Medical brain notes 25

  PROTEINS Introduction Proteins are complex organic nitrogenous substances found in animal and plant tissues. The term protein is derived from Greek : Proteios means primary or holding first place. Protein is the essential constituent of living cells. Protein make upto 12% of the protoplasm. They are not only responsible for comprising the structure of the cell but are concerned with every function of the cell including those of respiration, catalysis of reactions by enzymes, transport of materials, regulation of metabolism, and defense actions. The foods rich in proteins are known as body building foods.   Sources of protein Proteins are obtained from animal and plant sources. The animal sources of proteins include milk, egg, meat, fish, liver etc. Plant sources of proteins are pulses, nuts and cereals.

  Amino acids Amino acids are the simplest units of a protein molecule and they form the building blocks of protein structure. The general formula of an amino acid can be written as, An amino acid is an amino carboxylic acid. R is the side chain or residue and it represents the group other than -NH 2  and -COOH. It may be a hydrogen atom (H) or a methyl group (-CH 3 ) or an aliphatic group or an aromatic group or a heterocyclic group. The amino acids are classified based on the nature of R groups. D and L amino acids Based on the position of amino group on the asymmetric carbon atom, amino acids exist in two types. They are D and L amino acids The amino acid having the NH 2  group on the right is called D-amino acid. The amino acid having the NH 2  group on the left is called L-amino acid. These isomers are the mirror images of each other. All amino acids are α amino acids because the NH 2  group is attached to the a carbon atom which is next to the COOH group. ...

  Properties of amino acids   1. Physical properties Amino acids are coloureless, crystalline, generally soluble in water, in acid and in alkali but sparingly soluble in organic solvents.   2. Chemical properties i. Ionic forms of amino acids Amino acids bear atleast two ionizable weak acid groups, a - COOH and a -NH 2 . In solution, two forms of these groups, one charged and another uncharged, exist in protonic equilibrium: R-COOH and R-NH 3 +  represent the protonated or acidic partners in these equilibria. R-COO- and R-NH 2  are the conjugate bases of the corresponding acids.   ii. Zwitterion   Zwitter ionic structure of alanine Due to the presence of an acidic and a basic group in the same molecule, the amino acid exists largely as a dipolar ion or a zwitterion, which can react as an acid as well as a base. In zwitterion, the proton from the carboxylic group is transferred to the amino group. Thus a zwitterion carries both positive and negative cha...

  Classification of amino acids The amino acids are classified based on the nature of their R groups, in particular their polarity or tendency to interact with water at biological pH. The polarity of the R groups varies widely, from totally non polar to highly polar.   1. Non-polar, aliphatic R-group The R-group in this class of amino acids are non polar (or) hydrophobic. Six amino acids come under this class, which are  glycine, alanine, valine, leucine, isoleucine and   methionine. Glycine has the simplest structure. Methionine is one of the two sulphur containing aminoacids and has a non polar thio ether group in its side chain.   2. Aromatic R groups Phenylalanine, tyrosine  and  tryptophan,  with their aromatic side chains, are   relatively non-polar. All can participate in hydrophobic interactions. The hydroxyl group of tyrosine can form hydrogen bond with other compounds and it is an important functional group in some enzymes. Tyrosine...

  Physical properties   1. Colour and taste Proteins are colourless and usually tasteless. These are homogeneous and crystalline.   2. Solubility Solubility of proteins is influenced by pH. Solubility is lowest at isoelectric point and increased with increasing acidity of alkalinity.   3. Optical activity All protein solutions rotate the plane polarised light to the left i.e. these are levorotatory.   4. Colloidal nature Because of their giant size, the proteins exhibit many colloidal properties are: ·             Their diffusion rate is extermely low. ·             They may produce considerable light-scattering in solution, thus resulting in visible turbidity (Tyndall effect).   5. The comparatively week forces responsible for maintaining secondary, tertiary and quarternary structure of proteins are readily distrupted with resulting loss of biologi...

  Protein structure The architecture of protein molecule is complex but well organised. To understand this, a clear idea of certain basic details regarding the mode of arrangement of the structural units inside the molecule is necessary. Linder strom - Lang suggest four types of structural organisation for proteins. They are 1. Primary structure 2. Secondary structure 3. Tertiary Structure and 4. Quarternary strucutre   1. Primary structure The primary structure of protein is defined as the sequence of amino acid residues making up its polypeptide chain. The protein may be formed of one or more polypeptide chains. The amino acids are arranged in specific sequence in these polypeptide chain. The amino acid residues are linked by peptide bonds. The peptide bond is formed between the carboxyl group of one amino acid and the amino group of adjacent amino acid. Some times the adjacent polypeptide chains are linked by disulphide bonds. For example the primary structure of a protein ...