Cells enzyme organisation

Many enzymes in the cell are organised into sequences, so that the reactions they catalyse are integrated into pathways or processes. In these pathways, a precursor or substrate is converted to a product, e.g. glucose is converted to lactic acid amino acids are polymerised to form protein glutamine is converted to aspartate. These pathways have both a thermodynamic and a kinetic structure. The thermodynamic structure is presented in Chapter 2. The kinetic structure is described here. There are three basic facts that must be appreciated before the kinetic structure is explained. 

Cytokines are peptides that are produced and secreted by cells of the immune system. They organise the immune response to invasion by a pathogen by communicating between the different cells. They are synthesised in the immune cells as precursor proteins (pro-proteins) from which a peptide is removed by a proteolytic enzyme to produce the active cytokine, prior to secretion. This enzyme is a serine protease. Perhaps surprisingly, some viruses are capable of synthesising serpins which inhibit this enzyme in the immune cells, so that secretion does not occur and communication and integration of the immune response to the viral infection is lost. This is one of many biochemical mechanisms by which pathogens can reduce or overcome the defence mechanisms of the host (Chapter 17). 

Figure 17.29 Organisation of granules within the cytotoxic T-cell prior to release. The organisation ensures that the released enzymes plus perforin are very close to the infected cell so that killing is restricted to the infected cell. This minimises damage to neighbouring non-intected cells. Death is due to either lysis or apoptosis.

Very many technical and commercial factors are important such as the utility of the product (cost-benefit relationships), ease of scale-up, the productivity of the process etc. Process design, for instance, involves a series of choices, such as the use of isolated enzyme or intact microorganism, use of free or immobilised cell or enzyme, use of mutant or genetically modified cell, or batch or continuous processing etc. Such choices depend on other factors such as the availability and cost of precursors, product purity required, intended scale of operation and existing skills and equipment available within the Organisation. 

Enzymes are organised into metabolic pathways which collectively constitute metabolism. Two types of metabolism are found in cells, catabolism (breakdown pathways) and anabolism (synthetic pathways). Linking these two types of metabolic reactions are the intermediary reactions of central metabolism. Cells, which contain many complex polymers, thus have the means to generate and convert monomeric materials into the complex biological structure. The sources of these materials are the simpler components from the cell s environment, such as inorganic salts and glucose (Fig. 5.9). 

The term enzymes is coined to describe catalytic activity not bound to living cells (unorganised ferments). The name is extended later also to intracellular catalysts (organised ferments as defined by Pasteur). Kiihne... 

A caveat must be made at this point. This model is hypothetical, but it is the simplest that is consistent with the one linkage-one enzyme hypothesis and with the chemical evidence for the structures of xylans. Any further structural elaboration in the xylans must be primarily in their patterns of branching and that implies yet further intricacy in the biosynthesis of these branches. Interpretations other than those assumed here can be placed upon the data obtained by the partial degradation of xylans, but they do not alter one basic conclusion the side-chains of xylans are usually more complex in terms of biosynthesis than are the main core sequences. This suggests that the xylans may be examples of molecules in which encoded information of some sort is held in a three-dimensional array upon a non-informational substructure. In this case the helical core will support a somewhat brush-like array of side chains. Any information contained in the specific side chains probably relates to the construction and organisation of the cell wall, but in what way is unknown. 

The first satisfactory experimental proof that alcoholic fermentation is caused by a non-organised ferment (enzyme) in yeast was given by Buchner, who adopted for it Bechamp s name zymase. Buchner found that if the yeast cells are crushed by trituration with sand and kieselguhr and the paste is pressed in a cloth bag, a liquid free from yeast cells is obtained which produces alcoholic fermentation. The activity is not destroyed by evaporation to dryness at 30 -35° or precipitation by alcohol. Arthur Harden and others showed that a co-enzyme, separable from yeast-juice by dialysis, and an inorganic phosphate, are also involved in alcoholic fermentation, the process being apparently very complicated. 

Nature created multienzymatic systems to accomplish extremely efficient one-pot tandem catalysis. As in an assembly line, tens of enzymes are well organised to transform simple materials to complex molecules with perfect control of selectivity hy a series of coupled reactions in the cell. It has long been chemists endeavor to extend such coordinated catalytic action to artificial processes to make synthetic chemistry more sustainable. Nowadays, owing to the resource-intensive nature of the current synthetic industry, the development of tandem one-pot reactions, avoiding the use of costly and time-consuming protection-deprotection processes as well as purification procedures of intermediates, has become especially important and valuable because society is confronted with bottle-neck problems such as energy and time shortage and environmental pollution. 

The development of cell free systems has played an important role in attempts to decipher the mechanism of peptide antibiotic synthesis ". Most workers agree that this type of synthesis occurs in the complete absence of polynucleotides and that the amino acid sequence is determined by enzyme specificity and organisation in a multi-enzyme complex . The tyrocidines are produced in Bacillus brevis by an enzyme system which has been resolved into three complimentary fractions by Sephadex G-200 filtration a light and an intermediate component (molecular weight 100000 and 230000) which activate L-phenylalanine and L-proline respectively and a heavy fraction (460 000) which activates the remaining eight amino acids including L-phenylalanine. Each... 

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