Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Water
Amino Acids and Peptides

In terms of quantity, Proteins rank first among all macromolecules found in the living Cell, accounting for at least half of The Cell's dry weight. Proteins are present in all Cells and can be found in any cellular compartment. The Diversity of proteins is also remarkable; a single cell may contain hundreds of Different types of these macromolecules. Proteins perform a wide variety of biological Functions, serving as the molecular instruments through which Genetic information is ultimately expressed. Therefore, it is logical to begin The Study of biological macromolecules with proteins. (The name derives from a Greek word meaning "primary" or "first".)

The key to understanding The Structure of any of these thousands of different proteins lies in a small group of relatively simple molecules that act as building blocks. Across all proteins—whether from the most ancient bacterial lineages or higher organisms—the exact same set of 20 different Amino Acids is used, covalently linked to one another in a specific sequence unique to each protein. Owing to the distinctive properties of its side chain, each amino acid possesses a unique chemical identity; thus, this entire set of 20 Amino acids can be viewed as the alphabet of the Protein Structure "language."

In this chapter, we will also focus on Peptides—short chains consisting of two or more amino acids joined by covalent bonds. A truly remarkable property of cells is their ability to link the 20 amino acids in various combinations and sequences, producing Peptides and Proteins with entirely distinct properties and biological activities. From this same set of building blocks, various organisms are capable of producing products as diverse as Enzymes, Hormones, lens Proteins of the eye, feathers, spider silk, tortoise shells (Fig. 5-1), milk proteins, enkephalins (endogenous opioids produced by the body itself), Antibiotics, fungal toxins, and many Other Compounds endowed with specific biological activity.

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Fig. 5-1. The protein keratin is synthesized in all vertebrates. It serves as the primary structural component of Hair, scales, horns, wool, Nails, and feathers. Keratin is also a major component of the hard tortoise shell.