Definition of protein ,Role of protein ,Classification of protein ,protein deficiency disease
Definition of protein
The word protein is derived from the Greek word protios which means of first importance .Proteins are the most important constituents of all living matter they are the fundamental structural constituents of the body .
Proteins are high molecular weight polypeptides containing Alpha amino acids joined together by peptide linkage.
Composition :
Proteins are complex organic molecules. Their weight may range from 6000 to many Millions.
They contain the elements carbon ,hydrogen oxygen, nitrogen and sometimes Phosphorus and sulphur .
Role of proteins
• They act as enzymes and hormones
- Some transport proteins carry specific substances across membranes or body fluids .
- Storage proteins bind with specific substances like vitamins and minerals and mediate their cellular action .
- Proteins can be catabolized to release energy .
- Proteins exert osmotic pressure which helps in maintaining electrolyte and water balance .
Classification of proteins
Proteins are classified into the following groups :
• simple protein
• conjugated protein
• Derived protein
Simple protein
The simple protein simple sub-group are as follows :
- Albumin e.g. egg and serum
- Globulin
- Glutelin eg. Oryzenin of rice
- Prolamines eg. gliadin of wheat
- Sclero proteins eg. Keratin of hairs and nails ,collagen of bone and skin
- Histones eg. nucleo histones and globin of haemoglobin.
- Protamines eg. Cells of certain fish
Conjugated protein
These are simple proteins combined with non-protein group called prosthetic group. On the basis of prosthetic group the conjugated proteins are classified as follows
• Nucleo proteins eg. Virus proteins
• Phosphoproteins eg. casein of milk
• Glycoproteins eg. mucin of saliva
• Lipoproteins eg. blood and egg yolk
• Metalloproteins eg. ferritin which contains iron .
• Chromoproteins eg. heamoglobin and cytochromes .
3. DERIVED PROTEINS
There are two types :
1. Primary derived proteins
Example : metaproteins
2. Secondary derived proteins
Example : proteoses ,peptones ,peptides
Sources of proteins
The two main dietary sources of protein are
• Animal sources like milk ,meat, egg ,cheese et. These proteins contain all the the eight essential amino acids .
• vegetable sauces like pulses , cereals, beans, nuts, cakes of oilseeds etc. They are poor in essential amino acid.
° The requirement of protein for an Indian adult it is 1 gram per kg body weight
Functions of proteins
• proteins are necessary for body building since they are essential constituents of of all tissues.
• They are necessary for repair and maintenance of body tissues
• They are necessary for the synthesis of antibodies hemoglobin enzymes and hormones
• The help in cell mediated immunity and bactericidal action of leukocytes.
• Also there sources of energy (1gram yields 4 calories )
Structure of proteins
The structure of proteins is very complex but well organised. The structure of proteins can be conveniently considered under the following different levels of organisation .
1.primary structure
Primary structure refers to the sequence of amino acids which are linked through peptide bonds. The peptide Bond occurs between the carboxyl group of one amino acid and amino group .
2. Secondary structure
Secondary structure refers to the the quilling of the polypeptide chain into a helical structure
In this structure there is hydrogen bonding between neighbouring amino acid which occurs at the sites where the peptide chain is folded to form a helix .
Three Types of secondary structures are possible :
• Alfa-helical structure
• Reverse turn
• pleated structure
3. Tertiary Structure
Tertiary structure refers to the folding of protein chain to form tight and compact three dimensional structure .
It is only such as confirmation which is biologically active. Protein in this confirmation is called native protein.
Such an intricate folding on the tertiary structure brings closer the the amino acids which are for away.
This structure is further strengthened by the following forces :
• Hydrophobic interactions
• Hydrogen bonds
• Ionic bonds
• Disulfide bonds
4. Quaternary structure
It is the IV level of organisation in protein structure. It is shown by protein containing more than one polypeptide chain
These peptide chains are further strengthened by ionic bond hydrogen bond hydrophobic interactions etc.
Protein deficiency diseases
Various diseases are caused by dietary deficiency of proteins and amino acids some disease are caused by abnormal metabolism of proteins .
Dietary deficiency of proteins
Deficiency of protein in diet produce two important diseases in children. They are kwashiorkar and marasmus .
• Kwashiorkar
It is a disease of protein energy malnutrition. It occur in one year above children when they change from breast feeding to a diet low in protein .
The features are :
• Retarded growth and generalized edema
• Skin changes like pigmentation thickening cracks and ulceration
• Hair is thin and Spares the colour is radish to Grey.
Note :- diet rich in protein like milk egg and Soybean are helpful in the treatment of kwashiorkar .
• Marasmus
It occurs below 1 year in children .It occurs due to a diet very low in protein and calories . It occurs due to early termination of breast feeding and feeding with very dilute Cow's milk .
The features are :
• The child is very thin with no subcutaneous fat.
• Muscles are severely wasted .
• Head is large and Limbs are thin like sticks
• Weight of the child it is greatly reduced .
• Unlike kwashiorkar there is no oedema.
NOTE :- marasmus is is treated in the the same way as kwashiorkar by providing diet rich in protein .
Abnormal metabolism of proteins
There are following few such diseases :
• Phenylketonuria
• Alkaptonuria
• Albinism
• Tyrosinosis
Phenylketonuria
It is a metabolic disorder of protein .It occurs due to the absence of the enzyme phenylalanine hydroxylase . So phenylalanine cannot be converted to tyrosine .So this lead increased excretion of phenylalanine and it's catabolites like phenylpyruvic acid and phenyl lactic acid. The manifestation of this disease are mental retardation and seizures. Also diminished pigmentation are hair and skin can occur .
It is treated by giving a diet with very low levels phenylalanine .Tyrosine constitutes an essential amino acid in these patients and it must be provided in diet .
Alkaptonuria
It is a metabolic disorder associated with abnormal metabolism of tyrosine. It is caused by the absence of the enzyme homogentisate oxidase. So homogentisic acid accumulates in the tissues and blood and also it appears in urine. Urine containing homogentisic turns black in colour due to oxidation and exposed to air.
Alkaptonuria does not produce in clinical manifestation. In chronic patients it may produce darkening of the cartilage and tendons. This condition is called as ochronosis .
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