The types of chemical reaction considered here areoxidation, reduction, phosphorylation, hydrolysis, condensation, isomerization,deamination and carboxylation.
Oxidation Oxidation involves an increase in the oxidation number ofa species. This involves the addition of oxygen or the loss of hydrogen orelectrons. Ultimately the first two can always be viewed as equivalent toloss of electrons. Oxidation always occurs together with reduction as part ofa redox reaction. The substance producing the oxidation is termed the oxidant(electron acceptor) and is concomitantly reduced. Likewise the oxidisedspecies (electron donor) can be termed the reductant. (Atkins, 1990) There are many examples of oxidation reactions in thecatabolism of glucose. For example in the first stage of the glycolyticpathway leading from glucose to pyruvate, after the six carbon intermediatesare cleaved to generate glyceraldehyde 3-phosphate the enzyme glyceraldehydephosphate dehydrogenase catalyses the conversion of glyceraldehyde3-phosphate to 3-phosphoglycerol phosphate (utilizing the cofactor NAD andinorganic phosphate) This oxidation is the first time that reducing potentialin the form of NADH is generated in the breakdown of glucose (note the NAD iscorrespondingly reduced in the process).
Reduction Reduction is a decrease in the oxidation number of asubstance resulting from the gain of electrons as part of a redox reaction.This is often but not necessarily associated with the loss of oxygen orhydrogen. (See oxidation above). An example of a reduction reaction occurs in the finalstage of glycolysis under anaerobic conditions where pyruvate is reduced tolactate catalysed by the enzyme lactate dehydrogenase. The reducing potentialfor this reaction is provided by NADH and H+ and prevents thecells finite supply of NAD being tied up in the reduced form from reactionssuch as the oxidation example above. Again the reductant NADH is oxidised inthe process.
Phosphorylation Phosphorylation involves the addition of a phosphate (PO42-)or phospho (PO32-) from a donor to receptor moleculeusually by a nucleophilic displacement of the phosphorus atom by a lone pairon an electronegative heteroatom (e.g. O or N). (Cox, 2004) In the first reaction of the catabolism of glucose, theglucose molecule is phosphorylated by the high-energy phosphate compoundadenosine triphosphate (ATP) catalysed by the enzyme hexokinase. The hydroxylgroup on carbon atom 6 of the glucose nucleophilically attacks the terminalphosphate of ATP displacing a PO32- group, which isadded to the glucose releasing ADP. In addition to priming the molecule withenergy it keeps the glucose in the cytoplasm, as the glucose transporters arespecific for free glucose. (Cox, 2004)
Hydrolysis Hydrolysis is the reaction of a chemical species (moleculeor ion) with water. In biological systems this usually involves addition ofthe elements of water across a chemical bond to break the bond, resulting inan OH group attached to one atom of the hydrolysed bond and an H atom addedto the other atom. This can split the molecule into two separate molecules(see example) or can break a cyclical compound into a linear structure. The neurochemical transmitter acetylcholine is responsiblefor conduction of the motor neurone impulse across the synaptic gap. Toprevent continuation of the signal and tetanic paralysis of the muscle theacetylcholine is hydrolysed to acetate and choline by the enzymeacetylcholine esterase. This is an uncomplicated ester hydrolysis in whichwater nucleophilically attacks the carbonyl group of the acetate component ofthe ester. (Vander, 2001)
Condensation A condensation reaction occurs when two or more reactingspecies react to form a single product and eliminate a simple molecule in theprocess. Where the simple molecule is water the condensation reaction can bethought of as the opposite of the hydrolysis reaction. Likewise the tworeacting species can be separated but on the same molecule resulting in acyclization reaction. (McNaught, 1997) Peptide synthesis occurring on the ribosome and catalysedby its peptide synthetase activity is an example of a condensation reaction.Condensation takes place between the amino group of the added amino acid andthe carboxyl group of the growing peptide chain (activated by anaminoacyl-tRNA linkage) eliminating the elements of water. (Alberts, 1989).
Isomerization In an isomerization reaction the product of the reactionis an isomer of the reactant. In such a reaction there is no net change inthe stoichiometry of the molecular formula between reactants and products(though intermediate steps may involve extra atoms). The isomerization can bethe result of molecular or conformational rearrangements (McNaught, 1997) The second reaction of the glycolytic breakdown of glucoseis an example of an isomerization reaction. Glucose-6-phosphate is convertedto fructose-6-phosphate by the enzyme glucose phosphate isomerase. Thisproduces a more symmetrical molecule with a second available primary alcoholgroup for phosphorylation.
Deamination Deamination is the removal of an amine (NH2)group from a molecule. The nitrogen is usually removed as ammonia, the extrahydrogen coming from water leaving a ketone group in place of the amine. Thisreaction also increases the resulting oxidation number of the reactingspecies and is often termed oxidative deamination. (Cox, 2004) Oxidative deamination is an important reaction in thedegradation of amino acids especially in the liver. Glutamate, produced fromother amino acids by transamination, is converted into a-ketoglutarate and ammonia by the enzymeglutamate dehydrogenase in association with the cofactors NAD or NADP. Theammonia is ultimately excreted via the urea cycle. (Cox, 2004)
Carboxylation Carboxylation is the addition of a carboxylate group to amolecule. This is an important method for increasing the number of carbonatoms in a synthesis. The source of the carbon is typically carbon dioxidefor example in the reaction of a carbonated grignard reagent (or thebicarbonate ion in aqueous biological systems). The fixation of carbon dioxide by green plants is animportant example of a carboxylation reaction. In plants that use theCalvin cycle, CO2 is incorporated into 3-phosphoglycerate by theenzyme ribulose-diphosphate carboxylase. This carboxylates the five-carbonribulose sugar to produce a six-carbon intermediate, which is then hydrolysedto produce two 3 carbon molecules of 3-phosphoglycerate.