Structure and Catalytic Action

A prerequisite for the catalytic function of an enzyme is its native tertiary structure which is determined by the number and sequence of amino acids (primary structure) forming the molecule. Favoured by hydrogen bonds, parts of the polypeptide chain exist in an a-helical or a (3-sheet structure (secondary structure). Most enzymes are globular proteins, the tertiary structure of which may be fixed by disulfide bonds between cysteine residues. A famous example is lysozyme (Fig. 20), consisting of 129 amino acids. A defined three-dimensional structure is 

Within the mostly spherical, ellipsoidal, or kidney-shaped protein molecules a local cavity with a characteristic constitution and stereoconfiguration forms the catalytically active center (Fig. 21), where a chemically and spatially congruent substrate Clock-and-key principle ) is converted to a product. To a limited extent the protein structure is capable of adapting conformationally to the substrate. 

Enzymes accelerate the equilibrium formation of chemical reactions by a factor of 108-102° as compared with uncatalyzed reactions. Thus, urea is hydrolyzed at pH 8 and 20°C in the presence of urease about 1014 times faster than without catalysis, and the splitting of H2O2 is accel- 

This drastic decrease of activation energy, being equivalent to a large increase of the number of reactive substrate molecules in the activated intermediate state, mostly results from molecular tensions induced in both components by the enzyme-substrate complex formation. 

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