Introduction to Proteins

• Proteins are naturally occurring polymers made of amino acids linked by peptide bonds
• Proteins are the most abundant organic molecules in living systems
• The word protein is derived from the Greek word proteios, meaning holding the first place
• Proteins are nitrogenous organic compounds with large molecular weights
• Proteins are made from 20 alpha-amino acids
• A single amino acid unit is called a monomer; many monomers form polymers
• Proteins hydrolyze into amino acids
• Proteins are in every cell of the body and involved in most bodily functions

Functions of Proteins

• Structural proteins provide strength and structure to cells, tissues, organs, elastin, and collagen
• Enzymes catalyze biochemical reactions in the body, speeding up chemical processes
• Transport proteins transport metabolites, responsible for respiration
• Regulatory proteins regulate metabolic pathways, e.g., insulin
• Defense proteins protect the body from infection and toxins, e.g., antibodies and immunoglobulins
• Muscle proteins are required for mechanical work
• Proteins regulate pH, osmotic pressure, temperature, and electrolyte balance

Classification of Protein

• Simple, conjugated, and derived proteins are the classification of proteins

Simple Proteins

• Simple proteins are composed of only amino acid residues
• Globular proteins are spherical or oval and soluble in water
• Albumins and Globulins are globular proteins found in cells, e.g., serum albumin, ovalbumin (egg), lactalbumin (milk)
• Glutelins are mostly found in plants, e.g., glutelin (wheat) and oryzenin (rice)
• Prolamines are soluble in 70% alcohol, e.g., gliadin (wheat) and zein (maize)
• Histones are basic proteins with a high molecular weight, are heat coagulable and widely distributed in the body
• Globins are generally considered with histones
• Protamines are strongly basic, resemble histones but are smaller and soluble in NH4OH, found associated with nucleic acids
• Lectins are carbohydrate-binding proteins involved in cell-protein interaction, maintain tissue and organ structures

Fibrous Proteins

• Fibrous proteins are fiber-like and insoluble in water
• Collagens are connective tissue proteins that become soluble and digestible when boiled with water
• Elastins are found in elastic tissues like tendons and arteries
• Keratins are proteins of the skin, hair, and nails

Conjugated Proteins

• Conjugated proteins contain a non-protein moiety known as the prosthetic or conjugating group
• Nucleoproteins are formed by the combination of histone with RNA or DNA
• Chromo-proteins are soluble proteins combined with a chromophoric (colored) group, like haem and riboflavin
• Phosphoproteins contain a phosphate group along with amino acids, e.g., milk casein
• Glycoproteins consist of amino sugars, carbohydrate and amino acids, sulphate and sugar acids
• Lipoproteins are a combination of proteins with lipids, found in the brain and membrane
• Metalloproteins have various metals (Fe, Co, Zn) attached to simple proteins

Derived Proteins

• Derived proteins are denatured or degraded products of simple and conjugated proteins

Primary-Derived Proteins

• Primary-derived proteins are denatured, coagulated, or first hydrolyzed products
• Coagulated proteins are denatured by heat, acids, or alkalies, e.g., albumin (egg white)
• Proteans are the first products of protein hydrolysis by enzymes, dil. acids, alkalies, e.g., fibrin from fibrinogen
• Metaproteins are the second products of protein hydrolysis, made using stronger acids, e.g., acid and alkali metaproteins

Secondary-Derived Proteins

• Secondary derived proteins are degraded due to breaking peptide bonds in proteins

Proteins and its structure

• Proteins are polymers of alpha-amino acids
• Proteins have a complex structure divided into four levels

Primary Structure

• Primary structure is a linear sequence of protein that forms the backbone
• Primary structure is responsible for the proteins functions
• Each protein has a unique sequence of amino acids linked by peptide bonds; abnormal sequencing causes illnesses
• Peptide bonds form when amino and carboxyl groups of two amino acids interact

Secondary Structure

• Secondary structure is the shape of the polypeptide chain and arrangement in space by twisting
• Alfa-helix is the most common spiral structure where amino acids are tightly packed and coiled
• Formation of alfa-helix requires the lowest energy
• Beta-sheet has hydrogen bonds between neighboring polypeptide segments forming a sheet-like structure
• Beta-sheets can be parallel (same direction) or antiparallel (opposite direction)

Tertiary Structure

• Tertiary structure is the three-dimensional structure of proteins
• Hydrophobic side chains are held inside, hydrophilic groups outside
• This arrangement stabilizes the molecule

Quaternary Structure

• Quaternary structure involves two or more polypeptides held together by non-covalent bonds
• Two polypeptide chains are called oligomers, one is called a monomer
• Hemoglobin has four polypeptides
• Bonds can be hydrogen, ionic, or hydrophobic

Properties of Proteins

• Proteins form colloidal solutions in water due to their large size
• Molecular weight varies depending on the number of amino acids, ranges from 40-40,000
• Shape varies, Insulin has a globular shape, albumin oval, and fibrinogen elongated
• Zwitterions are ions carrying both positive and negative charges in different parts of the molecule
• Proteins get dehydrated and precipitate when salts like ammonium sulfate are added; dehydration also occurs with heavy metals
• Alcohol, tannic acid, and picric acid also dehydrate proteins, leading to precipitation
• Coagulation: It is a semisolid or solid precipitate of protein that is irreversible, e.g., albumin

Amino Acids

• Amino acids are organic compounds with two functional groups

Amino Groups

• Amino (-NH2)

Carboxyl Groups

• Carboxyl (-COOH)
• These functional groups are attached to alpha-carbon and called a-amino acids
• Over 300 amino acids exists in nature but only 20 are standard amino acids
• R represents a side chain attached to alpha carbon; each of the 20 amino acids has a different R group

Structural Classification of Amino Acids

• Aliphatic amino acids: e.g., alanine, glycine, valine, leucine, isoleucine
• Aromatic amino acids: e.g., phenylalanine, tyrosine, tryptophan
• Hydroxyl group-containing amino acids: serine, threonine
• Sulfur containing amino acids: e.g. cysteine, methionine
• Acidic amino acids: e.g. glutamic acid, aspartic acid
• Basic amino acids: e.g. histidine, lysine, arginine
• Imino acids: e.g. proline

Classification of Amino Acids Based on Polarity

• Nonpolar amino acids are hydrophobic and have no charge on the R group, e.g., alanine, valine, leucine
• Polar amino acids are hydrophilic and have a charge on the R group, e.g., glycine, serine, cysteine

Classification of Amino Acids – Essential and Non Essential

• Essential amino acids: These cannot be synthesized in the body and therefore be supplied through the diet; valine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, arginine, and histidine
• Nonessential amino acids: These are synthesized in the body and need not be consumed in the diet; glycine, alanine, serine, cysteine, aspartate, asparagine, glutamate, glutamine, tyrosine, proline

Properties of Amino Acids

• Amino acids are colorless and crystalline
• All are soluble in water, but the extent varies; R group determines solubility
• Polar aa are highly soluble in water, nonpolar are soluble in organic solvents
• Amino acids have high melting points, mostly above 200°C
• Taste: can be sweet (glycine), bitter (arginine), or tasteless (leucine)
• All amino acids except glycine are optically active

Functions of Amino Acids

• Amino acids are the building blocks of proteins, DNA instructs cells on amino acid sequences needed to build proteins
• Some amino acids produce neurotransmitters like GABA used in brain functions
• The body uses arginine to make nitric oxide, which lowers blood pressure by relaxing blood vessels

Reactions of proteins

• Heat Test: Protein solutions coagulate and lose biological activity, this is called denaturation of protein
• Test with trichloroacetic acid (TCA): TCA precipitates proteins from their solution when acid is added and the protein is denatured
• Biuret test: Biuret reagent contains copper sulphate and sodium Potassium tartrate. Proteins treated reduce the cu2+ ions to cu+ creating a violet color used to identify porteins
• Hydrolysis test: Proteins on hydrolysis give amino acids when treated with Sulphuric or Hydrochloric acid
• Xanthoproteic reaction: Nitration of aromatic amino acids gives a yellow colour which turns orange when alkali acids are used
• Millons test: Proteins react with mercuric sulphate in presence of sodium nitrite and sulphuric acid changing colour to red

Disorders of Protein Deficiency

• Protein deficiency in early childhood leads to diseases
• Kwashiorkor is a severe protein malnutrition disease that occurs at age 1-4
• Symptoms of Kwashiorkor are retarded growth, edema, skin alterations, hair changes, liver enlargement, vomiting, and diarrhea
• Causes of Kwashiorkor are poor maternal health, large family size, poor breastfeeding, environmental conditions, over diluted cow's milk
• Cure for Kwashiorkor is a diet rich in protein such as milk and eggs, and soya beans are the best alternative source of protein
• Marasmus is similar to Kwashiorkor but occurs in infants below 1 year of age
• Causes of Marasmus are nutritional deficiency in carbohydrates, proteins; and the stoppage of early breastfeeding
• Cure for Marasmus is a diet rich in proteins
• Nutritional oedema is swelling caused by insufficient protein intake
• Causes of Nutritional oedema are long continuous deprivation of proteins
• Symptoms in adults: Loss of weight, Anemia, Constant infection, Frequent loose stools, and Delay in healing wounds
• Cure: Diet rich in proteins, Soyabean, milk and eggs in the diet