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Explain the structure of enzymes and discuss their metabolic reactions. Explain the structure of enzymes and discuss their metabolic reactions.

Enzyme, a protein that catalyses a specific reaction. ‘Living cells contain thousands of different enzymes’ (Elliott, p.214), each of which catalyses (that is, accelerates without itself being changed) just one kind of reaction. In some of these reactions, small organic molecules such as amino acids, sugars, nucleotides, and lipids are broken down to provide energy for the cell. In other reactions, small molecules are built into complex macromolecules, such as proteins, DNA, RNA, and polysaccharides, or used to carry signals, or to control cell movements or gene expression. Enzyme-catalysed reactions are usually connected in series, so that the product of one reaction becomes the starting material, or substrate, for the next. These long linear reaction pathways are in turn linked to one another, forming a maze of interconnected reactions that enables the cell to survive, grow, and reproduce. Enzymes dictate the pattern of chemical changes in a cell and without them life as we know it would be impossible. Enzymes are biological catalysts. The lock and key theory was suggested in 1894 by Emil Fischer and properly described as follows "The specificity of an enzyme (the lock) for its substrate (the key) arises from their geometrically complementary shapes".

The lock and key theory is simply a way of describing how specific an enzyme is for its substrate. Just like a lock requires a specifically shaped key for it to work so does an enzyme. Each enzyme is a protein which is a polypeptide chain folded into a complex 3 dimensional structure. Part of that structure contains the active site which is where the enzyme can bind to the substrate on which it will perform some chemical reaction. Because each enzyme performs a specific task on a specific substrate the active centre of the enzyme can be considered to be the "lock" which requires the specific "key" or substrate to perform the function. Smaller keys, larger keys, or incorrectly positioned teeth on keys (incorrectly shaped or sized substrate molecules) do not fit into the lock (enzyme). Only the correctly shaped key (substrate) opens a particular lock. If we imagine the enzyme as the lock and the substrate the key - the key is inserted in the lock and if this is his right enzyme for the substrate the lock is turned, and the door is opened and the reaction proceeds.

The active site is the specific region of the enzyme which combines with the substrate. The products are released from the enzyme surface to regenerate the enzyme for another reaction cycle. ‘The active site has a unique geometric shape that is complementary to the geometric shape of a substrate molecule.’(Kent, p.198) This means that enzymes specifically react with only one or a very few similar compounds. Products are released

Products are released
Reactants approach enzyme
Reactants approach enzyme
Reactants bind to enzyme
Reactants bind to enzyme
Enzyme changes shape
Enzyme changes shape

The Induced fit theory - Enzymes act as biological catalysts. They are ‘globular proteins that have a specific shape within which there is a functional portion’(Berg, p.31) known as the active site. Enzymes lower the activation energy of a reaction, allowing it to proceed at a lower temperature than it would normally. In an enzyme controlled reaction, the general term for the substance on which the enzyme acts is substrate and the substances formed at the end of their action are known as the products. The enzyme molecule and the substance it acts on fit together very precisely, giving rise to the name lock and key theory of enzyme action. In practice, the enzyme is thought to change shape slightly and so mould itself to the shape of the substance it acts on. This is called the induced fit theory of enzyme action. See above diagram.

Enzymes are large proteins that...
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