Examples, biochemical reaction Chapter

Natural organophosphate esters are frequently dissymmetric, although only one particular isomer is usually involved in biochemical reactions (Chapter 11.1). Examples of relatively simple inorganic dissymmetric molecules are P4S3I2 and NajPn (Figure 13.7). 

In the meantime, the intense study of the simpler vesicle systems has unravelled novel, unsuspected physicochemical aspects - for example growth, fusion and fission, the matrix effect, self-reproduction, the effect of osmotic pressure, competition, encapsulation of enzymes, and complex biochemical reactions, as will be seen in the next chapter. Of course the fact that vesicles are viewed under the perspective of biological cell models renders these findings of great interest. In particular, one tends immediately to ask the question, whether and to what extent they might be relevant for the origin of life and the development of the early cells. In fact, the basic studies outlined in this chapter can be seen as the prelude to the use of vesicles as cell models, an aspect that we will considered in more detail in the next chapter. 

Bioreactors are the apparatus in which practical biochemical reactions are performed, often with the use of enzymes and/or living cells. Bioreactors that use living cells are usually called fermentors, and specific aspects of these are discussed in Chapter 12. Ihe apparatus applied to waste water treatment using biochemical reactions is another example of a bioreactor. Even blood oxygenators, that is, artificial lungs as discussed in Chapter 15, can also be regarded as bioreactors. 

Previous chapters in this volume have been concerned with chemical reaction engineering and refer to reactions typical of those commonplace in the chemical process industries. There is another class of reactions, often not thought of as being widely employed in industrial processes, but which are finding increasing application, particularly in the production of fine chemicals. These are biochemical reactions, which are characterised by their use of enzymes or whole cells (mainly micro-organisms) to carry out specific conversions. The exploitation of such reactions by man is by no means a recent development—the fermentation of fruit juices to make alcohol and its subsequent oxidation to vinegar are both examples of biochemical reactions which have been used since antiquity. 

Proteins that are reactants in biochemical reactions are also be included in BasicBiochemData2 examples included are cytochrome c, ferrodoxin, and thioredoxin. Later in Chapter 7 it is shown that the effect of pH on a biochemical reaction involving a protein can be calculated if the pKs of groups in the reactive site of the protein can be determined. 

Most chemical reactions give off heat and are classified as exothermic reactions. The rate of a reaction may be calculated by the Arrhenius equation, which contains absolute temperature, K, equal to the Celsius temperature plus 273, in an exponential term. As a general rule, the speed of a reaction doubles for each 10°C increase in temperature. Reaction rates are important in fires or explosions involving hazardous chemicals. A remarkable aspect of biochemical reactions is that they occur rapidly at very mild conditions, typically at body temperature in humans (see Chapter 3). For example, industrial fixation of atmospheric elemental nitrogen to produce chemically bound nitrogen in ammonia requires very high temperatures and pressures, whereas Rhizobium bacteria accomplish the same thing under ambient conditions. 

While the majority of these concepts are introduced and illustrated based on single-substrate single-product Michaelis-Menten-like reaction mechanisms, the final section details examples of mechanisms for multi-substrate multi-product reactions. Such mechanisms are the backbone for the simulation and analysis of biochemical systems, from small-scale systems of Chapter 5 to the large-scale simulations considered in Chapter 6. Hence we are about to embark on an entire chapter devoted to the theory of enzyme kinetics. Yet before delving into the subject, it is worthwhile to point out that the entire theory of enzymes is based on the simplification that proteins acting as enzymes may be effectively represented as existing in a finite number of discrete states (substrate-bound states and/or distinct conformational states). These states are assumed to inter-convert based on the law of mass action. The set of states for an enzyme and associated biochemical reaction is known as an enzyme mechanism. In this chapter we will explore how the kinetics of a given enzyme mechanism depend on the concentrations of reactants and enzyme states and the values of the mass action rate constants associated with the mechanism. 

One of the most important functions of proteins is their role as catalysts. (Until recently, all enzymes were considered to be proteins. Several examples of catalytic RNA molecules have now been verified. See Chapter 18.) Recall that living processes consist almost entirely of biochemical reactions. Without catalysts these reactions would not occur fast enough to sustain life. 

Fig. 33.28. Synthesis of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine. The multiple pathways reflect the importance of phospholipids in membrane structure. For example, phosphatidylcholine (PC) can be synthesized from dietary choline when it is available. If choline is not available, PC can be made from dietary carbohydrate, although the amount synthesized is inadequate to prevent choline deficiency. SAM is S-adenosyhnethionine, a methyl group donor for many biochemical reactions (see Chapter 40).

In this chapter, we have firstly discussed the general issue of reactions in liposomes from a technical viewpoint. Starting from this basic knowledge, that is general and can be applied to whatever chemical system, we have then discussed some relevant examples of complex biochemical reactions in liposomes and fatty add vesicles. This choice reflects the authors interest, more than representing the large spectrum of possible applications, or the more general field of water-soluble enzymes entrapped inside liposomes (recently reviewed by Walde and Ichikawa ). The use of liposomes as a tool to study membrane-proteins has also been skipped. ... 

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