Pyramid Science

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Monday, December 29, 2008


Cell metabolism involves many types of chemical reaction that often occur in a well defined sequence or metabolic pathway. The overall effect is to break down large molecules, usually nutrient molecules with the output of energy, then catabolism occurs through catabolic pathways or conversely anabolism: building up larger molecules from smaller ones. These usually require energy to be input. Enzymes are functional proteins that catalyse chemical reactions that do not normally occur or are too slow at body temperature. Catalysts participate in chemical reactions, but are not themselves changed or incorporated into products. Enzymes are usually tertiary of quaternary proteins of complex shape. Tertiary implies a three dimensional shape and quaternary the assembly of more than one protein in a cluster. Often their molecules contain a non-protein cofactor. Inorganic ions or vitamins may make up part of this and if it is an organic non-protein molecule is called a coenzyme.

A very important structural attribute of enzymes is the active site. This is a portion of the enzyme molecule that chemically "fits" the substrate molecule(s). The substrate is the molecule acted upon by the enzyme: the target molecule. The shape and electrochemical attractions of the active site complement some portion of the substrate(s). Often used is the lock and key model to describe enzyme action. The enzyme can lock (bind) or unlock substrates. The suffix -ase used with the root name of the chemical substance involved gives the name to the enzyme. Sucrase will be the enzyme that catalyses sucrose reactions. Sucrase may also be termed a hydrolase because it catalyses the hydrolysis of sucrose. Some enzymes are still known by their earlier trivial names (trypsin and pepsin). Oxidases, hydrogenases and dehydrogenases are all oxidation-reduction enzymes and release energy for muscular contractions. All physiological work relies on these enzymes. Hydrolases are hydrolysing enzymes usually active in digestion such as lipase, sucrase or maltase. Phosphorylating enzymes add or remove phosphate groups and are known as phosphorylases or phosphatases. The removal of carbon dioxide is by carboxylases or decarboxylases. Mutases or isomerases rearrange atoms within a molecule. Hydrases add water.

Enzymes are intracellular (majority) or extracellular. An important class of extracellular enzymes are the digestive enzymes and are all hydrolases. Generally, enzymes regulate cell function by regulating the metabolic pathways. Each reaction in the pathway requires one or more enzymes to permit that reaction to occur. The entire metabolic sequence can be turned on or off by the activation or inactivation of a single enzyme in the metabolic pathway. Most enzymes are highly specific in their action by acting only on a specific substrate. The configurations of the enzyme and the substrate make the active site unique. Various physical and chemical agents can easily disrupt enzyme action by altering the shape of the enzyme molecule. Such an agent is known as an allosteric effector. Enzyme inhibition may occur by distorting the active site or by giving it functional shape. Some may bind at an allosteric site on the enzyme molecule and thereby change the shape of the active site on a different part of the enzyme.

Antibiotic drugs, change in pH or temperature may act as allosteric effectors. Pepsin (protein-digesting enzyme gastric juice) operates within a low pH range (2-4) whereas trypsin (protein-digesting enzyme in pancreatic juice) operates within a higher pH range (6-8). Most enzymes work best in a fairly narrow range (40degC). Cofactors when they are added or removed from an enzyme molecule also have an allosteric effect.

End-product inhibition is a process where a chemical product at the end of a metabolic pathway binds to the allosteric site of one or more of the enzymes along the pathway that produced it thereby inhibiting synthesis of more product. This is a type of automatic negative feedback in the cell which prevents an accumulation of an extreme amount of a metabolic product. Most enzymes catalyse reactions in both directions, the rate and direction being governed by the law of mass action. An accumulation of a product slows the reaction and tends to reverse it. Enzymes are continually being destroyed and so have to be continually synthesised, even though they are not used up in the reactions they catalyse. Most enzymes are synthesized as inactive proenzymes. Kinases are the substances which activate enzymes. Commonly allosteric effects are the mechanism. Enterokinase changes the inactive trypsinogen into active trypsin by changing the shape of the molecule. A type of kinase (kinase A) within cells has been shown to activate enzymes that regulate certain pathways after a hormonal signal is received by the cell.


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