Protein-based Artificial Enzymes

Copyright 2005 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-31165-3 

---

There is another approach that is increasingly part of synthesis the use of enzymes as catalysts. This approach is strengthened by the new ability of chemists and molecular biologists to modify enzymes and change their properties. There is also interest in the use of artificial enzymes for this purpose, either those that are enzyme-like but are not proteins, or those that are proteins but based on antibodies. Catalytic antibodies and nonprotein enzyme mimics have shown some of the attractive features of enzymes in processes for which natural enzymes are not suitable. 

Electrocatalytic groups such as porphyrins and phthalocyanines that act as supramolecular hosts for different metals and mimic the active sites of various proteins are commonly used in amperometric sensors [66,67]. A biomimetic sensor based on an artificial enzyme or synzyme has been demonstrated [68]. The artificial enzyme used in this study was a synthetic polymer (quaternised polyethyleneimine containing 10% primary amines) which decarboxylated oxaloacetate. The product carbon dioxide was detected potentiometrically via a gas membrane electrode. 

A much more improved antibody catalyst for amide hydrolysis has been elicited very recently by a joint hybridoma and combinatorial antibody library approach. The measured with a primary amide substrate at pH 9 and 25 °C was 5x10" s" for this new antibody. This corresponds to a half-life of 4 h when the substrate is fully complexed to the active site. The half-lives for the amide hydrolysis catalyzed by the antibodies are much longer than that (10-30 min at pH 4.5-7 and 4 °C when the substrate is fully complexed to the active site) of the light chain of Gbn hydrolyzed by 39. The fastest protein cleavage recorded so far with artiHcial proteinases is the cleavage of chymotrypsin by a coordinatively polymerized bilayer membrane which will be discussed later in this review. The coordinatively polymerized bilayer membrane achieved half-life as short as 3 min at 4 °C and pH 5.5-9.5. This is several times faster than the hydrolysis of Gbn by 39. In terms of utility in practical applications, however, 39 is more useful than the artificial enzyme based on the bilayer membrane due to the immobile nature of the former as well as the intrinsic instability of bilayer membranes. 

The electron-transfer rate between large redox protein and electrode surface is usually prohibitively slow, which is the major barricade of the electrochemical system. The way to achieve efficient electrical communication between redox protein and electrode has been among the most challenging objects in the field of bioelectrochemistry. In summary, two ways have been proposed. One is based on the so-called electrochemical mediators, both natural enzyme substrates and products, and artificial redox mediators, mostly dye molecules and conducted polymers. The other approach is based on the direct electron transfer of protein. With its inherited simplicity in either theoretical calculations or practical applications, the latter has received far greater interest despite its limited applications at the present stage. 

The enzyme-based biosensor has come through three steps (1) with oxygen for the media (2) with artificial intermediate for media and (3) without media and based for the direct electron transfer of redox proteins. The following is an example ... 

---