Reactions biological

There are two main types of biological reaction, aerobic and anaerobic ... 

Biological reactions in vivo rarely operate under conditions even remotely approaching those of reversibility. For a living organism, the rate of a process is usually more important than the attainment of equilibrium, and large driving... 

Section 15 11 Oxidation of alcohols to aldehydes and ketones is a common biological reaction Most require a coenzyme such as the oxidized form of nicotin amide adenine dmucleotide (NAD" )... 

Many biological reactions involve initial binding of a carbonyl compound to an enzyme or coenzyme via mine formation The boxed essay Imines in Biological Chemistry gives some important examples... 

Other processes explored, but not commercialized, include the direct nitric acid oxidation of cyclohexane to adipic acid (140—143), carbonylation of 1,4-butanediol [110-63-4] (144), and oxidation of cyclohexane with ozone [10028-15-5] (145—148) or hydrogen peroxide [7722-84-1] (149—150). Production of adipic acid as a by-product of biological reactions has been explored in recent years (151—156). 

Fig. 2. Steps by which oxygen is transferred from the gas phase to the biological reaction site. Terms are defined in the text.

As in all biological reactions, nitrification is a function of temperature. For municipal wastewaters, a minimum sludge age of 3.5 d is required at 20°C while the minimum sludge age must be increased to 12 d at 10°C in order to achieve nitrification. 

Chira.lHydrogena.tion, Biological reactions are stereoselective, and numerous dmgs must be pure optical isomers. Metal complex catalysts have been found that give very high yields of chiral products, and some have industrial appHcation (17,18). The hydrogenation of the methyl ester of acetamidocinnamic acid has been carried out to give a precusor of L-dopa, ie, 3,4-dihydroxyphenylalanine, a dmg used in the treatment of Parkinson s disease. 

Since there may well be chemical or biological reactions happening on or in the solid phase, depending upon the size of the process participants, macro- or micro-scale effects may or may not be appropriate to consider. 

TABLE 25-39 Electron Acceptors and End Products for Biological Reactions... 

The predominant activity in the study of enzymes has been in relation to biological reactions. This is because specific enzymes have bodi controlled and catalyzed syndietic and degradation reactions in all living cells. Many of diese reactions are homogeneous in the liquid phase (i.e., type 3 reactions). 

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... 

FIGURE 5.12 Enzymes are classified according to the specific biological reaction that they catalyze. Cells contain thousands of different enzymes. Two common examples drawn from carbohydrate metabolism are phos-phofrnctokinase (PFK), or, more precisely,... 

FIGURE 14.3 A 90% yield over 10 steps, for example, in a metabolic pathway, gives an overall yield of 35%. Therefore, yields in biological reactions must he substantially greater otherwise, unwanted by-products would accumulate to unacceptable levels. 

Figure 5.8 An energy diagram for a typical, enzyme-catalyzed biological reaction (blue curve) versus an uncatalyzed laboratory reaction (red curve). The biological reaction involves many steps, each of which has a relatively small activation energy and small energy change. The end result is the same, however.

A Comparison between Biological Reactions and Laboratory Reactions... 

Yet another difference is that laboratory reactions are often done using relatively small, simple reagents such as Br2, HC1, NaBH4, OO3, and so forth, while biological reactions usually involve relatively complex "reagents" called coenzymes. In the hexokinase-catalyzed phosphorylation of glucose just shown,... 

One final difference between laboratory and biological reactions is in their specificity. A catalyst might be used in the laboratory to catalyze the reaction of thousands of different substances, but an enzyme, because it can bind only a specific substrate molecule having a specific shape, will catalyze only a specific reaction, it s this exquisite specificity that makes biological chemistry so remarkable and that makes life possible. Table 5.4 summarizes some of the differences between laboratory and biological reactions. 

Table 5.4 A Comparison of Typical Laboratory and Biological Reactions Laboratory reaction Biological reaction...

Predict the produces of each of the following biological reactions by interpreting the flow of electrons as indicated by the curved arrows ... 

In biological reactions, the situation is different from that in the laboratory. Only one substrate molecule at a time is present in the active site of the enzyme where reaction takes place, and that molecule is held in a precise position, with coenzymes and other necessary reacting groups nearby. As a result, biological radical reactions are both more controlled and more common than laboratory or industrial radical reactions. A particularly impressive example occurs in the biosynthesis of prostaglandins from arachiclonic acid, where a sequence of four radical additions take place. The reaction mechanism was discussed briefly in Section 5.3. 

A large number of biological reactions involve prochiral compounds. One of the steps in the citric acid cycle by which food is metabolized, for instance, is... 

In acidic solution at low pH, a carboxylic acid is completely undissociated and exists entirely as RCO2H- In basic solution at high pH, a carboxylic acid is completely dissociated and exists entirely as RC02 - Inside living cells, however, the pH is neither acidic nor basic but is instead buffered to nearly neutral pH—in humans, to pH = 7.3, a value often referred to as physiological pH. In what form, then, do carboxylic acids exist inside cells The question is an important one for understanding the acid catalysts so often found in biological reactions. 

An enzyme—usually a large protein—is a substance that acts as a catalyst for a biological reaction. Like all catalysts, an enzyme doesn t affect the equilibrium constant of a reaction and can t bring about a chemical change that is otherwise unfavorable. An enzyme acts only to lower the activation energy for a reaction,... 

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