Enzyme, catalytic active size

Most enzymes consist of several identical or different subunits. It is known that subunits of similar activity but different origin, which therefore differ in size and amino acid composition and sequence, replace each other in oligomeric enzymes, leading to the formation of enzyme chimeras of catalytic activity 47). The feasibility... 

As we demonstrate in this chapter, enzymes can be extremely active electrocatalysts at ambient temperatures and mild pH, and have significantly higher reaction selectivity than precious metals. The main disadvantage in applying redox enzymes for electrocatalysis arises from their large size, which means that the catalytic active site density is low. Enzymes also have a relatively short hfetime (usually not more than a few months), making them more suited to disposable applications. 

Usually, activities of enzymes (hydrogenases included) are investigated in solutions with water as the solvent. However, enhancement of enzyme activity is sometimes described for non-aqueous or water-limiting surroundings, particular for hydrophobic (or oily) substrates. Ternary phase systems such as water-in-oil microemulsions are useful tools for investigations in this field. Microemulsions are prepared by dispersion of small amounts of water and surfactant in organic solvents. In these systems, small droplets of water (l-50nm in diameter) are surrounded by a monolayer of surfactant molecules (Fig. 9.15). The water pool inside the so-called reverse micelle represents a combination of properties of aqueous and non-aqueous environments. Enzymes entrapped inside reverse micelles depend in their catalytic activity on the size of the micelle, i.e. the water content of the system (at constant surfactant concentrations). 

The enzyme consists of a single polypeptide chain of Mr 13 680 and 124 amino acid residues.187,188 The bond between Ala-20 and Ser-21 may be cleaved by subtilisin. Interestingly, the peptide remains attached to the rest of the protein by noncovalent bonds. The modified protein, called ribonuclease S, and the native protein, now termed ribonuclease A, have identical catalytic activities. Because of its small size, its availability, and its ruggedness, ribonuclease is very amenable to physical and chemical study. It was the first enzyme to be sequenced.187 The crystal structures of both forms of the enzyme were solved at 2.0-A resolution several years ago.189,190 Subsequently, crystal structures of many complexes of the enzyme with substrate and transition analogues and products have been solved at very high resolution.191 Further, because the catalytic activity depends on the ionizations of two histidine residues, the enzyme has been extensively studied by NMR (the imidazole rings of histidines are easily studied by this method—see Chapter 5). 

Protein is formed mainly of polymerised amino-acids. The primary structure, unlike that of synthetic polymers, is non-repetitive and, for its production, requires a chemical template stored in the structure of the DNA molecule. The sizes of proteins vary considerably (in a range of molecular weights from 6000 to 1,000,000). Proteins fulfill many roles within the cell, the most important of which is that of catalysis. Proteins which have catalytic activity are called enzymes whilst other proteins have important roles in storage, transport, protection (antibodies), as chemical messengers (hormones) and in structure 17,, 8). 

The sol-gel procedure enables encapsulation of enzymes in optically transparent, porous silicate matrices, under mild room-temperature conditions. The small pores prevent microbial degradation and, due to the biomolecule size, they will not diffuse out of the polymer network. The physical encapsulation avoids self-aggregation effects as well as protein unfolding and denaturalization. At the same time, the catalytic activity is maintained as the enzymes are able to react with small substrates that can transfer across or within the support, assuring continuous transformations [75]. 

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