PROTEIN

Proteins were first described by the Dutch chemist Gerardus Johannes Mulder and named by the Swedish chemist Jöns Jacob Berzelius in 1838. Mulder in 1838 used the term proteins for the high molecular weight nitrogen-rich and most abundant substances present in animals and plants.

Introduction

Proteins are the buildings of muscle mass. Amino acid is monomer of proteins and these are organic compound containing a carboxyl group (COOH) and amino group (NH2). e.g., glycine, lysine.

Proteins are predominantly constituted by 5 major elements. These are namely as Carbon, Hydrogen, Oxygen, Nitrogen, and Sulphur.

Classification of proteins

Proteins are classified into 2 groups-

1) Fibrous protein

2) Globular protein

Fibrous proteins are insoluble in common solvent, such as water, diluted salt solution, diluted acid solution, and organic solvent. e.g.

Keratin - skin, hair and nails

Collagen - white connective tissue of bone

Elastin - yellow elastic fibre

Globular proteins are soluble in water. It is of 2 types:

1) Simple globular protein

        - soluble in distilled water. e.g., albumin, histone.

        - insoluble in distilled water. e.g., globulin.

2) Conjugate globular protein

        - globular proteins are combination with the non-proteins. e.g., haemoglobin.

Classification of proteins on the basis of function

There are 2 types of functions of protein.

a) Structural functions:

        - certain proteins perform brick and mortar roles and are primarily responsible for  structure and strength of body. e.g.,

            collagen and elastin (found in bone matrix, vascular system and other organs)

            α-keratin (alpha-keratin), present in epidermal tissues.

b) Dynamic functions:

        - the dynamic functions of proteins are more diversified in nature. These include proteins acting as enzymes, hormones, blood clotting factors, immunoglobulins, membranes receptors, storage proteins, besides their function in genetic control, muscle contraction, respiration, etc.

       - proteins performing dynamic functions are appropriately regarded as the working horses of cell.

Classification of proteins on the basis of structure

1. Primary structure - linear sequence of amino acids forming the backbone of proteins.

2. Secondary structure - spatial arrangement of protein by twisting of polypeptide chain.

3. Tertiary structure - 3D structure (3 dimensional structure) of a functional protein.

4. Quaternary structure - some of the proteins are composed of 2 or more polypeptide chains referred to as subunits. Spatial arrangement of these subunits is known as quaternary structure.

Dietary sources

Meat, fish, milk, egg, cheese, beans and peas.

Digestion of proteins

In Mouth, protein is unchanged, due to absence of proteases enzyme in saliva.

In Stomach, protein converts into polypeptides and amino acids, in the presence of pepsin. Pepsin is protease proenzyme.

In Small Intestine, Pancreatic digestive enzymes perform the majority of protein digestion. The major proteolytic enzymes include trypsin, chymotrypsin, elastase, and carboxypeptidase. These enzymes of the exocrine pancreas digest proteins down to short chains of a few amino acids, termed "Oligopeptides". The final stage of protein digestion occurs on the brush border of the small intestine epithelium. Here, membrane-bound peptidases complete digestion of oligopeptides to either single amino acids or dipeptides and tripeptides. The end result of protein digestion is the production of single amino acids or dipeptides and tripeptides which are amenable to epithelial absorption.

Absorption of proteins

Free amino acids, dipeptides and to some extent tripeptides are absorbed by intestinal epithelial cells. Di-peptides and Tri-peptides, after being absorbed are hydrolysed into free amino acids in cytosol of epithelial cells. The activities of dipeptidases are high in these cells. Small intestine possess an efficient system to absorb free amino acids.

Fate of amino acids

Amino group of amino acid + alpha ketoglutarate is convert into Glutamate.

Then, glutamate convert into NH3 and alpha ketoglutarate.

Later, NH3 convert into Urea through Urea cycle and excreted through urine.

Nitrogen Equilibrium

Dietary protein is almost an exclusive source of Nitrogen to body. Nitrogen balance represents the protein utilization and its loss from the body.

Nitrogen balance is determined by comparing the intake of Nitrogen and excretion of Nitrogen.

A normal healthy adult is in a Nitrogen Equilibrium,

daily dietary intake (I) is equal to the loss through urine (U), feces (F) and sweat (S).

                                                I=F+S+U

There are 2 types of Nitrogen equilibrium (or nitrogen balance).

1. Positive nitrogen balance - nitrogen intake is higher than the output, and observed in growing children, pregnant women or during recovery after serious illness.

2. Negative nitrogen balance - nitrogen output is higher than the input, and prolonged negative nitrogen balance may be lead to death. Observed in children suffering from Kwashiorkor or Marasmus diseases. It may occur due to inadequate dietary intake of protein.

Formation of Ammonia

NH3 is being liberated in the metabolism of amino acids and other nitrogenous compounds. Production of NH3 occurs from amino acids, biogenic amines, amino group of purines and pyrimidines and by the action of intestinal bacteria on urea. There are some reactions which generate ammonia into body. e.g., Glutamate = alpha ketoglutarate + NH3

Detoxification of NH3

In liver, CO2 and NH3 forms carbophosphate which then enter into urea cycle. Most of NH3 produce in blood is taken to the liver for the conversion of urea.

Toxicity of NH3

 Elevation of blood ammonia conc. is harmful for the brain. Higher conc. leads to comma, and finally death.

Hyperammonia - increased level of NH3 in blood, and may be genetic or acquired impairement in urea synthesis cycle.

Formation of Urea

The process of formation of urea is also known as urea cycle, was discovered by Hans Krebs and Kurt Henseleit in 1932.

The urea cycle (also known as the ornithine cycle) is a cycle of biochemical reactions that produces urea (NH₂)₂CO from ammonia (NH₃). This cycle occurs in ureotelic organisms. The urea cycle converts highly toxic ammonia to urea for excretion.

Image of Urea

Formation of Uric Acid

Uric acid is a waste product created by/during breakdown of Purines. Uric acid is cleaned out of blood by Kidneys, and passes out of body along with urine.

Formation of Creatinine

Creatinine is a waste product produced by muscles from the breakdown of a compound called creatine. Creatinine is removed from the body by the kidneys, which filter almost all of it from the blood and release it into the urine.

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