CLASSIFICATION OF PROTEINS

Functions / importance of Proteins Type Examples Structural tendons, cartilage, hair, nails Contractile muscles Transport hemoglobin, albumin Storage ferritin Hormonal eg. insulin, growth hormone Enzyme eg. hydroxylases Protection immunoglobulins Energy 4.1 k.cal/gm

Classification of proteins based on physical properties

CLASSIFICATION OF PROTEINS BASED ON PHYSICAL PROPERTIES Examples I. FIBROUS PROTEINS Insoluble in aqueous solutions elongated molecules often consisting of several coiled polypeptide chains 1. Collagens Can be converted into soluble gelatins by boiling contain large amounts of hydroxy-proline and hydroxylysine but no cysteine or tryptophan The major proteins of connective tissues 2. Elastin Similar to collagens but cannot be converted to gelatins by boiling Proteins of tendons and arteries 3. Keratins Contain large amounts of cysteine Hair, wool, nails (Hair is about 14% cysteine)

CLASSIFICATION OF PROTEINS BASED ON PHYSICAL PROPERTIES Examples II GLOBULAR PROTEINS Soluble in aqueous solutions spherical or ellipsoidal in shape Albumins Readily soluble in pure water coagulated by heat function as carriers for hydrophobic molecules Serum albumin, egg albumin 2. Globulins Insoluble or only slightly soluble in pure water very soluble in aqueous salt solutions can be coagulated by heat Enzymes and antibodies 3. Histones Basic proteins contain large amounts of arginine and lysine soluble in pure water Histones in chromatin 4. Protamines Very basic proteins contain large amounts of arginine but no tryptophan or tyrosine Found in sperm cell chromosomes

Classification of proteins based on function

Classification of proteins based on function Properties Examples CATALYTIC PROTEINS Enzymes Catalyze chemical reactions Lactate dehydrogenase (LDH) amylase pyruvate dehydrogenase II. NONCATALYTIC PROTEINS 1. Carrier proteins Carry molecules or ions through the bloodstream Hemoglobin, albumin 2. Receptor proteins Bind hormones and neurotrans mitters to cell membranes The insulin receptor 3. Membrane transport proteins Carry molecules across cell membranes Na+ K+ Atpase, which transports K+ ions into cells and pumps Na+ ions out of cells 4. Structural proteins Form extracellular structures such as hair and nails Collagen keratin 5. Contractile proteins Extend or contract to produce movement of muscles cells or subcellular parts Myosin, tubulin

Classification of proteins based on function……. cont Properties Examples 6. Proteins hormones Messenger molecules that direct the activities of various cells and organs Insulin, adreno corticotropic hormone (ACTH), growth hormones 7. Antibodies Bind to foreign substances and activate their elimination from the body Anti-Rh, Anti –A (antibodies in Rh factor and to blood group A)

Another Classification Simple Conjugated Derived

I SIMPLE PROTEINS Water soluble , coagulated by heat eg. albumin, lactalbumin, ovalbumin Insoluble in water, soluble in dilute salt solutions. Heat coagulable to variable extent eg lact globulins, immune globulins, myosine Rich in histidine. Unite with heme to form hemoglobin Soluble in 70-80% ethanol and insoluble in water eg. gliadin of wheat and zein of maize Soluble in water but not in ammonium hydroxide. Present in nucleus Like histones but present in sperm cells. Soluble in ammonium hydroxide. Also called scleroproteins eg. collagen , elastin and keratin Albumin Globulin Globin Prolamines Histones Protamines Albuminoids

II CONGUGATED PROTEINS Types Prosthetic Group Properties Example Nucleoproteins Nucletic acid (DNA,RNA) Large, compact complexes Chromatin, ribosomes Mucoproteins* Carbohydrate More than 4% carbohydrate by weight Human chorionic gonadotrophin, a hormone used to test for pregnancy Glycoproteins * Lipoproteins Lipid Less than 4% carbohydrate Water soluble Antibodies Serum lipoproteins Proteolipids Not very water soluble, soluble in nonpolar solvents Cell membranes Hemoproteins Heme group Characteristic color Hemoglobin, cytochrome c Metalloproteins Metal ion (Fe3+, Zn2+ Mg2+, Mn2+ ) Require a metal ion to function Carbonic anhydrase *The distinction between mucoproteins and glycoproteins is somewhat arbitrary

III Derived Proteins Primary Derived Secondary Derived Eg denatured proteins Polypeptides, oligopeptides, proteosis, peptones

Peptides Amino acids can be polymerized to form chains: Two amino acids  dipeptide  one peptide bond. Three amino acids  tripeptide  two peptide bonds. Four amino acids  tetrapeptide  three peptide bonds. Few (2-20 amino acids)  oligopeptide. More (>20 amino acids)  polypeptide.

Peptide Bond (amide bond)

Peptide bond In proteins, amino acids are joined covalently by peptide bonds, which are amide linkages between the á-carboxyl group of one amino acid and the á-amino group of another. For example, valine and alanine can form the dipeptide valylalanine through the formation of a peptide bond. Peptide bonds are not broken by conditions that denature proteins, such as heating or high concentrations of urea

Characteristics of the peptide bond: The peptide bond has a partial double-bond character, that is, it is shorter than a single bond, and is rigid and planar . This prevents free rotation around the bond between the carbonyl carbon and the nitrogen of the peptide bond. However, the bonds between the á-carbons and the á-amino or á-carboxyl groups can be freely rotated (although they are limited by the size and character of the R-groups). This allows the polypeptide chain to assume a variety of possible configurations..

Each amino acid in a chain makes two peptide bonds. The amino acids at the two ends of a chain make only one peptide bond. The amino acid with a free amino group is called amino terminus or NH2-terminus. The amino acid with a free carboxylic group is called carboxyl terminus or COOH-terminus.

PEPTIDES AND PROTEINS Oligopeptide :a few amino acids ≤ 100 Carboxyl terminal- C-terminal Amino terminal- N-terminal- Oligopeptide :a few amino acids ≤ 100 Polypeptide : many amino acids ≥ 100