The classification of natural products can be by chemical structure (e.g. alkaloids), by biological properties (e.g. anti-inflammatory properties) or by the types of enzymic reactions they have undergone (e.g. glycosides). Biosynthesis of natural products by enzymes will vary between primary metabolites (very specific since structural integrity is essential) and secondary metabolites (less specific and a range of substrates can be acted upon to give several products with the same core). Enzymes can operate in ‘novel’ environments—non-optimal reaction conditions. Extremophiles can produce more stable enzymes to give novel applications (e.g. polymerase chain reaction to amplify DNA). Properties of enzymes (e.g. thermal, pH or ionic stability) can be altered or the reaction being catalysed may change if the gene for the enzyme is mutated in a controlled fashion. Can lose enzyme activity due to unforeseen consequences of amino acid changes (e.g. misfolding, altered substrate specificity). Enzymes operate through a transition state so if an antibody is raised to a stable transition state analogue then this antibody may operate as an enzyme (abzyme). A similar effect may be found with molecular imprinting of a transition state analogue using a globular, cross-linked protein.