Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Proteins: Covalent Structure and Biological Functions
Proteins perform a multitude of diverse biological functions

Let us first examine in general terms the entire range of biological Functions performed by Proteins. Indeed, one of the primary objectives of modern biochemistry is to understand how the Amino acid sequences of different proteins enable them to carry out such diverse functions. We can divide proteins into several major classes (Table 6-1) according to their biological functions.

a. Enzymes

The most diverse and highly specialized Class of proteins consists of enzymes, which possess catalytic activity. Nearly all Chemical Reactions Involving organic Biomolecules present within The Cell are catalyzed by enzymes. To date, more than 2,000 different enzymes have been discovered in various organisms; each of these enzymes serves as a catalyst for a specific chemical reaction.

Table 6-1. Classification of Proteins based on their biological functions

b. Transport proteins

Blood Plasma transport proteins bind and carry specific molecules or ions from one organ to another. Hemoglobin, contained within erythrocytes, binds oxygen as blood passes through the Lungs and delivers it to peripheral Tissues, where the oxygen is released and used to oxidize dietary components—a process that generates energy. Blood plasma contains Lipoproteins responsible for transporting Lipids from the Liver to other Organs. Cell membranes contain yet another type of transport protein capable of binding glucose, Amino Acids, and other nutrients and carrying them across the membrane into the cell.

c. Nutrient and storage proteins

The seeds of many plants store nutrient proteins that are consumed during the Cytology/cytology/16.html">Early stages of embryonic development. The best-known Examples of such proteins are found in the seeds of wheat, corn, and rice. Nutrient proteins also include egg albumin, the major component of egg white, and casein, the principal milk protein. Ferritin, found in animal tissues, serves to store iron.

d. Contractile and motile proteins

Certain proteins endow a cell or Organism with The ability to contract, change shape, or move. Actin and myosin are filamentous proteins that function in the contractile system of Skeletal Muscle as well as in many non-muscle Cells (Section 2.13). Another example is tubulin, the protein that makes up microtubules, which are essential components of Cilia and flagella (Section 2.14) used by cells for locomotion.

e. Structural proteins

Many proteins form fibers that are twisted together or arranged in flat sheets; they perform supportive or protective functions by holding biological structures together and giving them strength. The major component of Cartilage and tendons is the fibrous protein collagen, which possesses very high tensile strength. Tanned leather consists almost entirely of pure collagen. Ligaments contain elastin, a structural protein capable of stretching in two dimensions. Hair, Nails, and feathers consist almost exclusively of the tough, insoluble protein keratin. The main component of silk threads and spider webs is the protein fibroin.

f. Defensive proteins

Many proteins protect the organism against invasion by other organisms or safeguard it from injury. IMMUNOGLOBULINS, or antibodies produced by vertebrates, are specialized proteins synthesized in lymphocytes; they have the ability to recognize Bacteria, Viruses, or foreign proteins of other species that have entered the organism, and subsequently neutralize them or bind to them, causing precipitation. Fibrinogen and thrombin are proteins involved in blood clotting, protecting the organism from blood loss when The Vascular System is damaged. Snake venoms, Bacterial toxins, and toxic plant proteins such as ricin apparently also serve protective functions.

g. Regulatory proteins

Certain proteins participate in the Regulation of cellular or physiological activity. These include numerous Hormones, such as insulin, which regulates glucose METABOLISM (a deficiency of insulin leads to Diabetes Mellitus); growth hormone, synthesized in the Pituitary Gland; and parathyroid hormone, which regulates The transport of Ca²⁺ ions and phosphates. Other regulatory proteins, known as repressors, control enzyme Biosynthesis in bacterial cells.

h. Other proteins

There are many other proteins with rather unusual functions that make them difficult to classify. Monellin, a protein produced by an African plant, has an intensely sweet taste. It has been studied as a non-toxic, non-fattening substance that could be used as a sugar substitute to sweeten foods. The blood plasma of certain Antarctic fish contains antifreeze proteins that protect their blood from freezing. The "hinges" at the wing attachments of various insects are composed of resilin, a protein with nearly ideal elasticity.

It is remarkable that all these proteins, so diverse in their Properties and functions, are constructed from the exact same 20 amino acids.