Proteins  (Page 6/5)

Introduction

The characteristic specific properties of native proteins we attribute to their uniquely defined configurations. The denatured protein molecule we consider to be characterized by the absence of a uniquely defined configuration.

Proteins are one of the most abundant organic molecules in living systems and have the most diverse range of functions of all macromolecules. That diversity of function is due to a tremendous diversity of "uniquely defined" structures. Proteins may be structural, regulatory, contractile, or protective; they may serve in transport, storage, or membranes; or they may be toxins or enzymes. Each cell in a living system may contain thousands of proteins, each with a unique function. Their structures, like their functions, vary greatly. They are all, however, polymers of amino acids, arranged in a linear sequence. But that simple linear sequence is just the beginning of the story.

Types and functions of proteins

The primary types and functions of proteins are listed in [link] . We will consider some of these categories in some detail, but the others will be left for later discussion.

Enzymes , which are produced by living cells, are catalysts in biochemical reactions (like digestion) and are usually complex or conjugated proteins. Each enzyme is specific for the substrate (a reactant that binds to an enzyme) it acts on. The enzyme may help in breakdown, rearrangement, or synthesis reactions. Enzymes that break down their substrates are called catabolic enzymes, and often this is a hydrolysis reaction. Enzymes that build more complex molecules from their substrates are called anabolic enzymes, and often this is condensation reaction or dehydration synthesis. It should be noted that all enzymes increase the rate of reaction and, therefore, are considered to be organic catalysts. An example of an enzyme is salivary amylase, which hydrolyzes (breaks down) its substrate amylose, a component of starch, producing the simple disaccharide known as maltose along with other simpler sugars.

Hormones are chemical-signaling molecules, usually small proteins or steroids, secreted by endocrine cells that act to control or regulate specific physiological processes, including growth, development, metabolism, and reproduction. For example, insulin is a protein hormone that helps to regulate the blood glucose level.

Structural proteins are some of the more familiar proteins encountered everyday. Hair, fingernails, and feathers are largely composed of proteins called keratins. Your skin contains large quantities of proteins called collagens and elastins. Other structural proteins are found in bone, in muscle, in connective tissue, etc.

Storage proteins are used by some organisms to store energy over the long term, just as carbohydrates and lipids are the preferred energy storage molecules for other organisms. Casein, a protein found in milk, is one example. Zein proteins found in wheat grains provide energy for the developing wheat embryo, but also are critical in helping bread dough to rise and hold its shape. Egg albumin is an energy source for bird embryonic development. And proteins found in legumes, such as soybeans and other beans, nourish the embryos of those plants, as well as billions of humans around the world.