toluene and dichloromethane containing 5 to 60% buffer. The enzymatic reaction with dimethylformamide, dioxane and tetrahydrofuran was monophasic however, the reaction with toluene and dichloromethane was biphasic. The general schematic of the reaction is given in Figure 1. 

Fig. 6. Schematic representation of an enzymatic reaction in a reverse micellar system under steady state conditions and excess of empty micelles. E,S, and P represent the enzyme the substrate, and the product, respectively. The circles represent the water pools of the reverse micelles (cross-section) (from Ref. [88])... 

Figure 10.21 Schematic representation of enzymatic reaction using enzyme-immobilized microreactors. The substrate was pumped through the enzyme-immobilized microreactors using a syringe pump. The reaction was carried...

Figure 3 Schematic representation of a one-intermediate enzymatic reaction mechanism including different ionized forms of the enzyme.

Figure 19.2. Schematic overview of conversion pathways ieading to the formation of the main fiavor compounds by iactic acid bacteria in fermented foods. Enzymes extraceiiuiar enzymes or intraceiiuiar enzymes reieased from iysed ceiis broken arrows chemicai (non enzymatic) reactions red, boid fiavor compounds biue, itaiics main enzymes invoived.

When lipases are used for enzymatic conversions, the enzyme is mainly active at a phase boundary, which can effectively be provided by a membrane. Additionally, for conversions requiring two phases (e.g. fat splitting [84—86] and esterifications [87]), the membrane also keeps the two liquid phases (an oil and an aqueous phase, respectively) separated. This is schematically depicted in Fig. 13.11. The equilibrium reactions involved are... 

Figure 9.8 Schematic presentation of an enzymatic esterification reaction in a homogeneous system which may contain a water-miscible solvent to decrease the water activity and in a two phase system containing one aqueous phase and one organic phase. The enzyme is present in the aqueous phase.

The reaction is remarkable for a number of reasons. It is readily reversible and is catalyzed by an enzyme (fumarase) at nearly neutral conditions (pH s 7). Without the enzyme, no hydration occurs under these conditions. Also, the enzymatic hydration is a completely stereospecific antarafacial addition and creates L-malic acid. The enzyme operates on fumaric acid in such a way that the proton adds on one side and the hydroxyl group adds on the other side of the double bond of fumaric acid. This is shown schematically in Figure 10-9. 

Figure 1.1 Schematic of a representative enzymatic assay. The reaction mixture is prepared (Mix Preparation) and the reaction can be started (Initiation) by the addition of the enzyme. During the reaction (Incubation), samples are removed at intervals labeled h, t2, and r3, and the reaction is stopped (Termination) by inactivating the enzyme. The incubation mixture is fractionated (the illustration shows a traditional chromatographic column), and the product is isolated from the substrate (Separation). In this assay, a radiochemical was used as the substrate and therefore the amount of product that formed is determined by its collection, the addition of scintillation fluid, and the measurement of radioactivity by scintillation counting (cpm Detection). The progress of the reaction is given by the amount of radioactive product recovered (Data Reduction).

An EFC consists of two electrodes, anode and cathode, connected by an external load (shown schematically in Figure 5.1). In place of traditional nonselective metal catalysts, such as platinum, biological catalysts (enzymes) are used for fuel oxidation at the anode and oxidant reduction at the cathode. J udicious choice of enzymes allows such reactions to occur under relatively mild conditions (neutral pH, ambient temperature) compared to conventional fuel cells. In addition, the specificity of the enzyme reactions at the anode and cathode can eliminate the need for other components required for conventional fuel cells, such as a case and membrane. Due to the exclusion of such components, enzymatic fuel cells have the capacity to be miniaturized, and consequently micrometer-dimension membraneless EFCs have been developed [7]. In the simplest form, the difference between the formal redox potential (F ) of the active site of the enzymes utilized for the anode and cathode determines the maximum voltage (A ) of the EFC. Ideally enzymes should possess the following qualities. 

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