Enzyme-Based Organic Synthesis. Cheanyeh Cheng

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1 Introduction

      1.1 Discovery and Nature of Enzyme

      Although the historical discovery of enzyme can be sourced back to Spallanzani as early as in 1783 with his noting to the liquefied meat by gastric juice of hawks [1], the discovery of enzyme is in general ascribed to the first “isolation” of an enzyme by two chemists, Anselme Payen and Jean‐François Persoz, who worked at a sugar factory in Paris. In 1833, they obtained a substance from the malt extract called diastase (now known as amylase) that can hydrolyze starch to soluble sugar. Next year, Schwann succeeded in extracting the first enzyme from animal source, pepsin, which digests meat from stomach wall [2]. Later, he also identified trypsin, a peptidase in digestive fluids. By 1837, Jön Berzelius made a remarkable foresight for the catalytic nature of all these biological diastases. In the 1950s, Louis Pasteur acknowledged that sugar fermentation by yeast to produce alcohol is catalyzed by “ferments.” Then, in 1860, Berthelot obtained an alcohol precipitate from yeast that can convert sucrose to glucose and fructose and concluded that there was much such ferment in yeast. Not until 1878, the name enzyme, which means “in yeast,” was proposed by Frederick W. Kühne for these biological catalysts. The catalytic activity of enzyme was proved by Eduard Bücher in 1987 using yeast extract for catalytic alcohol fermentation. One year later, Duclaux proposed that all enzymes should give the suffix “ase” for an easily recognition [3].

The intensive studies of enzymes and proteins were both performed by biochemists in the 1800s. However, not until 1926 the protein nature of enzyme was seriously considered by biochemists that the jack bean urease that was recognized as a protein was first crystallized and recrystallized by James Sumner showing the catalytic ability for hydrolysis of urea to CO₂ and NH₃ [4, 5]. However, the crystal structure of urease which in essence is a nickel‐containing enzyme as known nowadays was obtained by Andrew Karplus from Klebsiella aerogenes [6, 7] almost 70 years later after Sumner’s work. Sumner’s conclusion was widely accepted in the 1930s, after John Northrop and Moses Kunitz crystallized pepsin, trypsin, and other digestive enzymes and found to be proteins. Due to the prosperous development of separation and purification technology and corresponding instrumentation, hundreds of enzymes had been purified and discovered in the middle of nineteenth century. Nevertheless, the sequencing of proteins and enzymes was not until the work of William H. Stein et al [8]. who first complete the sequence of ribonuclease A (an enzyme with only 124 amino acids) in 1960. Later, in 1972, William H. Stein and Stanford Moore shared the Nobel Prize in chemistry.

1.2 Enzyme Structure and Catalytic Function

      1.2.1 Enzyme Structure

      With the exception of a small group of catalytic RNA molecules, all enzymes are protein. The protein nature of enzyme has been elucidated about a century ago that has led fast and broad progress in chemistry, biochemistry, and biology, in addition, led the development of many new fields such as enzymology, bioorganic chemistry, and molecular biology. In order to understand enzyme and how its function as a catalyst, one must know the enzyme structure first. Since enzyme is a kind of protein, its structure follows the four‐level structure of protein, namely, the primary structure, the secondary structure, the tertiary structure, and the quaternary structure.

      Protein is a polymer of amino acids that is referred to as peptides or proteins. Peptides are chains of amino acids joined through a substituted amide linkage, termed a peptide bond, which is formed by dehydration to remove the elements of water from the α‐carboxyl group of one amino acid and the α‐amino group of another. When a few amino acids (usually, less than 10) are joined in this fashion, the structure is called an oligopeptide. When many amino acids are joined, the product is called a polypeptide. Proteins that may have thousands of amino acid residues are polypeptides. Therefore, “protein” and “polypeptide” are sometimes used interchangeably. However, molecules with molecular weight below 10 000 are generally, referred to as polypeptides. In a peptide, the amino acid residue at one end with a free α‐amino group is the amino‐terminal (or N‐terminal) residue, while at the other end, the residue with a free carboxyl group is the carboxyl‐terminal (C‐terminal) residue [9].

Since proteins are large macromolecules, the complexity of their 3D structure cannot be described easily like small molecules. Therefore, four levels of structure are used to define the complete 3D structure of protein. The primary structure is the amino acid sequence of a polypeptide chain that describes the order of all covalent bonds, mainly peptide bonds and disulfide bonds, linking amino acid residue in the polypeptide as illustrated in Scheme 1.1. The importance of primary structure is in determining the secondary, tertiary, quaternary structures of proteins, and thus their biological functions, which can be demonstrated by the hereditary disease sickle‐cell anemia of human.