Chapter 3 AMINO ACIDS, PEPTIDES, AND PROTEINS
Proteins mediate virtually every process that takes place in a cell, exhibiting an almost endless diversity of functions. To explore the molecular mechanism of a biological process, a biochemist almost inevitably studies one or more proteins. Proteins are the most abundant biological macromolecules, occurring in all cells and all parts of cells. Proteins also occur in great variety; thousands of different kinds may be found in a single cell. Proteins are the molecular instruments through which genetic information is expressed — the important final products of the information pathways discussed in Part III of this book.
Cells produce proteins with strikingly different properties and activities by joining a common set of 20 amino acids in many different combinations and sequences. From these building blocks, different organisms can make such widely diverse products as enzymes, hormones, antibodies, transporters, light-harvesting complexes in plants, the flagella of bacteria, muscle fibers, feathers, spider webs, rhinoceros horn, antibiotics, and myriad other substances that have distinct biological functions (Fig. 3-1). Among these protein products, the enzymes are the most varied and specialized. As the catalysts of almost all cellular reactions, enzymes are one of the keys to understanding the chemistry of life, and thus they provide a focal point for any course in biochemistry.
FIGURE 3-1 Some functions of proteins. (a) The light produced by fireflies is the result of a reaction involving the protein luciferin and ATP, catalyzed by the enzyme luciferase (see Box 13-1). (b) Erythrocytes contain large amounts of the oxygen-transporting protein hemoglobin. (c) The protein keratin, formed by all vertebrates, is the chief structural component of hair, scales, horn, wool, nails, and feathers. The black rhinoceros is extinct in the wild because of the belief prevalent in some parts of the world that a powder derived from its horn has aphrodisiac properties. In reality, the chemical properties of powdered rhinoceros horn are no different from those of powdered bovine hooves or human fingernails.
Protein structure and function are the topics of this and the next three chapters. Here, we begin with a description of the fundamental chemical properties of amino acids, peptides, and proteins. We also consider how a biochemist works with proteins. The material is organized around four principles:
In every living organism, proteins are constructed from a common set of 20 amino acids. Each amino acid has a side chain with distinctive chemical properties. Amino acids may be regarded as the alphabet in which the language of protein structure is written.
In proteins, amino acids are joined in characteristic linear sequences through a common amide linkage, the peptide bond. The amino acid sequence of a protein constitutes its primary structure, a first level we will introduce within the broader complexities of protein structure.
For study, individual proteins can be separated from the thousands of other proteins present in a cell, based on differences in their chemical and functional properties arising from their distinct amino acid sequences. As proteins are central to biochemistry, the purification of individual proteins for study is a quintessential biochemical endeavor.
Shaped by evolution, amino acid sequences are a key resource for understanding the function of individual proteins and for tracing broader functional and evolutionary relationships.
In every living organism, proteins are constructed from a common set of 20 amino acids. Each amino acid has a side chain with distinctive chemical properties. Amino acids may be regarded as the alphabet in which the language of protein structure is written.
In proteins, amino acids are joined in characteristic linear sequences through a common amide linkage, the peptide bond. The amino acid sequence of a protein constitutes its primary structure, a first level we will introduce within the broader complexities of protein structure.
For study, individual proteins can be separated from the thousands of other proteins present in a cell, based on differences in their chemical and functional properties arising from their distinct amino acid sequences. As proteins are central to biochemistry, the purification of individual proteins for study is a quintessential biochemical endeavor.
Shaped by evolution, amino acid sequences are a key resource for understanding the function of individual proteins and for tracing broader functional and evolutionary relationships.