TABLE OF CONTENTS Biocatalysis refers to catalysis by enzymes. The enzyme may be introduced into the reaction in a purified isolated form or as a whole-cell micro-organism. Enzymes are highly complex proteins, typically made up of 100 to 400 amino acid units. The catalytic properties of an enzyme depend on the actual sequence of amino acids, which also determines its three-dimensional structure. In this respect the location of cysteine groups is particularly important since these form stable disulfide linkages, which hold the structure in place. This three-dimensional structure, whilst not directly involved in the catalysis, plays an important role by holding the active site or sites on the enzyme in the correct orientation to act as a catalyst. Some important aspects of enzyme catalysis, relevant to green chemistry, are summarized in Table 4.3.
| Property | Green chemistry relevance |
|---|---|
| Fast reactions due to correct catalyst orientation | Faster throughput |
| Orientation of site gives high stereospecificity | Possibility for asymmetric synthesis |
| High degree of substrate specificity due to limited flexibility of active site | High degree of selectivity |
| Water soluble | Opportunity for aqueous-phase reactions |
| Naturally occurring | Non-toxic, low hazard catalysts |
| Natural operation under conditions found in body | Energy-efficient reactions under moderate conditions of pH, temperature, etc. |
| Possibility for tandem reactions when using whole organisms | Possibility for carrying out sequential one-pot syntheses |
Since enzymes are composed of amino acids they may be assumed to act as either acid or base catalysts through groups such as -COOH, -NH
