Читать книгу Enzyme-Based Organic Synthesis - Cheanyeh Cheng - Страница 12

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.

Scheme 1.1 The primary structure of a polypeptide chain linked by the peptide bond shows the sequence of amino acids.

Part of the very long chain polypeptide can be coiled or folded into units by amino acid residues within a short distance to form recurring structural patterns of secondary structure such as the α‐helix of α‐keratin. The helix is a part of the tertiary structure that is the overall 3D arrangement or folding of a polypeptide. An example of the tertiary structure is myoglobin, a globular protein with 153 amino acid residues. The secondary structure refers to the spatial arrangement of amino acid residues that are adjacent in the primary structure, whereas tertiary structure includes longer‐range aspects of the primary structure. When a protein has two or more polypeptide subunits that are associated with each other or one another, their arrangement in space is referred to as quaternary structure. Hemoglobin consisting of four polypeptide subunits is the most well‐known protein with a complex quaternary structure.