Chapter 4 :- Proteins

Introduction:- Proteins are compounds found in all living cells, in animals and plants. They play a variety of important roles and are essential to maintain the structure and function of all lifeforms. The word ‘protein’ is derived from the Greek word protos, meaning “primary” or “first”. Proteins are vital for the growth and repair, and their functions are endless. Each and every property that characterizes a living organism is affected by proteins, whether it is a bacteria or a human body.

Proteins perform many functions which are essential for life. The building blocks of proteins are the twenty naturally occurring amino acids. The chemical and physical structure of amino acids and proteins, describe the topology proteins and discuss an important enzyme and penicillin amidase. These amino acids are liberated when proteins are hydrolyzed. Proteins are the polymers of -amino acids.

 

• Proteins are made of long chains of amino acids.
• Proteins are composed of carbon, hydrogen, oxygen and nitrogen arranged as the strands of amino acids.
• The digestive system breaks down protein containing food into individual amino acids.
• The body then resembles the amino acids in different orders to make new proteins, which can be used for growth, repairing tissues, hormones and as enzymes.
• As proteins are made of amino acids joined together by peptide bonds amino acids can be called the basic molecules of life.

(A.) Basic structure and properties:-

Structure of Proteins-

The 20 amino acids commonly found in proteins are joined together by peptide bonds. The complexity of protein structure is best analyzed by considering the molecule in terms of four organizational levels, namely primary, secondary, tertiary and Quaternary.

Primary structure:-

Primary structure is the order in which the amino acid are covalently linked together. The primary structure is the one-dimensional first step in specifying the three-dimensional structure of a protein. Understanding the primary structure of proteins is important because many genetic diseases result in proteins with abnormal amino acid sequences, which cause improper folding and loss or impairment of normal function.

Secondary structure:-

Secondary structure is the arrangement in space of the atoms in the peptide backbone. The α-helix and β-pleated sheet arrangements are two different types of secondary structure. In many proteins the folding of parts of the chain can occur independently of the folding of other parts. Such independently folded portions of proteins are referred to as domains or super-secondary structure.

Tertiary structure:-

Tertiary structure includes the three-dimensional arrangement of all the atoms in the protein, including those in the side chains and in any prosthetic groups. A specific three-dimensional shape of a protein resulting from interactions between R groups of the amino acid residues in the protein.

Quaternary structure:-

Many proteins consist of a single polypeptide chain, and are defined as monomeric proteins. However, others may consist of two or three polypeptide chain that may be structurally identical or totally unrelated. The arrangement of these polypeptide subunits is called the quaternary structure of the protein. All the subunits are held together by the non-covalent interaction like hydrogen bond, ionic bond and hydrophobic bond.

Properties of Protein:-

A protein is a biological macro molecule composed of one or more chain of amino acids linked by peptide bonds. In general, we speak of protein when the string contains more than 50 amino acids. For smaller sizes, we speak of peptide and polypeptide, but more often they are simply “small protein”.

Solubility in Water

1. The relationship of proteins with water is complex. The secondary structure of proteins depends largely on the interaction of peptide bonds with water through hydrogen bonds.
2. Hydrogen bonds are also formed between protein (alpha and beta structures) and water. The protein-rich static ball are more soluble than the helical structures.
3. At the tertiary structure, water causes the orientation of the chains and hydrophilic radicals to the outside of the molecule, while the hydrophobic chains and radicals tend to react with each other within the molecule (cf. hydrophobic effect).
4. The solubility of proteins in an aqueous solution containing salts depends on two opposing effects on the one hand related to electrostatic interactions (“salting in”) and other hydrophobic interactions (salting out).

(B.) Types of proteins based on the origin (plant/animal):-

Classification of protein:-

1. On the basis of their quality( i.e. amino acids present in them) / function

• Complete proteins – these proteins contain all essential amino acids in sufficient proportion and amount to meet the body’s growth need and for repair of tissue cells.

Complete protein food have high biological value (BV). It is defined as the amount of absorbed nitrogen retained in the body. Eg. – egg, milk, meat.

• Partially complete proteins : those proteins in which one or more  essential amino acids are present in inadequate amounts. They cannot  synthesize tissues but can maintain life.

Found in plant foods- like cereals. Pulses, nuts, oil seeds. A combination of one incomplete protein with other in diet can provide increased amount of essential amino acids. Eg. Cereals are deficient in lysine and pulses are deficient in methionine.

• Incomplete proteins : in these one or more essential amino acids are completely lacking. Thus these proteins are incapable of growth and repair of body cells. They cannot maintain life. Eg. Gelatin, zein in corn.

 

2. On basis of origin

1. Animal protein-  

• Egg- contain 13-14% protein about 2/3 of which is present in egg white and rest in egg yolk. Major proteins are ovalbumin, conalbumin and ovomucoid.
• Meat- it is the edible muscle of cattle sheep and swine. 8.9-26.6g/100grm protein. Major proteins are actin and myosin.
• Milk –cow milk contain 3.5% of protein. 1.1-4.39 g/100grm. Casein insoluble in water (80% of total milk). Whey soluble protein of milk (20% of total milk).

2.) Plant/ Vegetable protein-

• Pulses – contain on an average more than 20% protein. 9.6-43.2g/100grm. Proteins are albumin and globulin.
• Ceral grains contain protein ranging from 6 to 20 %. 4.7-12.5g/100grms. Proteins are albumin, globulin, gluten which comprises of gliadin and glutenin.
• Fresh vegetable are not good source of not good source of proteins may contain less than 1%protein.

(C.) Effect of heat on proteins (Denaturation,Coagulation):-

Denaturation:- Denaturation is defined as any non-proteolytic modification in the original structure of the native proteins, giving rise to definite changes in physical, chemical, and biological properties. Denaturation is brought about by the following:

1. Denaturing agents such as acids,alkalis, salts.
2. Increase in temperature 
3. Extensive beating 

Stages in Heat Denaturation:-

1. Unfolding of helix of the protein molecules as the cross-links holding the helix is disrupted. 
2. R groups are exposed. Rebonding takes place between adjacent R groups of protein molecules leading to aggregation of the molecules, bringing about increased viscosity. This is the first stage in denaturation which involves structural alteration (1st stage).

When sufficient proteins have united, the protein molecules are no longer dispersed as a sol. At this stage, the protein is said to have coagulated (second stage),i.e., water is held in the capillary spaces formed by united protein molecules and the coagulated proteins form a 'gel'.

If the liquid separates from the coagulated protein, the protein is said to be 'precipitated ' or 'flocculated', i.e., 'curdling ' takes place (third stage of denaturation). These stages can be observed while cooking scrambled eggs. If it is overcooked, liquid separates out.

Effects of Denaturation:-

Properties of denatured proteins are completely different from their native form.

1. Denatured proteins are easily attacked by proteolytic enzymes,e.g., cooked meat is more easily digested than raw meat. 
2. They slow decreased solubility, e.g., cooked egg white is not soluble in water. 
3. They lose their biological activity as enzymes are destroyed, e.g., browning does not take place in boiled potato.
4. Denatured proteins lose their ability to crystallize. 
5. There is an increase in viscosity of food. 
6. Heat denaturation results in improved flavour and texture, e.g., cooking improves flavour in meat,and eggs give structure and improve texture of cakes. 
7. Denaturation of food is irreversible unless it occurs under very mild conditions. 

Coagulation:- Coagulation is a vital chemical process in which proteins in a liquid state is changed to a solid state and it can only occur 

after the substance has gone through denaturation. 
However, coagulation can only occur in the presence of heat.
When a protein substance is going through coagulation, the strands of protein or amino acids start to vibrate from the applied heat and therefore making the strands move around and tangle with the rest of the protein strands, resulting in clumps of protein. 

A linking example is steak. 
When steak first touches a hot surface like a heated frypan, the proteins are denatured.
However, as the heat applied to the steak prolongs, it starts to coagulate and result  in a thoroughly cooked piece of meat.

Effect of heat on Coagulation/milk proteins:-

1. Heat denatures and coagulates serum or whey proteins which settle to the bottom of the container. 
2. When milk is heated in an uncovered vessel a skin forms on top. This is because of evaporation of water and concentration of casein, which blocks calcium salts and milk fats. The skin holds steam and makes milk boil over. A foam minimizes skin formation and this is the reason why hot coffee, cocoa,etc.,are whipped to induce a foam or served with whipped cream. Casein is otherwise insensitive to heat.
3. The colour and flavour of condensed milk is because of browning caused by lactose and milk protein. 

(D.) Functional properties of proteins (Gelation, Emulsification, Foamability, Viscosity):-

FOAMABILITY:-

Proteins as that present in egg –like ovomucin, ovoglobulin & conalbumin can be beaten into a form. Egg white is a viscous sol with proteins dispersed in it. As air is incorporated into the liquid, the proteins molecules collect   at the air –water interface. When more air is incorporated, the water layer gets thinner and protein molecules get stretched and unwind from their coiled structure.

Surface denaturation takes place exposing the reactive ‘R ‘groups along the protein molecules. These groups unite & give rigidity to the foam. Surface denaturation makes the foam rigid and when heat is applied the proteins coagulate forming a permanent foam

Stages of fresh egg white foam formation

1) Foamy stage 2) Soft peak stage 3) Stiff peak stage 4) Dry stage

• Foamy stage- bubbles are formed on surface, but a bit liquid remains at the bottom of the bowl. Air bubbles are large, opalescent, showing changing colors. Foam is very unstable; mixture is still fluid. Foam at this stage is used for clarifying, emulsifying & thickening of foods. Acid salt & vanilla are added this stage.
• Soft peak stage – As beating whipping is continued more air is beaten into the white, causing a whiter, opaque appearance. Air bubbles becomes more small, all egg white exist as foam slight drainage occur on standing foam is soft and flow slowly. Peaks just bend over. Such foams are suitable for folding into a batter and used for soft meringues or for incorporation with other ingredients in most products.
• Stiff peak stage- Air cells become fine. Foam become stiff & very white. Peaks stand up straight. Stability of foam is excellent, but the foam is not easily folded into other ingredients. Foam at this stage are used for hard meringues, soufflé, omelets.
• Dry peak stage-When beating is continued beyond stiff peak stage dry foam is produced, it is very white but dull. Breaks into small flakes, not stable. At this stage, the egg white have little utility in food preparation.

GELATION:-

Gelatin is a partially degraded protein prepared form collagen (commonly used as gelling as agent). Collagen is the intercellular cementing substance between cells. Skin, ligaments and bones (cattle, chicken, pigs & fish) are hydrolyzed by dilute acid or alkali, breaking collagen molecules into shorter fibrous molecules called gelatin. Gelatin contains a large proportion of amino acids which have a great affinity for water. The long thin fibers of gelatin help in forming firm gels at low temperature.

It may be used as a stabilizer, thickener or texturizer in foods such as yogurt, cream cheese and margarine. Used in fat reduced foods to stimulate the mouthfeel of fat and to create volume without adding calories.

VISCOSITY:- 

It is a resistance to the flow of a liquid. It is a measure of the resistance of a fluid to deformation under shear stress. It describes a fluid’s internal resistance to flow and maybe thought of as a measure of fluid friction. Thus water is thin having low viscosity while vegetable oil is thick having a high viscosity

EMULSIFICATION :– 

The breakdown of fat globules in the duodenum into tiny droplets, which provides a larger surface area on which the enzyme pancreatic lipase can act to digest the fats into fatty acids and glycerol. Emulsification is assisted by the action of the bile salts

Emulsifying properties of proteins basically depend on two effects:

(1) a substantial decrease in the interfacial tension due to the adsorption of the protein at the oil-water interface and

(2) the electrostatic, structural and mechanical energy barrier caused by the interfacial layer that opposes the destabilization processes. The knowledge on the biochemical and physicochemical characteristics, interfacial behaviors and emulsifying properties of proteins will help us understand the structure-function relationship of the emulsifying proteins.

(E.) Commercial uses of proteins in different food preparations ( like Egg gels, Gelatin gels, Cakes, Confectionary items, Meringues, Souffles, Custards, Soups, Curries, etc.) :-

1. Form foams 
2. Bind water and form viscous sols and gels 
3. Get coagulated by heat
4. Exhibit emulsifying properties 
5. Show enzymatic activity 

Key Terms:-

• Coagulation:- Solidification of a liquid protein on heating. 
• Collagen:- Protein in bones, skin,and connective tissue which is completely hydrolyzed to gelatin by heat during cooking. Collagen fibres predominate in meat.
• Denaturation:- Relaxation of the tertiary structure to the secondary structure, accompanied by decreased solubility of a protein. 
• Organoleptic:- Qualities of a food which appeal to the sense of taste,smell, etc.
• Rigor mortis:- Stiffening of muscles immediately after animals have been slaughtered. 
• TVP :-Textured vegetable protein