Types of reactions in metabolism

The breakdown of materials from the environment and their conversion into usable forms take place through a series of catabolic pathways and the process is one of 

Fatty acids. Lipids, Secondary polyketides Mevalonate, Steroids, Carotenoids, Terpenes 

Anabolism and catabolism are complementary to one another, and may be compared with the biologically mediated cycling of carbon (and other materials) in the environment (Fig. 5.2). However, there is an area of metabolism where the two types of metabolism coincide and become indistinguishable. These pathways are known as intermediary metabolism and are the central part of cellular metabolism where the basic requirements for growth are met. At this metabolic crossroads, materials are either rearranged into synthetic precursors or are oxidised to generate energy. 

The metabolism of an organism is in most cases related to growth and reproduction and it is then termed primary metabolism. In many cases, however, metabolism can be uncoupled from growth by limiting some nutrient or by environmental control. This non-growth associated metabolism of an organism is called secondary metabolism. 

The most obvious product of primary metabolism is the biomass itself or the fermentation products of anaerobic organisms. Table 5.13 shows primary metabolites (biochemical intermediates and products) of industrial interest. 

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A quote from a biochemistry text IS instructive here This IS not an easy reaction in or game chemistry It is how ever a very important type of reaction in metabolic chemistry and is an integral step in the oxidation of car bohydrates fats and several ammo acids G L Zubay Biochemistry 4th ed William C Brown Publishers 1996 p 333... 

This is not an easy reaction in organic chemistry. It is, however, a very important type of reaction in metabolic chemis-... 

Many of the simplest chemical reactions involve only an interchange of atoms or ions between reactants, or perhaps only the dissociation of one reactant into two parts. In such reactions, there is no change in the electrical charge of any of the atoms involved. This chapter deals with another type of reaction, in which one or more electrons are transferred between atoms, with the result that some of the atoms involved do have their electrical charges changed. These reactions are known as electron-transfer reactions. You can appreciate their importance when you realize that every battery used in electronic devices and machines, every impulse involved in nerve transmission, every metabolic reaction that produces energy in biological systems, photosynthesis, and combustion processes (to mention but a few examples) requires electron-transfer reactions. 

The coupling of reaction sequences through the adenylate system is entirely different from the two types of stoichiometric relationships discussed above. The stoichiometry of each reaction in which ATP is regenerated or used is, of course, fixed by the nature of the reaction, but the number and types of reactions in which ATP is involved have been determined by evolutionary processes, rather than by simple chemical necessity. Indeed the use of ATP as a coupling agent and the evolutionary adjustments of stoichiometric relationships for maximal metabolic advantage are at the very center of biological function. This fact is underscored by the participation of ATP in every extended metabolic sequence. 

Pyridoxal phosphate mainly serves as coenzyme in the amino acid metabolism and is covalently bound to its enzyme via a Schiff base. In the enzymatic reaction, the amino group of the substrate and the aldehyde group of PLP form a Schiff base, too. The subsequent reactions can take place at the a-, (3-, or y-carbon of the respective substrate. Common types of reactions are decarboxylations (formation of biogenic amines), transaminations (transfer of the amino nitrogen of one amino acid to the keto analog of another amino acid), and eliminations. 

Two important examples of reductive metabolism of xenobiotics are the reductive dehalogenation of organohalogen compounds, and the reduction of nitroaromatic compounds. Examples of each are shown in Figure 2.13. Both types of reaction can take place in hepatic microsomal preparations at low oxygen tensions. Cytochrome P450 can catalyze both types of reduction. If a substrate is bound to P450 in the... 

In vivo, under steady-state conditions, there is a net flux from left to right because there is a continuous supply of A and removal of D. In practice, there are invariably one or more nonequilibrium reactions in a metabolic pathway, where the reactants are present in concentrations that are far from equilibrium. In attempting to reach equilibrium, large losses of free energy occur as heat, making this type of reaction essentially irreversible, eg. 

A few of the reactions carried out by the monooxygenase system of methanotrophic bacteria are summarized in Figure 2.9, and it is on account of this that methylotrophs have received attention for their technological potential (Lidstrom and Stirling 1990). An equally wide metabolic potential has also been demonstrated for cyclohexane monooxygenase, which has been shown to accomplish two broad types of reaction one in which formally nucleophilic oxygen reacts with the substrate, and... 

The ways in which energy in the form of ATP is produced and utilized constitute bioenergetics, and will be discussed in greater detail at the end of this chapter. However, before turning to a selection of metabolic pathways, we outline some fundamental notions concerning redox reactions followed by a brief description of the central role of ATP in metabolism as an acceptor and donor of phosphoryl groups, and finally a summary of the types of reactions that we will encounter as we wend our way along a sample of some of the pathways of intermediary metabolism. 

The true significance of ring opening and ring formation reactions in metabolism is not always recognized and, to the best of our knowledge, these two types of reaction have never been reviewed per se in a systematic manner. Here, we offer a preliminary classification of these reactions in an attempt to clarify this complex field. 

The coenzyme tetrahydrofolate (THF) is the main agent by which Ci fragments are transferred in the metabolism. THF can bind this type of group in various oxidation states and pass it on (see p. 108). In addition, there is activated methyl, in the form of S-adenosyl methionine (SAM). SAM is involved in many methylation reactions—e. g., in creatine synthesis (see p. 336), the conversion of norepinephrine into epinephrine (see p. 352), the inactivation of norepinephrine by methylation of a phenolic OH group (see p. 316), and in the formation of the active form of the cytostatic drug 6-mercaptopurine (see p. 402). 

As their name implies, the A-acetyltransferase (NAT) enzymes catalyze to a drug molecule the conjugation of an acetyl moiety derived from acetyl coenzyme A. Examples of this type of reaction are depicted in Figure 4.1. The net result of this conjugation is an increase in water solubility and increased elimination of the compound. The NATs identified to date and involved in human drug metabolism include NAT-1 and NAT-2. Little overlap in substrate specificities of the two isoforms appears to exist. NAT-2 is a polymorphic enzyme, a... 

The chemical changes that occur during drug metabolism are usually caused by oxidation, reduction, hydrolysis, or conjugation of the original compound.28,52 60 Examples of each type of reaction are listed in Table 3-1. Each type of reaction and the location of the enzymes catalyzing the reaction are also discussed here. 

Other forms of vanadium have been implicated in the stimulation of the plasma membrane vanadate-dependent NAD(P)H oxidation reaction. Decavanadate has been shown to be a more potent stimulator of the vanadate-dependent NADH oxidation activity than added orthovanadate [30,31], Interestingly, decavanadate reductase activity has been found to be an alternative activity of an NADP-specific isocitrate dehydrogenase [32], Diperoxovanadium derivatives have also been shown to be involved in this type of reaction [33,34], Decavanadate may play a role in the biological role of vanadium, as it is found in yeast cells growing in the presence of orthovanadate [8] and is a potent inhibitor of phosphofructokinase-1, the control step of glycolysis, and other metabolic reactions [35],... 

Mammalian metabolism itself involves biotransformations of (generally hydrophobic) drugs in liver cells, kidney, and other organs to more polar, hydrophilic derivatives in order to allow excretion from the body. This process involves two types of reactions, classified as Phase I (functionalization) and Phase II (conjugation). This has been intensively reviewed and is covered by standard textbooks in pharmacology and pharmacy. 

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