Proteins

 PROTEINS:

 The word " protein" means that which is of  first importance. Indeed they are of the greatest  importance in human  nutrition. Proteins are composed of carbon, hydrogen, oxygen, nitrogen and sulphur in varying  amounts. Some proteins also contain phosphorus and iron and occasionally  other elements. Proteins differ from carbohydrates and fat in the respect that they contain nitrogen.Protein are made up of simpler substances, called amino acids. These are the building blocks of protein.



These are large molecules made up of one or more chain of   amino acids in specific order. Almost 43 per cent of dry weight of human bodies is due to proteins. 

Proteins are polypeptides. They are linear chains of amino acids linked by peptide bonds . 

 Each protein is a polymer of amino acids. As there are 20 types of amino acids ( e.g., alanine, cysteine, proline, tryptophan, lysine, etc.), a protein is a heteropolymer has only one type of monomer repeating 'n' number of times . This information about the amino acid content is important as later in your nutrition lessons, you will learn that certain amino acids are essential for our health and they have to be supplied through our diet. Hence, dietary proteins are the source of essential amino acids. Therefore, amino acids can be essential or non__ essential. The latter are those which our body can make, while we get essential amino acids through our diet/ food. Proteins carry out many functions in living organisms, some transport nutrients across cell membrane, some fight infectious organisms, some are hormones, some are enzymes, etc. Collagen is the most abundant protein in animal world and Ribulose bisphosphate Carboxylase__ Oxygenase ( RuBisCO) is the most abundant protein in the whole of the biosphere.

   The name " protein", meaning " preeminent" or " first" is well chosen by Berzelius³ ( 1838) because the proteins play a very significant role in the structure and metabolism of the cell ( G. Proteios = first place). 

Percentage:

The proteins form about 12% of the cell contents⁴ . They are next to water as a component of the cell.

Structure of proteins :

Proteins have four structural levels: primary, secondary, tertiary and quaternary. 

1): Primary Structure:  

The linear sequence of amino acid units in a polypeptide chain is called the primary (1°) Structure of a protein molecule. This sequence is determined by the sequence of  nucleotide triplets in the DNA of the cell's nucleus or nucleoid. The enzyme ribonuclease is a protein with primary structure only. So is myoglobin. A protein having a single polypeptide chain is called  monomeric protein. 

   The work of finding out the amino acid sequence of proteins was pioneered by the double Nobel prize winner Frederick Sanger. He worked out the amino acid sequence of boving insulin which has 51 amino acids units.

2): Secondary Structure: 

A polypeptide chain is often coiled into a regular spiral, called the alpha helix, to have a 3__ dimensional form. The amino acids are so placed that their side chains extend outward from the spiral. The helical structure is maintained by a series of regularly spaced intramolecular hydrogen bonds formed between the carboxyl ( __ CO) and amino ( __ NH) of the amino acid units in the successive turns of the spiral. The alpha coil may at places be much less regular, forming random coils. Two or more polypeptide chains may join together by intermolecular hydrogen bonds and may bend into parallel folds to form a  ß_ pleated sheet. The alpha helix, random coil and ß__ pleated conformations are termed the secondary (2°) Structure of proteins. Keratin of hair and myosin of muscle have helical structure, and fibroin, the protein in silk fibres produced by insects and spiders  had pleated structure. The protein lysozyme occurs in both helical and pleated forms.

Position of certain amino acids is crucial for the form and function of the proteins. A change of just one amino acid alters the normal haemoglobin to the haemoglobin of sickle __ cell anaemia. 


3): Tertiary Structure: 

The helical polypeptide molecule may fold on itself and assume a complex but specific form__ spherical, rod__ like or any form in between these. These geometrical shapes are known as the tertiary (3°) Structure of protein molecules. The coils and folds of the polypeptide molecule are so arranged as to hide the nonpolar amino acids chains inside and to expose the polar acids chains inside and to expose the polar acids chains. The tertiary structure of a protein brings distant amino acid sidechains nearer to form active sites of enzyme proteins. The tertiary structure is maintained by weak bonds, such as hydrogen, ionic disulphids* and hydrophilic __ hydrophobic bonds, formed between one part of a polypeptide and another. The biological activity of a protein  molecule depends largely on the specific tertiary structure. This functional form of protein is called native state. This structure is easily disrupted ( denatured) by pH, temperature and chemicals, stopping the function of proteins. Tertiary structure is found in globular proteins. 

4) Quaternary Structure: 

Certain proteins consist of a bundle of two or more polypeptide chains, each having appropriate primary, secondary and tertiary structure. Such proteins have a quaternary (4°) Structure. Insulin, collagen has two polypeptide chains linked by sulphur bridges.Collagen has three helical polypeptides supercoiled to form a rope __ like structure of great strength. Haemoglobin has four helical polypeptide chains, two œ__ chains and two ß chains.The liver protein ferritin is formed of 20 identical polypeptides. The enzyme pyruvate dehydrogenase complex has 72 polypeptides. Individual polypeptide chains of a quaternary protein are called subunits and the active protein itself is termed multimer.

   A protein having two or more polypeptide chains is termed oligomeric protein. 

   Primary to Higher Structures. Once amino acids are linked up in proper order to form th primary structure, the subsequent structures follow automatically, governed by the attractive and repulsive forces within different parts of the polypeptide. 

Functions:

The proteins have a number of functions. 

(i) Structural Components : Many proteins primarily serve as building material of cells and tissues.  They are called the structural proteins. They are essential for growth, repair and reproduction. They are components of__ 

(a) Cell organelles such as membranes, ribosomes, mitochondria, plastids, microfilaments, microtubules and chromosomes. Walls of pollen grains contain proteins. 

(b) Tissues,e.g., matrix of connective tissues contains collagen and elastin protein fibres, matrix of cartilage contains the protein chondrin, and the matrix of bone has the protein ossein.

(c) Protective coverings,such as horny layer of skin, and exoskeletal structures, viz., scales, feathers, hair, wool, nails, claws, hoofs and horns, are all formed of the white  insoluble proteins fibroin. Cobweb of spiders is also formed of fibroin.


(ii) Metabolic Regulation: Certain proteins form enzymes, some coenzumes, and many hormones ( insulin, parathormone) and regulate metabolism. They are  called functional proteins. 



(iii) Fuel:  The proteins may act as fuel to supply energy when glucose or fats are in short supply.

 (iv) Transport : The proteins also serve as physical carriers of other functionally important smaller molecules. A protein ' globin', for example, carries  the pigment 'heme', the two forming haemoglobin. The heme transport oxygen  in the body. Myoglobin in muscle cells stores oxygen. The œ__ globulin of blood transport thyroxine and bilirubin. The ß__ globulin carrier vitamins A, D and K and cholesterol and ions in the blood. Serum albumen transport fatty acids and lipids in the blood. In plants, P__ protein present in the sieve tubes is vibratile.It is believed to help  in the transport of organic compounds through phloem. Protein carrier molecules ( permeases) in cell membranes actively transport molecules across the membranes. 

(v) Visual Pigments.Rhodopsin of rod cells and iodopsin of cone cells of retina are proteins. They enable animals to see.

 (Vi) Movements: Myosin and actin make  muscle fibres contractile to bring about movements. These are called contsractile proteins.

(vii) Defence: Some proteins acts as antibodies which defend the organism against foreign materials entering the body.

(viii) Raw Materials: Proteins may break down into amino acids to provide materials to synthesize new proteins or other compounds ( carbohydrates and fats).

(ix) Biological Buffers: Proteins also assist in maintaining a balance of acidity and alkalinity in the cell by combining with excess acids and bases. They are, thus, biological " buffers" against acids and bases that might harm the cell.

(x) Receptor Molecules: The receptor molecules of the cells are proteins. The hormone molecules bind to these receptors to produce their effects in the cells.

(xi) Toxins:Many toxins of animals are proteins, e.g., snake venom. The ricin of castor oil plant, Ricinus Communis, is a highly toxic protein. If eaten, acts as a violent irritant and may be fatal. Bacterial toxins are also proteins. 

(xii) Support:Collagen and keratin provide rigidity and support to the tissues, organs and body. 

(xiii) Osmotic Pressure:Protein contents of  a cell influence the osmotic pressure in it.

(xiv) Blood Clothing :The proteins thrombin and fibrinogen help in blood clotting to check bleeding from injuries. 

(xv) Regulation of Gene Action:The repressors that regulate gene action are proteins in nature.

(xvi) Pollination:Pollen grains have specific proteins in the wall to help them in pollinating the stigmas of the flowers of specific plants only.

(xvii) Storage Proteins:These include ovalbumen of egg white, glutens of grains, casein of milk. Ferritin stores iron. It also regulates iron transport from the intestinal lumen to the plasma.

(xviii) Perception of Pain and Pleasure:Small peptides called enkephalins, having only 5 amino acids each, bind to specific receptor sites in the brain,  and influce the perception of pain and Pleasure. 


(xix) Blood Transfusion:Compatibility of antigens on the RBCs with antibodies in the plasma determines the type of blood group fit of blood groups fit for transfusion. Both antigens and antibodies are proteins. 

(xx) Sweetest Substance: Monelline, a protein derived from an African berry is 2000 times sweeter than sucrose. Being nontoxic and noncaloric, it may prove suitable for the diabetics. 


(xxi) Mucus: Muscus secreted by mucous glands protects the delicate cellular lining of alimentary canal from digestive enzymes and friction with food.

Composition:

The proteins are composed of carbon, hydrogen, oxygen, nitrogen and sulphur.Certain proteins may contain phosphorus, iron or other elements also. The proteins are linear,unbranched polymers of amino acids.Twenty amino acids commonly occur in natural proteins. In a protein molecule, the amino acid units are linked together by peptide bonds( ____ NH____ CO) formed between the amino and the carboxyl groups of successive amino acids. The order in which amino acids occur is specific for a particular polypeptide. The protein molecules are very large and highly complex, and have very high molecular weight⁵.

Insulin⁶( human) has 22 and 31 amino acids in it's two polypeptide chains, spinach protein ferredoxin has 97 amino acids, and the human serum albumin has 582 amino acids.

Variety:

The proteins show an unlimited variety because a polypeptide may have any number of twenty amino acids present in any proportion and arranged in any sequence. Even a small polypeptide with 50 amino acids units can have 20⁵⁰ different sequences. There are 1000 to 2000 types of proteins in a bacterial cell, and over 10,000 types of proteins in the human body. 

Properties:

The proteins have the following important properties __ 

( i)  Macromolecules:Proteins are very large molecules. Their molecular weight varies from the minimum of 4500 for adrenocorticotrophin to the maximum of 4,600,000 for pyruvate dehydrogenase. 

 (ii) Solubility: The proteins, due to the large size of their molecules, from colloidal solutions and do not diffuse through cell membranes. They are, therefore, hydrolysed  during digestion to their constituent amino acids by breakdown of the peptide bonds. The amino acids have relatively small and can diffuse through the cell membranes. 

(iii) Permeation: Proteins cannot pass through cell membranes. They enter or leave the cells by endocytosis and exocytosis. Normally, each cell prepares its own proteins by polymerisation of amino acids.

 (iv) Amphoteric: The proteins are amphoteric. The amino group is basic and can combine with acids, the carboxyl group is acidic and can combine with bases.Becauses of having both the groups, the proteins are very important buffers in the cells.

(v) Reactivity: Dur to the presence of different groups on the side chains of their amino acids, the proteins may  bind to a  variety of inorganic ions, chemical groups and organic molecules in the cells. 

(vi) Denaturation and Renaturation: Extreme changes in temperature or pH disrupt the bonds that maintain tertiary protein structure, leading to loss of the functional activity of proteins. This change is called denaturation. This is why no organism can survive temperatures above 45___ 55°C. Colour and appearance of an egg change on cooking due to denaturation of its proteins. Egg albumin becomes opaque during cooking because the denatured proteins are insoluble and solidify. 

   A denatured protein may spontaneously refold into its original structure provided conditions are suitable. This is called renaturation.This shows that tertiary structure is determined by primary structure. 

*Disulphide bond forms between two sulphur atoms.

Types: 

The proteins may be classified in three ways on the basis of the shape, components or nature of their molecules. 

A: Shape of Molecules: Proteins are of 2 types according to the shape of their molecules: fibrous and globular. 

(i) Fibrous Proteins: The fibrous proteins have polypeptide chains spirally wound to form fibres. They have secondary structure. They are insoluble in water, are tough and are contractile. They are nonenzymatic and structural in role. They common examples are collagen of connective tissue, actin and myosin of muscles, Keratin of scales, feathers, hair claws, nails, horns and hoofs, silk in spider web.


Fibrinogen is a fibrous but soluble protein of blood plasma. It forms insoluble fibrin when blood clots.

(ii) Globular Proteins: The globular proteins have polypeptide chains coiled about themselves to form spherical molecules. They are noncontractile and have tertiary structure. They may be enzymatic or nonenzymatic. Small globular proteins, such as histones, are usually soluble in water and are not coagulated by heat. Large globular proteins are insoluble and are coagulated by heat. These include egg albumen, serum globulins, haemoglobin of erythrocytes, glutens of wheat and rice, most of the enzymes and many hormones. 

B: Components of Molecules: The proteins are of three types according to their constituents: simple, conjugated and derived. 

(i) Simple Proteins: The are formed of amino acids only. They include albumens of egg, serum and milk; globulins of egg, serum and milk; collagen of connective tissues; keratin of hair, feathers, claws, nails, hoofs, horns and scales; gluten of wheat and rice ; histones, protamines and enzymes. 

(ii) Conjugated Proteins: These are composed of a protein molecule joined to a nonprotein group called the prosthetic group.These include__.

(a) Nucleoproteins: The prosthetic group is nucleic acid. Deoxyribonucleoproteins ( DNPs) of chromosomes and ribonucleoproteins ( RNPs) of ribosomes are examples.

 (b) Chromoproteins: The prosthetic group is some pigment. Common examples are haemoglobin, haemocyanin,cytochromes, visual purple, and flavoprotein.

 (c) Phosphoproteins:  The prosthetic group is phosphoric acid. Examples: casein of milk,vitellin of egg yolk.

(d) Glycoproteins: The prosthetic group is a carbohydrate. The glycoproteins occur in blood plasma, egg albumen, saliva,mucus and blood group substances. 

(e) Lipoproteins: The prosthetic group is mainly phospholipid. The lipoproteins occur in membranes, milk( casein), blood serum and egg yolk (vitellin).

(f)Metalloproteins: The prosthetic group is some metal ion. The enzyme carbonic anhydrase is an example. It contains zinc ions besides amino acids in its molecule. Ferritin is another example. It is iron and protein complex.

(iii) Derived Proteins: These are formed by partial breakdown of natural proteins in digestion or otherwise. They include proteoses, proteoses, peptones and short peptides formed in protein digestion; are soluble in water and diffusible. Proteoses are intermediate products of protein hydrolysis between protein and peptones.


C : Nature of Molecules: The proteins are of 2 types according to the nature of their molecules: acidic and basic.

(i) Acidic Proteins: These have  acidic amino acids ( aspartic acid, glutamic acid), exist as anions and behave as acids. Examples: most blood proteins. 

(ii) Basic Proteins: These are rich in basic amino acids ( lysine, arginine), exist as cations and behave as  acids. Examples: histones.
Proteins Specificity:Some  Conjugated Proteins and  Their Prosthetic Group 

Evolutionary Relationships shown by Polypeptide sequence


Protein Specificity :


Every cell has hundreds of different proteins.Each kind of cell has some proteins unique to itself, and these proteins make it ditinct from all other types of cells. Every species likewise has a set of proteins found in no other species. Similar species share many of their proteins, whereas dissimilar species share fewer.Study of proteins, thus,shows the evolutionary relationship among species.

  The principle of protein specificity has practical application in blood transfusion, tissue grafting and organ transplants which succeed only in animals that have similar proteins.


Sources : 

There are two main dietary  sources of proteins:_

(1) Animal sources: milk, eggs, meat fish, etc.

(2) Plant sources : Puleses, cereals, nuts, etc.

  From  nutritional standpoint, animal proteins are superior to vegetable proteins because  they contain all the " essential  amino acids" needed by the body. But animal proteins are relatively  expensive and on religious grounds, certain communities  in  India do not take animal  protein. The intake of animal protein is very low in many parts of the world, including India.


 On  the other hand, vegetable proteins are cheaper and readily available. Pulses such as Arhar, Chana and Moong contain as much as 25 per cent of protein, because of which they are called " poor man's meat". SOYABEAN is unique among pulses because  it contains over 40 per cent of protein, and that too of good quality. Cereals such as rice and wheat also contribute substantial quantities  of protein in vegetarian diets, because of their sheer bulk.In general, vegetable proteins are deficient  in one or more of the essential amino acids needed by the body; hence they  are rated inferior to animal proteins. However,  by proper combination of two or more vegetables foods ( rice and dhal), it is possible to produce a mixture capable of supplying  all the essential  amino acids comparable to good animal protein, and yet cheaper than animal protein.


This is called " supplementary  action" of proteins. The protein content of some foods is given 







Protein requirements 

It is customary  to express protein requirements in terms of body weight. The Indian Council of Medical Research  in 2010, recommended 1.0g. protein/ kg.body weight  for  an Indian adult, assuming  NPU of 65 for the dietary proteins .Daily allowance recommended by the ICMR for various population groups are given 





Effects of protein deficiency 

 The effects of protein deficiency may be summarized as below: 

(1) During pregnancy: Still _ birth, low birth weight, anaemia. 

2) Infamcy and early childhood:Marasmus, kwashiorkor, mental retardation,  stunted growth and development. 

3) Adults: Loss of weight, underweight, poor musculature,  anaemia, increased susceptibility  to infection, frequent loose  stools, general lethargy, incapacity to sustained work, delay in wound healing , cirrhosis of liver, oedema, ascites, etc.

Protein

Protein requirements  vary from  individual to individual. Apart from age, sex and  other physiological variables,  factors  like infection, worm infestation, emotional  disturbances and stress  situations  can affect  a  person's  requirement. 

 Assessment  of protein

(a)  Protein quality 

    The quality  of  a protein is assessed  by comparison  to the " reference protein" which is usually  egg protein. Two methods of assessment of protein quality  need be mentioned: 

  (i)  Amino acid  score : It is a measure  of  the  concentration  of each  essential  amino  acid in the test  protein expression  as a percentage  of  that amino acid in the  reference  protein.

            Number of mg of one 
             amino acid per g of
             protein 
Amino 
acid   =_________________________×100
score   Number of mg of the 
           same amino acid g of 
              egg protein. 

 The amino acid ( or chemical ) score is somewhere between  50  and 60 for starches, and 70 and 80 for animal foods. 


(ii) Net  protein  utilization ( NPU) : 
It is a product  of digestibility  coefficient  and biological value divided by 100. The NPU gives a more  complete expression  of protein quality  than  the amino acids  score. It is a biological method that requires special laboratory   facilities. 

                  Nitrogen retained by
                 the body
NPU=______________________________×100
           Nitrogen intake 

  In calculating  protein quality, 1 gram of protein is assumed to be equivalent  to 6.25 g of N.

  The protein requirement varies with the NPU of dietary  protein. If the NPU is low, the protein requirement is high and vice  versa. The NPU of the protein of Indian diets varies between  50 and  80.


 (b) Protein quantity 

   The protein content of  many Indian foods has been determined  and published in food composition tables. One way of evaluating  food as source of protein is to determine  what per cent of their energy value is supplied by their protein content. This is known as Protein _ Energy  Ratio (PE ratio or percentage).

                   Energy from protein
PE per =________________________×100
cent       Total energy in diet 

  
 The protein _ energy percentage value of some commonly used foods 

  If the PE is less than 4 per cent, the subject  will be unable to eat enough to satisfy  protein requirements. It is recommended that protein should account for approximately  15 to  20 per cent of the total daily energy intake.

Dietary  intakes

  It is customary to express protein requirement in terms of grams per kg of body weight. This principle applies  to all age groups, although absolute additions in units of grams of protein per day are made for  pregnancy  and lactation.

   The ICMR Expert Group suggested an intake of one gram of protein per kg  of body weight  for adult males and females,  assuming  a NPU  of 65 for  the dietary  protein. The protein intakes for individuals  of different  ages and physiological  states 

Vulnerable  groups 

 The protein requirement  for women are increased during pregnancy . For 10 kg gestational   weight  gain the requirement  increased  by 1, 7 and 23 g/ day in Ist, 2nd and 3rd  trimesters respectively; and during lactation  by about 13 g/ day ( during 0_ 6 months), over and above their normal  requirements. 

 Young  children ( 0_ 6 years) require proportionately  more protein for each kilogram  of body weight than adults. They are more vulnerable  to malnutrition. 

  The ICMR  Expert Group has not made any recommendations  for the elderly .It seems reasonable, in our present state of ignorance , to assume that the requirement of the aged are not less than that for young adults, because is less efficient  in the elderly. 

 All estimates  of protein requirement are valid only when the energy  requirements are fully met. If the total energy intake is inadequate  some dietary  protein will be diverted  to provide energy. It is now accepted  that there are no body protein  stores which can be filled up by a high protein intake.

At present  there is no evidence  that higher intake of protein confer greater benefit although the possibility cannot be ruled out. Most people, if they can, apparently  choose to eat more protein than the physiological  requirement. The question remains whether  high protein intake far from being beneficial  may actually  be harmful .

Amino  acid  requirements 

The protein intake must also satisfy  the need for essential  amino acids. The 2007 WHo Expert Committee Report on energy and protein  requirements gives current estimates of amino acid requirements (in mg/kg per day) for adults

 New tissue  cannot be formed unless all the essential  amino acids ( EAA) are present in the diet. The requirement of EAA decreases sharply as one advances in age. The quality of the diet is far more critical  for the infant than for adult.









































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