Chemistry of Proteins and Amino acids Subscribe
Date: 03. November 2017
Introduction to Proteins
In this lecture educator explains the Chemistry of Proteins and Amino acids. In the first section of the lecture Nitrogenous substances found in egg and milk coagulate on heating. JJ Berzelius suggested the term ‘proteins’ for these substances. Word protein is derived from Greek word Proteios means “primary”, or “holding first place” or “pre-eminent” because Berzelius thought them to be most important of biological substances.
Proteins are unbranched polymers of L- α-amino acids. Composed of C, H, O and N (16 wt%). Small amounts of S and P. Trace amounts of I, Cu, Mn, Zn and Fe, Large molecules with a high molecular weight ranging from 5000 to 25,00,000. and Hydrolysis of proteins yields constituent amino acids. Biomedical Importance of Proteins is that they provide fundamental basis for the structure and function of life, Main source of nitrogen in the body, Main structural and functional components of cytoskeleton. All enzymes (Biochemical catalysts) are proteins, Immunoglobulins are defense proteins. Structural proteins are Collagen & elastin. Contractile proteins are actin & myosin. Transport proteins are albumins. Storage proteins is Ferritin. Respiratory pigments are Haemoglobin & myoglobin.
Introduction to Amino Acids
Basic monomeric unit of protein is called amino acid. Amino group is NH2, Carboxylic acid group is COOH. Hydrogen atom attached to carbon located next to the – COOH group, Side chain –R varies. There are 20 standard amino acids. Nonstandard amino acids include: compounds that are similar to basic structure of amino acids but do not occur in proteins and D-amino acids.
20 Standard Amino Acids are normally isolated from animal and plants. They Can be Classified According to: 1. Reaction in solution: Acidic, basic and neutral, 2. Side chain R, 3. Presence of Imine group, 4. Presence of Sulphur and 5. Availability of amino acids through nutrition: Essential, Non-essential. Non-Standard Amino Acids has two groups: Compounds similar to basic structure of amino acids but do not occur in proteins and D-amino acids: Amino acids obtained from bacteria and antibiotics and in brain tissues of animals.
Functions and Properties of Amino Acids
In the third section of this lecture educator explains the Functions of Amino Acids. Besides being a constituent of proteins, amino acids have a variety of functions: Specialized products are Tyrosine and they forms hormones like thyroid hormones, (T3, T4), epinephrine, norepinephrine and a pigment called melanin, Tryptophan can synthesize niacin & serotonin and Glycine is used in the synthesis of haem. Glycine and Cysteine are used as detoxicants of specific substances. Methionine is used for transmethylation of substances. Source of sulphur are Cystine and methionine.
Properties of Amino Acids are that they are Amphoteric Nature: Amino acids behave as proton donors or proton acceptors depending on pH of solution. It involves Ampholytes and Isoelectric point. Physical Properties are that it is Colourless, crystalline, water soluble, high melting points, high dielectric constant, and has high dipole moment. Chemical Properties includes I- Due to Carboxylic group that involves formation of esters, Reduction to amino alcohol, Formation of amines by decarboxylation and Formation of amides and Optical isomerism. II- Due to Amino Group involves formation of salts with acids, Formation of acyl derivatives and Oxidation. N-terminal residue can be identified by using a reagent that bonds covalently with its α-NH2 group.
Classification and Properties of Proteins
Proteins are classified on the basis of Shape and Size, Functional Properties and Solubility and Physical Properties. Types of Proteins Based on Solubility and Physical Properties are Simple Proteins: Subclassified on basis of solubility and heat conjugability, Properties depend on shape and size are Protamines, histones, albumins, globulins, gliadins, glutelins, scleroproteins or albuminoids. Conjugated Proteins are made up of simple proteins and prosthetic group, Protein part: apoprotein, entire molecule: holoprotein and Nucleoproteins, mucoproteins, glycoproteins, chromoproteins, lipoproteins, metalloproteins.
Derived Proteins are proteins formed from simple and conjugated proteins, Primary derived proteins are proteans, metaproteins, coagulated proteins and Secondary derived proteins are proteoses, albumoses, peptones, and peptides.
Properties of Proteins are that they are tasteless except derived proteins which are bitter, Odourless but turn brown on heating and smell like burning feather, Molecular Weight: High molecular weights, Viscosity: Depends on shape where fibrous protein solutions are more viscous than globular protein solution, Hydration: Swelling up in presence of water, Heat conjugation: Heat causes coagulation / denaturation, Amphoteric Nature: Ampholytes, act as both acids and bases, depending on solution pH. This property is used for separation of proteins by electrophoresis, Precipitation: Helps in isolation of proteins, deproteinization of blood and other biological fluids and extracts.
When two amino acids are joined together by one peptide bond, such a structure is called as dipeptide. A third amino acid can form a second peptide bond through its free –COOH end and is called tripeptide. Chains with fewer than 50 amino acid residues are called peptides or oligopeptides. Peptide linkage holds amino acids together in a specific sequence and number to form a protein.
Proteins Structure, Denaturation and Purity
In the last section of this lecture educator explains the Proteins Structure, Denaturation & Purity. There are 4 structural organizations of proteins; Primary protein structure that is sequence of a chain of amino acids, Secondary protein structure that is local folding of the polypeptide chain into helicesor sheets, Tertiary structure of proteins is the three dimensional folding pattern of a protein due to side chain interactions, Quaternary Protein structure involves protein consisting of more than one amino acids chain. Secondary structure of protein has further five divisions.
Diseases resulting from Altered Protein Conformation are Prions Disease (The basic defect involves alteration of α-helical structure into β-pleated sheet) and Alzheimer's Disease (β-amyloid protein undergoes a conformational transformation from a soluble α-helix rich state to a state rich in β-sheets which aggregates to form plaques).
Denaturation may be defined as a disruption of the secondary, tertiary, and wherever applicable quaternary organization of a biologically active protein molecule due to cleavage of noncovalent bonds. Physical Denaturing Agents are Heat, UV light, ultrasound, and high pressure. Even violent shaking can denature the protein and Chemical Denaturing Agents are Organic solvents, acids/alkalies, urea and various detergents.
Alterations in Protein Molecules after Denaturation are Chemical Alteration: decreased solubility, precipitation at isoelectric point and previously inactive chemical groups are exposed, Physical Alteration: Increased viscosity and decreased rate of diffusion, and Biological Alteration: Increased digestibility by proteolytic enzymes, Enzymal and hormonal inactivity and biological inactivity. At the end educator explains the Purity of Proteins and Aminoacidurias in detail.