Reactants, biological concentrations

The actual biological concentrations of the reactants (ligands, iron, complexes, H202) are also important and need to be considered when assessing the redox properties of the formed complex. 

Biological reactions nearly always occur in the presence of enzymes as catalysts. The enzyme catalase, which acts on peroxides, reduces the E for the reaction from 72 kJ/mol (uncatalyzed) to 28 kJ/mol (catalyzed). By what factor does the reaction rate increase at normal body temperature, 37.0°C, for the same reactant (peroxide) concentration Assume that the collision factor. A, remains constant. 

In the genuine low-temperature chemical conversion, which implies the incoherent tunneling regime, the time dependence of the reactant and product concentrations is detected in one way or another. From these kinetic data the rate constant is inferred. An example of such a case is the important in biology tautomerization of free-base porphyrines (H2P) and phtalocyanins (H2PC), involving transfer of two hydrogen atoms between equivalent positions in the square formed by four N atoms inside a planar 16-member heterocycle (fig. 42). 

As an example of two reactions that are coupled, look at the phosphorylation reaction of glucose to yield glucose 6-phosphate plus water, an important step in the breakdown of dietary carbohydrates. The reaction of glucose with HOPO 2- does not occur spontaneously because it is energetically unfavorable, with AG° = + 13.8 kj/mol. (The standard free-energy change for a biological reaction is denoted AG0 and refers to a process in which reactants and products have a concentration of 1.0 M in a soiution with pH = 7.)... 

The number calculated in (b) for the concentration of H+ in blood, 4.0 X 10-8 Af, is very small. You may wonder what difference it makes whether [H+] is 4.0 X 10-8M,4.0 X 10-7Af, or some other such tiny quantity. In practice, it makes a great deal of difference because a large number of biological processes involve H+ as a reactant, so the rates of these processes depend on its concentration. If [H+] increases from 4.0 X 10-8M to 4.0 X 10-7M, the rate of a first-order reaction involving H+ increases by a factor of 10. Indeed, if [H+] in blood increases by a much smaller amount, from 4.0 X 10-8 Af to 5.0 X 10-8 M (pH 7.40----- 7.30),... 

Most biological reactions fall into the categories of first-order or second-order reactions, and we will discuss these in more detail below. In certain situations the rate of reaction is independent of reaction concentration hence the rate equation is simply v = k. Such reactions are said to be zero order. Systems for which the reaction rate can reach a maximum value under saturating reactant conditions become zero ordered at high reactant concentrations. Examples of such systems include enzyme-catalyzed reactions, receptor-ligand induced signal transduction, and cellular activated transport systems. Recall from Chapter 2, for example, that when [S] Ku for an enzyme-catalyzed reaction, the velocity is essentially constant and close to the value of Vmax. Under these substrate concentration conditions the enzyme reaction will appear to be zero order in the substrate. 

Reaction-diffusion systems have been studied for about 100 years, mostly in solutions of reactants, intermediates, and products of chemical reactions [1-3]. Such systems, if initially spatially homogeneous, may develop spatial structures, called Turing structures [4-7]. Chemical waves of various types, which are traveling concentrations profiles, may also exist in such systems [2, 3, 8]. There are biological examples of chemical waves, such as in parts of glycolysis, heart... 

In amperometry, we measure the electric current between a pair of electrodes that are driving an electrolysis reaction. One reactant is the intended analyte and the measured current is proportional to the concentration of analyte. The measurement of dissolved 02 with the Clark electrode in Box 17-1 is based on amperometry. Numerous biosensors also employ amperometry. Biosensors8-11 use biological components such as enzymes, antibodies, or DNA for highly selective response to one analyte. Biosensors can be based on any kind of analytical signal, but electrical and optical signals are most common. A different kind of sensor based on conductivity—the electronic nose —is described in Box 17-2 (page 360). 

Which experimental method should be used depends on the type of reactor and how it will be operated, and if clean or process water is to be used for the measurement. Nonsteady state methods are generally simpler and faster to perform if kLa is to be determined in clean water without reaction. For processes that are operated at steady state with a reaction, determination of kLa using steady state methods are preferred, since continuous-flow processes need not be interrupted and operating conditions similar to the normal process conditions can be used. This is especially important for systems with reactions because the reaction rate is usually dependent on the concentration of the reactants present. They are thus often applied for investigations of the mass transfer coefficient under real process conditions with chemical reactions kLa(02) or biological activity kLa(02), e. g. in waste water treatment systems. 

Techniques used to study fast reactions all monitor concentration spectroscopically, as described in Major Technique 2. For instance, suppose we were studying the effect of a chlorofluorocarbon on the concentration of ozone, a blue gas. We could use a spectrometer to monitor the absorption responsible for the color and interpret the intensity of absorption in terms of the molar concentration of 03 molecules. In the stopped-flow technique, solutions of the reactants are forced into a mixing chamber very rapidly and the formation of products is observed spectroscopically (Fig. 13.3). This procedure is commonly used to study biologically important reactions. 

Thermodynamics only identify whether a particular reaction mixture has a tendency to form products, but do not indicate whether that tendency will ever occur in a biologically appropriate time scale. To have a real idea if that reaction occurs, it would be necessary to know the rate of the chemical reactions that is, making a kinetic analysis. For a given reaction 13, the reaction rate (v) is proportional to the molar concentration of the reactants (A, B) raised to a simple power (a, (1). These values are called partial orders with respect to each of the species participating in the reaction. The rate constant (k) is essentially defined by the thermodynamic characteristics of the species under reaction and... 

It is important to note that no biological compound can ever be said to be universally high in energy. Each reaction has a free energy, or AG°, which is measured at a standard, arbitrarily defined, concentration. AG° does not determine whether the corresponding chemical reaction runs in the forward or reverse direction, however. This is determined as well by the concentrations of the reactants and the products, and the direction in which the state is out of equilibrium. This is captured by AG, which reflects both AG° as well as the concentration of the reactants and products. 

For a single, irreversible step in a chemical reaction, i.e., an elementary chemical process, the rate of the reaction is proportional to the concentrations of the reactants involved in the process. The constant of proportionality is called the rate constant, or the unitary rate constant to highlight the fact that it applies to an elementary process. A subtlety that may be introduced into rate expressions is to use chemical activities (see Chap. 10) and not simply concentrations, but activity coefficients in biological systems are generally taken to be near 1. 

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