In living systems

Most chemically reacting systems tliat we encounter are not tliennodynamically controlled since reactions are often carried out under non-equilibrium conditions where flows of matter or energy prevent tire system from relaxing to equilibrium. Almost all biochemical reactions in living systems are of tliis type as are industrial processes carried out in open chemical reactors. In addition, tire transient dynamics of closed systems may occur on long time scales and resemble tire sustained behaviour of systems in non-equilibrium conditions. A reacting system may behave in unusual ways tliere may be more tlian one stable steady state, tire system may oscillate, sometimes witli a complicated pattern of oscillations, or even show chaotic variations of chemical concentrations. 

The reactions that amino acids undergo in living systems include transamination and decarboxylation... 

Dou ble hel ix (Section 28 8) The form in which DNA normally occurs in living systems Two complementary strands of DNA are associated with each other by hydrogen bonds be tween their base pairs and each DNA strand adopts a helical shape... 

Biologists are quite sure that reagents are not homogeneously mixed in living systems. Chemical physicists striving to understand the ultrafast, primary processes of chemical change know that uniform concentration is too cmde an approximation for their purposes. Nonetheless, the assumption of a weU-stirred mixture is so pervasive that kineticists rarely point it out in their reports. 

Histamine is the biological amine, playing an important role in living systems, but it can also cause unnatural or toxic effects when it is consumed in lai ge amounts. It can occur with some diseases and with the intake of histamine-contaminated food, such as spoiled fish or fish products, and can lead to undesirable effects as headache, nausea, hypo- or hypertension, cai diac palpitations, and anaphylactic shock syndrome. So, there is a need to determine histamine in biological fluids and food. 

The reverse reaction also occurs in living systems NADFl reduces acetaldehyde to ethanol in the presence of alcohol dehydrogenase. In this process, NADH serves as a hydride donor and is oxidized to NAD" while acetaldehyde is reduced. 

We ll see numerous examples of both reaction types in the following sections. Keep in mind that in vivo reactions (reactions in living systems) are enzyme-catalyzed and occur at far greater rates than those for the sane transfonnations caiiied out in vitro ( in glass ) in the absence of enzymes. In spite of the rapidity with which enzyme-catalyzed reactions take place, the nature of these transfonnations is essentially the sane as the fundfflnental processes of organic chemistry described throughout this text. 

The most striking thing about proteins is the diversity of their roles in living systems silk is a protein, skin and hair- are mostly proteins, many hormones are proteins, a protein cariies oxygen from the lungs to the tissues where it is stored by another protein, and all enzymes are proteins. 

The balance of this chapter will be devoted to several classic and representative enzyme mechanisms. These particular cases are well understood, because the three-dimensional structures of the enzymes and the bound substrates are known at atomic resolution, and because great efforts have been devoted to kinetic and mechanistic studies. They are important because they represent reaction types that appear again and again in living systems, and because they demonstrate many of the catalytic principles cited above. Enzymes are the catalytic machines that sustain life, and what follows is an intimate look at the inner workings of the machinery. 

Iron is the most important transition element involved in living systems, being vital to both... 

Whatever the detailed biogenetic steps, there is almost general agreement that j3-earboline derivatives originate in living systems from tryptophan or tryptamine. 

Whereas j3-carboline derivatives abound in nature, other carbolines are virtually unknown in living systems. The only exception is a benz-8-carboline derivative, cryptolepine (78). Two alternative biogenetic... 

Aromatic alkylations occur in numerous biological pathways, although there is of course no MCI3 present in living systems to catalyze the reaction. Instead, the carbocation electrophile is usually formed by dissociation of an organodiphosphate, as we saw in Section 11.6. The dissociation is typically assisted by complexation to a divalent metal cation such as Mg2+ to help neutralize charge. 

Enzymes are proteins that catalyze chemical reactions in living systems. Such catalysts are not only efficient but are also extremely selective. Hence, enzymes combine the recognition and amplification steps, as needed for many sensing applications. 

In this chapter we will discuss the various forms of myosin and the roles they play in living systems. We will compare and contrast the function and regulation of myosin activity in different cellular environments. Finally, we will examine the clinical aspects of myosin strucmre and function. 

Study of the action of AR on . coli precA luxCDABE-Anvp has confirmed it photoprotective effects and has shown features of such activity in live systems. Surprising was the interrelation between preservation of viability of AR-processed bacterial cells in the conditions of a long and intensive UV-irradiation and depression of activity their reparing SOS-systems. It has assumed AR action and the SOS-answer as alternative "passive" and "active" mechanisms for protection of bacterial cells DNA at various intensivity of UV-irradiation. 

Siik is just one exampie of macromoiecuies, aiso known as poiymers. Macromoiecuies are the subject of this chapter. The principies introduced in Chapters 9-11 help to explain the properties of these molecules, many of which are carbon-based. In this chapter, we outline the principles of the stmcture and synthesis of the major classes of macromoiecuies and describe the properties that give these chemical substances central roles in industrial chemistry and biochemistry. We describe the components from which macromoiecuies are constmcted, some important industrial polymers, and the macromoiecuies found in living systems. 

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