proteins

Proteins
Proteins are large biomolecule or macromolecules consisting of one or more long chains of amino acid residues.
A linear chain of amino acid residue is called a polypeptide.
A protein contains at least one long polypeptide.
The twenty amino acids commonly found in proteins are joined together by peptide bonds.
Functional proteins have four levels of structural organization:
1- Primary structure: the linear structure of amino acid in the poly peptide chain.
2- Secondary structure: hydrogen bonds between peptide group chains in an alpha-helix or beta sheet.
3- Tertiary structure: three dimension structure of alpha helixes and beta helixes folded.
4- Quaternary structure: three-dimential structure of multiple polypepties and how they fit together.

Note: Understanding the primary structure of proteins is important because many genetic diseases results in protein with abnormal amino acid sequences which cause improper folding and loss or impairment of normal function.

General reactions of proteins
1- The biuret test for peptide bonds.
Is a chemical test used for detecting the presence of peptide bonds.
Cupric ions chelate with peptide bonds of proteins in alkaline medium to produce a pink or violet color. The intensity of the color is proportional to the peptide bonds.
Advantage: the biuret method is simple one step process and is the most widely used method for plasma protein estimation.
Disadvantage: sensitivity of the method is less and it is unsuitable for estimation of proteins in small quantity.
2- Denaturation of proteins by heat and extreme pH.
Denaturation is a process in which protein loss the quaternary, tertiary and secondary structure which is present in their native state by application of some external stress or compound such as a strong acid or base, a concentrated inorganic salt, an organic solvent (e.g alchole,chloroform) ,radiation or heat
If protein in a living cell are denaturated this result in disruption of cell activity and possibly cell death.
Primary structure is not altered during the process of denaturation.
Protein when heated at isoelectric point, some proteins will denaturated irreversibly to produce thick floating conglomerate called coagulum. This process is called heat coagulation.
Heat can be used to disrupt hydrogen bonds and non-polar hydrophobic interactions, this occur because heat increase the kinetic energy and cause the molecules to vibrate so rabidly and violently that the bonds are disrupted.
Albumin is easily coagulated and globin to a lesser extent.
3- Precipitation by heavy metal ions.
In alkaline medium proteins have net negative charge or anions.
To such a solution if salts of heavy metals are added positively charged metal ions can complex with protein molecules and metal proteinase are precipitated.
Salts of iron, copper, zinc, lead, cadmium and mercury are toxic because they tend to precipitate normal proteins of the gastrointestinal wall.
4- Precipitation by acidic agents.
Tungstic acid, phosphotungstic acid, trichloroacetic acid, picric acid, sulphosalicylic acid and tannic acid are powerful protein precipitating agents.
These acids lower the pH of medium when protein carries net positive charges. These protein contains are electrostatically complexed with negatively charged ions to form protein-tugstate, protein –picrate, etc. and thick flocculent precipitate is formed. In clinical laboratory phosphotungstic or trichloroacetic acid are usually used for precipitating protein.