Answer
Proteins are chemical compounds made up of peptide chains, which are sequences of amino acids. Amino acids are small organic molecules with carbon, hydrogen, nitrogen and oxygen atoms. A peptide or polypeptide consists of two or more amino acids joined by peptide bonds, formed through dehyration synthesis. In addition to the liner backbone, amino acids also have side chains (R groups) which chemically characterize each amino acid.
Protein synthesis involves four different levels of organization : first there is the formation of the peptide or polypeptide chain or chains:his is the primary structure--the molecules of some proteins have only one peptide chain; second, a polypeptide chain may form coils (alpha helices in myoglobin) or sheets (beta sheets in antibodies). These coils and pleats of the secondary structures are stabilized mainly by hydrogen bonds. The tertiary structure of proteins is the three dimensional (3D) structure; this develops by further twisting and coiling of the polypeptide chain to give spheroidal or globular conformations. The 3D shapes are stabilized by several kinds of bonds, for example, disulfide bonds, hydrogen bonds, ionic bonds and hydrophilic forces, Tertiary structure is the criterion for classifying protein into the two large categories of fibrous proteins(keratin, desmin) and globular proteins (hemoglobin).The fourth level of organization is the quaternary level. This type of structure develops when two or more polypeptide chains (sub-units) link by binding to form large complex molecules. These aggregates of polypeptide are also stabilized by hydrogen bonds, disulfide bonds and salt bridges. Some proteins with quaternary structure are collagen, hemoglobin and insulin.
Proteins are important structural molecules of the body : examples are collagen, keratin, myosin. They are also chemical messengers as in hormones like oxytocin, vassopressin, thyroxine; they are enzymes that catalyze chemical reactions e.g ATP synthase, insulin, amylase, and they are transport proteins like hemoglobin and serum albumin.
Peptide bond. A peptide bond( or amide bond) is a chemical link that joins two or more amino acids together to form a peptide chain ( dipeptide, oligopeptide, polypeptide). Peptides or polypeptides are formed by dehydration synthesis reactions in which a hydroxyl ion is removed from the carboxyl end of one amino acid and a hydrogen ion is removed from the amino end of the other and the two joined to for a peptide with the elimination of water. Peptide bonds link up amino acids to form the primary linear backbones of peptides ( proteins) Peptide bonds may join just a few amino acids in an oligopeptide or as many as 30,00 in the protein connectin.
Work Step by Step
The formation of peptide linkages is a condensation reaction in which water is eliminated. The reaction is endergonic, requiring several thousands of joules of energy per mole of product. The enzyme that catalyzes the addition of an amino acid to a growing peptide chain is peptidyl transferase. Polypeptides and proteins are very stable substances; because the hydrolytic breaking of peptide bonds releases energy, the reaction is very slow, which is fortunate for living beings.
