PROTENIS

Proteins

Proteins are large organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in a protein is defined by a gene and encoded in the genetic code. Although this genetic code specifies 20 "standard" amino acids plus selenocysteine and - in certain archaea - pyrrolysine, the residues in a protein are sometimes chemically altered in post-translational modification: either before the protein can function in the cell, or as part of control mechanisms. Proteins can also work together to achieve a particular function, and they often associate to form stable complexes.

Like other biological macromolecules such as polysaccharides and nucleic acids, proteins are essential parts of organisms and participate in every process within cells. Many proteins are enzymes that catalyze biochemical reactions and are vital to metabolism. Proteins also have structural or mechanical functions, such as actin and myosin in muscle and the proteins in the cytoskeleton, which form a system of scaffolding that maintains cell shape. Other proteins are important in cell signaling, immune responses, cell adhesion, and the cell cycle. Proteins are also necessary in animals' diets, since animals cannot synthesize all the amino acids they need and must obtain essential amino acids from food. Through the process of digestion, animals break down ingested protein into free amino acids that are then used in metabolism.

Three possible representations of the three-dimensional structure of the protein triose phosphate isomerase. Left: all-atom representation colored by atom type. Middle: simplified representation illustrating the backbone conformation, colored by secondary structure. Right: Solvent-accessible surface representation colored by residue type (acidic residues red, basic residues blue, polar residues green, nonpolar residues white).

Classes of proteins

Historically based on SOLUBILITY of PROTEINS...

Two classes - SIMPLE & COMPLEX

SIMPLE PROTEINS: on hydrolysis include only amino acids:

1. Albumins - soluble in water (distilled), globular, most enzymes

2. Globulins - soluble in dilute aqueous solutions; insoluble in pure distilled water

3. Prolamins - insoluble in water; soluble in 50% to 90% simple alcohols

4. Glutelins - insoluble in most solvents; soluble in dilute acids/bases

5. Protamines - not based upon solubility; small MW proteins with 80% Arginine & no Cysteine

6. Histones - unique/structural: complexed w DNA, high # basic aa's - 90% Arg, Lys.

7. Scleroproteins - insoluble in most solvents fibrous structure - architectural proteins of cartilage & connective tissue.

Collagen = high Glycine, Proline, & no Cysteine when boiled makes gelatin

Keratins = proteins of skin & hair high basic aa's (Arg, His, Lys), but Cys.

Complex Proteins: on hydrolysis yield amino acids + other molecules

lipoproteins - (+ lipids) blood, membrane, & transport proteins

glycoproteins - (+ carbohydrates) antibodies, cell surface proteins

nucleoproteins - (+ nucleic acids) ribosomes & organelles

Common terminology:

dipeptide = 2 amino acids

tripeptide = 3 amino acids

peptide = short chain of amino acids (20-30)

polypeptide = many amino acids (up to 4,000)

protein = polypeptide with well defined 3D structure