Unit XVII proteins and peptides

Proteinis a highly complex substance that is present in all living organisms. Proteins are of great nutritional value and are directly involved in the chemical processes essential for life. The importance of proteins was recognized by the chemists in the early 19th century who coined the name for these substances from the Greek proteios, meaning “holding first place.” Proteins are species-specific; that is, the proteins of one species differ from those of another species. They are also organ-specific; for instance, within a single organism, muscle proteins differ from those of the brain and liver.

A protein molecule is very large compared with molecules of sugar or salt and consists of many amino acids joined together to form long chains, much as beads are arranged on a string. There are about 20 different amino acids that occur naturally in proteins. Proteins of similar function have similar amino acid composition and sequence.

Amino acid is any of a group of organic molecules that consist of a basic amino group (в?’NH2), an acidic carboxyl group (в?’COOH), and an organic R group (or side chain) that is unique to each amino acid.

Plants can synthesize all of the amino acids; animals cannot, even though all of them are essential for life. Plants can grow in a medium containing inorganic nutrients that provide nitrogen, potassium, and other substances essential for growth. They utilize the carbon dioxide in the air during the process of photosynthesis to form organic compounds such as carbohydrates. Animals, however, must obtain organic nutrients from outside sources. Nonruminant animals, including humans, obtain proteins principally from animals and their products – e.g., meat, milk, and eggs. The seeds of legumes are increasingly being used to prepare inexpensive protein-rich food.

Nowadays it is evident that protein molecules are produced in cells by the stepwise alignment of amino acids and are released into the body fluids only after synthesis is complete. And this process is impossible without enzymes, which are the catalysts of all metabolic reactions, enable an organism to build up the chemical substances necessary for life—proteins, nucleic acids, carbohydrates, and lipids—to convert them into other substances, and to degrade them. All enzymes identified thus far are proteins. Life without enzymes is not possible. There are several protein hormones with important regulatory functions. In all vertebrates, the respiratory protein hemoglobin acts as oxygen carrier in the blood, transporting oxygen from the lung to body organs and tissues. A large group of structural proteins maintains and protects the structure of the animal body.

The common property of all proteins is that they consist of long chains of О±-amino (alpha amino) acids. In protein molecules the О±-amino acids are linked to each other by peptide bonds between the amino group of one amino acid and the carboxyl group of its neighbor.

Proteins usually are almost neutral molecules; that is, they have neither acidic nor basic properties.

Biochemists often refer to four distinct aspects of a protein's structure:

Primary structure: the amino acid sequence. A protein is a polyamide.

Secondary structure: regularly repeating local structures stabilized by hydrogen bonds. The most common examples are the alpha helix, beta sheet and turns. Because secondary structures are local, many regions of different secondary structure can be present in the same protein molecule.

Tertiary structure: the overall shape of a single protein molecule; the spatial relationship of the secondary structures to one another. Tertiary structure is generally stabilized by nonlocal interactions, most commonly the formation of a hydrophobic core, but also through salt bridges, hydrogen bonds, disulfide bonds, and even posttranslational modifications. The term "tertiary structure" is often used as synonymous with the term fold. The tertiary structure is what controls the basic function of the protein.

Quaternary structure: the structure formed by several protein molecules (polypeptide chains), usually called protein subunits in this context, which function as a single protein complex.

Proteins can be informally divided into three main classes: fibrous proteins, globular proteins and membrane proteins.

Fibrous (structural) proteins. As the name implies, these substances have fiber-like structures, and serve as the chief structural material in various tissues. Corresponding to this structural function, they are relatively insoluble in water and unaffected by moderate changes in temperature and pH. Subgroups within this category include: Collagens and Elastins, the proteins of connective tissues, tendons and ligaments; Keratins, proteins that are major components of skin, hair, feathers and horn; Fibrin, a protein formed when blood clots.

Globular Proteins. Members of this class serve regulatory, maintenance and catalytic roles in living organisms. They include hormones, antibodies and enzymes and either dissolve or form colloidal suspensions in water. Such proteins are generally more sensitive to temperature and pH change than their fibrous counterparts.

Membrane proteins often serve as receptors or provide channels for polar or charged molecules to pass through the cell membrane