Micro-organisms as catalysts

Enzymes also act as catalysts, but they differ from micro-organisms, as shown in the few simple points given below. 

There are many reactions in which the products formed often act as catalysts for the reaction. The reaction rate accelerates as the reaction continues, and this process is referred to as autocatalysis. The reaction rate is proportional to a product concentration raised to a positive exponent for an autocatalytic reaction. Examples of this type of reaction are the hydrolysis of several esters. This is because the acids formed by the reaction give rise to hydrogen ions that act as catalysts for subsequent reactions. The fermentation reaction that involves the action of a micro-organism on an organic feedstock is a significant autocatalytic reaction. 

Within the chemical industry, micro-organisms and enzymes are often used as catalysts. It is possible for a unit operation in an essentially chemical production process to be a biochemically catalysed step giving rise to a mixed chemical/biochemical production process. The products of these reactions include organic chemicals, solvents, polymers, pharmaceuticals, and purfumes. Mixed chemical/biochemical production processes are continuously innovated and optimised, mainly for economical reasons. 

Enzymes isolated from micro-organisms have many desirable properties as catalysts for the synthesis of industrial chemicals, but there are associated problems ... 

Biocatalysis refers to catalysis by enzymes. The enzyme may be introduced into the reaction in a purified isolated form or as a whole-cell micro-organism. Enzymes are highly complex proteins, typically made up of 100 to 400 amino acid units. The catalytic properties of an enzyme depend on the actual sequence of amino acids, which also determines its three-dimensional structure. In this respect the location of cysteine groups is particularly important since these form stable disulfide linkages, which hold the structure in place. This three-dimensional structure, whilst not directly involved in the catalysis, plays an important role by holding the active site or sites on the enzyme in the correct orientation to act as a catalyst. Some important aspects of enzyme catalysis, relevant to green chemistry, are summarized in Table 4.3. 

P 21 ] Palladium on alumina was employed as catalyst [26]. Hydrogen and organic reactant were mixed in the micro mixer and fed to a Merck Superformance HPLC column of 100 mm length and 5 mm inner diameter, which was used as a hydrogenator. No further details are given in [67] or [26]. 

Enzymes are frequently used as catalysts to promote specific reactions in free solution. They are typically required in small amounts and are attractive in that they obviate both the need to provide the nutritional support which would be required for micro-organisms to perform the same conversion, and the possible subsequent removal of those microbes. Furthermore, the enzyme need not necessarily be of microbial origin so that a wider choice of operating conditions and characteristics may be available. 

A microbial transformation is the conversion of one substance (substrate) to another (product) by a micro-organism. It is a chemical reaction, catalyzed by a particular cellular enzyme or by an enzyme originally produced within cells. Most such enzymes are necessary for the normal functioning of the biological processes of cellular metabolism and reproduction. In microbial transformation, however, these enzymes simply act as catalysts (biocatalysts) for chemical reactions. In addition to their natural substrates, many of these enzymes can utilize other structurally related compounds as substrates and therefore occasionally catalyze unnatural reactions when foreign substrates are added to the reaction medium. Thus, microbial transformation constitutes a specific category of chemical synthesis. 

Whole cell catalysts do not need immobiUzation, especially when mycelial micro-organisms are involved, since their morphological structure allows for easy filtration and re-utihzation. Carboxylesterases bound to the mycelia of molds have been advantageously employed as biocatalysts in water and/or organic solvents the first report of the use of fungal myceha in organic solvent dates back to 1978... 

All the substances that cause these unpleasant and possibly lethal effects are chemicals, albeit manufactured by a plant, micro-organism, or animal. They may be simple irritant chemicals such as the formic acid in ant bites (formica is the Latin for ant), or complex protein molecules such as is found in bee venom. Proteins are relatively large molecules, one of the main building blocks of the body and also the main component of enzymes (biological catalysts). The venom of animals such as snakes often contains enzymes which degrade flesh. Mushrooms and toadstools are another source of poisonous chemicals, for example the Death Cap mushroom found in Britain which can be lethal if eaten. 

Make a list of at least five commonly found features of micro-organisms that would benefit their use as catalysts for organic synthesis. 

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