A.7 Enzymes
Enzymes are large protein molecules (apoenzymes), which act as catalysts for
almost all the chemical reactions that occur in living organisms. The structures
of a number of enzymes contains groups of metal ions, known as metalclusters,
coordinated to the peptide chain. These enzymes are often referred to as
metalloenzymes. Many enzymes require the presence of organic compounds
(co-enzymes) and/or metal ions and inorganic compounds (co-factors) in order
to function. These composite active enzyme systems are known asholoenzymes.
apoenzymeþco-enzyme and=or co-factor¼holoenzyme
(inactive)(inactive)(active)
Enzymes are found embedded in cell walls and membranes as well as occuring
in the various fluids found in living organisms. A number of enzymes are
produced in specific areas of the body by the metabolism of inactive protein
precursors known asproenzymesorzymogens.
proenzymeÐactive form of the enzyme
This allows the body to produce the active form of the enzyme only in the
region of the body where it is required.
Enzymes are broadly classified into six major types (Table A7.1). Those that
catalyse the same reactions but have significantly different structures are known
asisofunctionalenzymes.
The International Union of Biochemistry has recommended that enzymes
have three names, namely a systematic name, which shows the reaction being
catalysed and the type of reaction based on the classification in Table A7.1, a
recommended trivial name and a four figure Enzyme Commission code (EC
code). Nearly all systematic and trivial enzyme names have the suffix -ase.
Systematic names show, often in semi-chemical equation form, the conversion
the enzyme promotes and the class of the enzyme. Trivial names are usually
based on the function of the enzyme but may also include or be based on the
name of the substrate. However, some trivial names in current use are historical
and bear no relationship to the action of the enzyme or its substrate, for example,
pepsin and trypsin are the names commonly used for two enzymes that catalyse
the breakdown of proteins during digestion. The Enzyme Commission’s code is
unique for each enzyme. It is based on the classification in Table A7.1 but further
subdivides each class of enzyme according to how it functions. The full code is
252 APPENDIX 7 ENZYMES