Biochemical reactions constants

Clearly, Aid is equal to the heat transferred in a constant pressure process. Often, because biochemical reactions normally occur in liquids or solids rather than in gases, volume changes are small and enthalpy and internal energy are often essentially equal. 

For biochemical reactions in which hydrogen ions (H ) are consumed or produced, the usual definition of the standard state is awkward. Standard state for the ion is 1 M, which corresponds to pH 0. At this pH, nearly all enzymes would be denatured, and biological reactions could not occur. It makes more sense to use free energies and equilibrium constants determined at pH 7. Biochemists have thus adopted a modified standard state, designated with prime ( ) symbols, as in AG°, AH°, and so on. For values determined... 

Generally, in an equation of a chemical reaction rate, the rate constant often does not change with temperature. There are many biochemical reactions that may be influenced by temperature and the rate constant depends on temperature as well. The effect of temperature on... 

Tlie biochemical reaction rate followed the Monod rate model with a Monod rate constant of ks = 6.2 X 10 6g-cm 3 and a specific growth rate of vmlx 6.67 X 10 7g-cm 3-s. Design the bioreactor with a suitable heat transfer area. 

Homeostasis involves maintaining a relatively constant intracellular and intra-organ environment despite wide flucmations in the external environment via appropriate changes in the rates of biochemical reactions in response to physiologic need. 

Although the restrictions of constant T and P are stringent, they apply to many important chemical processes, including reactions that occur in the human body, which has a nearly constant temperature of 37 °C and nearly constant pressure of 1 bar. Any biochemical reaction that occurs in the body occurs under conditions in which the immediate surroundings are at constant T and P. Figure 14-16 shows three examples of systems at constant T and P. 

All enzymatic reactions are initiated by formation of a binary encounter complex between the enzyme and its substrate molecule (or one of its substrate molecules in the case of multiple substrate reactions see Section 2.6 below). Formation of this encounter complex is almost always driven by noncovalent interactions between the enzyme active site and the substrate. Hence the reaction represents a reversible equilibrium that can be described by a pseudo-first-order association rate constant (kon) and a first-order dissociation rate constant (kM) (see Appendix 1 for a refresher on biochemical reaction kinetics) ... 

We seek to describe the time-dependent behavior of a metabolic network that consists of m metabolic reactants (metabolites) interacting via a set of r biochemical reactions or interconversions. Each metabolite S, is characterized by its concentration 5,(f) > 0, usually measured in moles/volume. We distinguish between internal metabolites, whose concentrations are affected by interconversions and may change as a function of time, and external metabolites, whose concentrations are assumed to be constant. The latter are usually omitted from the m-dimensional time-dependent vector of concentrations S(t) and are treated as additional parameters. If multiple compartments are considered, metabolites that occur in more than one compartments are assigned to different subscripts within each compartment. 

Different from conventional chemical kinetics, the rates in biochemical reactions networks are usually saturable hyperbolic functions. For an increasing substrate concentration, the rate increases only up to a maximal rate Vm, determined by the turnover number fccat = k2 and the total amount of enzyme Ej. The turnover number ca( measures the number of catalytic events per seconds per enzyme, which can be more than 1000 substrate molecules per second for a large number of enzymes. The constant Km is a measure of the affinity of the enzyme for the substrate, and corresponds to the concentration of S at which the reaction rate equals half the maximal rate. For S most active sites are not occupied. For S >> Km, there is an excess of substrate, that is, the active sites of the enzymes are saturated with substrate. The ratio kc.AJ Km is a measure for the efficiency of an enzyme. In the extreme case, almost every collision between substrate and enzyme leads to product formation (low Km, high fccat). In this case the enzyme is limited by diffusion only, with an upper limit of cat /Km 108 — 109M. v 1. The ratio kc.MJKm can be used to test the rapid... 

Most individual biochemical reactions are reversible and are therefore quite correctly considered to be chemical equilibria, but cells are not closed systems fuel (e.g. a source of carbon and, in aerobic cells, oxygen) and other resources (e.g. a source of nitrogen and phosphorus) are continually being added and waste products removed, but their relative concentrations within the cell are fairly constant being subject to only moderate fluctuation. Moreover, no biochemical reaction exists in isolation, but each is part of the overall flow of substrate through the pathway as a whole. 

If the reaction conditions are fixed (and standard), the value for AG° must be a constant for any given biochemical reaction. The value for AG° may be seen as a benchmark the further away AG is from AG° the further away the real reaction is from standard conditions. 

Because cryosolvents must be used in studies of biochemical reactions in water, it is important to recall that the dielectric constant of a solution increases with decreasing temperature. Fink and Geeves describe the following steps (1) preliminary tests to identify possible cryosolvent(s) (2) determination of the effect of cosolvent on the catalytic properties (3) determination of the effect of cosolvent on the structural properties (4) determination of the effect of subzero temperature on the catalytic properties (5) determination of the effect of subzero temperature on the structural properties (6) detection of intermediates by initiating catalytic reaction at subzero temperature (7) kinetic, thermodynamic, and spectral characterization of detected intermediates (8) correlation of low-temperature findings with those under normal conditions and (9) structural studies on trapped intermediates. 

Whenever reporting equilibrium constants, detailed information concerning the reaction conditions should always be indicated. Alberty has also presented an important review of biochemical thermodynamics in which he discusses the apparent equilibrium constant for biochemical reactions (K ) in terms of sums of reactant species. 

Processes at constant pressure. Chemical and biochemical reactions are much more likely to be conducted at constant pressure (usually 1 atm) than they are at constant volume. For this reason, chemists tend to use the enthalpy H more often than the internal energy E. 

Since enthalpy changes can be obtained directly from measurement of heat absorption at constant pressure, even small values of AH for chemical and biochemical reactions can be measured using a micro-calorimeter.1112 Using the technique of pulsed acoustic calorimetry, changes during biochemical processes can be followed on a timescale of fractions of a millisecond. An example is the laser-induced dissociation of a carbon monoxide-myoglobin complex.13... 

A chemist suspects that manganese(III) might be involved in an unusual biochemical reaction and wants to prepare some of its compounds. Could aqueous potassium permanganate be used to oxidize manganese(II) to manganese(III) If so, what would be the equilibrium constant for the reaction ... 

P) and volume (V) of the system. AH is the amount of heat absorbed from the surroundings if a reaction occurs at constant pressure and no work is done other than the work of expansion or contraction of the system. (The work done when a system expands by AV against a constant pressure P is P AV. This type of work is generally not very useful in biochemical systems.) In most biochemical reactions, little change occurs in either pressure or volume, so the difference between AH and AE is relatively small. 

Biotransformation is the process by which chemical substances undergo chemical or biochemical reactions in organisms. The rate of transformation usually is expressed in terms of a rate constant or half life. 

This introductory chapter describes the thermodynamics of biochemical reactions in terms of equilibrium constants and apparent equilibrium constants and avoids references to other thermodynamic properties, which are introduced later. 

Biochemical textbooks often add a H+ on the right-hand side, but this is stoichiometrically incorrect when the pH is held constant, as we will see in the next section. It is also wrong, in principle, as we will see in Chapter 4, since hydrogen atoms are not balanced by biochemical reactions because the pH is held constant. The statement that the pH is constant means that in principle acid or alkali is added to the reaction system as the reaction occurs to hold the pH constant. In practice, a buffer is used to hold the pH nearly constant, and the pH is measured at equilibrium. 

The equations and calculations described in this chapter are very useful, but so far we have not discussed thermodynamic properties other than equilibrium constants. The other properties introduced in the next three chapters provide a better understanding of the energetics and equilibria of reactions. We will consider the basic structure of thermodynamics in Chapter 2 and then to apply these ideas to chemical reactions in Chapter 3 and biochemical reactions in Chapter 4. 

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