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                                                                            Protein properties

Name – Priyanka

Roll. No.- 221155081015

 All proteins contain carbon, hydrogen, oxygen, and nitrogen, the presence of nitrogen distinguishing them from carbohydrates and fats. On an average protein contain 16% nitrogen. Some proteins also have Sulphur and a few proteins have phosphorus and other elements may be present. Protein is colorless and tasteless. Cells contain a very large number of proteins. All proteins typically range in size from a spherical structure of a crystal to a long fibrillar structure. The protein’s molecules are amphoteric type, that is they act as acids and alkalis both like amino acids. According to the net charges, these biomolecules can form a salt with both cations and anions. Proteins are formed by the repetition of amino acids, linked by peptide bonds, any protein has –NH₂ and –COOH terminal groups. –NH₂ group is known as N-terminal and –COOH group is called C-terminal. When boiled with amino acids they yield amino acids in varying molar ratios. To date 20 amino acids have been isolated from proteins, but occasionally a protein will contain an amino acid not commonly found in most other proteins.

                             Physical Properties of Proteins

Color and Taste-
Proteins are colorless and usually tasteless. These are homogeneous and crystalline.

Shape and Size-
The proteins range in shape from simple crystalloid spherical structures to long fibrillar structures. Two distinct patterns of shape have been recognized.
A. Globular proteins- These are spherical in shape and occur mainly in plants, esp., in seeds and in leaf cells. These are bundles formed by folding and crumpling of protein chains. e.g., pepsin, edestin, insulin, ribonuclease etc.
B. Fibrillar proteins- These are thread-like or ellipsoidal in shape and occur generally in animal muscles. Most of the studies regarding protein structure have been conducted using these proteins. e.g., fibrinogen, myosin etc.

Molecular Weight-
The proteins generally have large molecular weights ranging between 5 × 103 and 1 × 106. It might be noted that the values of molecular weights of many proteins lie close to or multiples of 35,000 and 70,000.

Colloidal Nature-

Because of their giant size, the proteins exhibit many colloidal properties, such as; Their diffusion rates are extremely slow and they may produce considerable light-scattering in solution, thus resulting in visible turbidity (Tyndall effect).

Denaturation-
Denaturation refers to the changes in the properties of a protein. In other words, it is the loss of biologic activity. In many instances the process of denaturation is followed by coagulation— a process where denatured protein molecules tend to form large aggregates and to precipitate from solution.

Amphoteric Nature-
Like amino acids, the proteins are amphoteric, i.e., they act as acids and alkalise both. These migrate in an electric field and the direction of migration depends upon the net charge possessed by the molecule. The net charge is influenced by the pH value. Each protein has a fixed value of isoelectric point (pl) at which it will move in an electric field.

Ion Binding Capacity-
The proteins can form salts with both cations and anions based on their net charge.

Solubility-
The solubility of proteins is influenced by ph. Solubility is lowest at isoelectric point and increases with increasing acidity or alkalinity. This is because when the protein molecules exist as either cations or anions, repulsive forces between ions are high, since all the molecules possess excess charges of the same sign. Thus, they will be more soluble than in the isoelectric state. The solubility of a protein is the thermodynamic manifestation of the equilibrium between protein-protein and protein-solvent interactions.

                             Protein-Protein + Solvent-Solvent = Protein-Solvent

The major interactions that influence the solubility characteristics of proteins are hydrophobic and ionic in nature. Hydrophobic interactions promote protein-protein interactions and result in decreased solubility, whereas ionic interactions promote protein-water interactions and result in increased solubility. Based on solubility characteristics, proteins are classified into four categories.

1) Albuminsare -those that are soluble in water at pH 6.6 (e.g. serum albumin, ovalbumin, and α-lactalbumin)

 2) Globulinsare- those that are soluble in dilute salt solutions

  at pH 7.0 (e.g., glycinin, phaseolin, and β-lactoglobulin)

 3) Glutelinsare those that are soluble only in acid (pH 2) and

  alkaline (pH 12) solutions (e.g., wheat glutelins)

 4) Prolaminesare those soluble in 70% ethanol (e.g., zein and

  gliadins). Both prolamines and glutelin's are highly

  hydrophobic proteins.

Optical Activity-
All protein solutions rotate the plane of polarized light to the left, i.e., these are levorotatory.

           Chemical Properties of Proteins

Hydrolysis-
Proteins are hydrolyzed by a variety of hydrolytic agents.
A. By acidic agents: Proteins, upon hydrolysis with conc. HCl (6–12N) at 100–110°C for 6 to 20h, yield amino acids in the form of their hydrochlorides.
B. By alkaline agents: Proteins may also be hydrolyzed with 2N NaOH.

Reactions involving COOH Group-
A. Reaction with alkalise (Salt formation)
B. Reaction with alcohols (Esterification)
C. Reaction with amines

Reactions involving NH2 Group-
A. Reaction with mineral acids (Salt formation): When either free amino acids or proteins are treated with mineral acids like HCl, the acid salts are formed.
B. Reaction with formaldehyde: With formaldehyde, the hydroxy-methyl derivatives are formed.
C. Reaction with benzaldehyde: Schiff ‘s bases are formed
D. Reaction with nitrous acid (Van Slyke reaction): The amino acids react with HNO2 to liberate N2 gas and to produce the corresponding α-hydroxy acids.
E. Reaction with acylating agents (Acylation)
F. Reaction with FDNB or Sanger’s reagent
G. Reaction with dansyl chloride

Reactions involving both COOH AND NH2 Group-
A. Reaction with triketohydrindene hydrate (Ninhydrin reaction)
B. Reaction with phenyl isocyanate: With phenyl isocyanate, hydatic acid is formed which in turn can be converted to hydantoin.
C. Reaction with phenyl isothiocyanate or Edman reagent
D. Reaction with phosgene: With phosgene, N-carboxyanhydride is formed
E. Reaction with carbon disulfide: With carbon disulfide, 2-thio-5-thiozolidone is produced

Reactions involving R Group or Side Chain-
A. Biuret test
B. Xanthoproteic test
C. Millon’s test
D. Folin’s test
E. Sakaguchi test
F. Pauly test
G. Ehrlich test

Reactions involving SH Group-
A. Nitroprusside test: Red color develops with sodium nitroprusside in dilute NH4.OH. The test is specific for cysteine.
B. Sullivan test: Cysteine develops red colour in the presence of sodium 1, 2-naphthoquinone- 4-sulfonate and sodium hydrosulphite.

Protein properties in food chemistry (Functional properties of proteins) -

“Functionality” of food proteins is defined as “those physical and chemical properties which affect the behaviour of proteins in food systems during processing, storage, preparation and consumption”. Food preferences by human beings are based primarily on sensory attributes such as texture, flavour, colour, and appearance. The sensory attributes of a food are the net effect of complex interactions among various minor and major components of the food. Proteins generally have a great influence on the sensory attributes of foods. For example

1)  The sensory properties of bakery products are related to

the viscoelastic and dough-forming properties of wheat gluten.

2) The textural and succulence characteristics of meat

products are largely dependent on muscle proteins (actin,

myosin, and several water-soluble meat proteins).

3) The textural and curd-forming properties of dairy products

  are due to the unique colloidal structure of casein micelles.

4) The structure of cakes and the whipping properties of

dessert products depend on the properties of egg-white

proteins.

Organoleptic properties -

The organoleptic or sensory properties of proteins include colour, flavour, taste & odour. The sensory attributes of foods are achieved by complex interactions among various functional ingredients.

Gelation-

A gel is an intermediate phase between a solid and a liquid. Technically, it is defined as “a substantially diluted system which exhibits no steady state flow”. It is made up of polymers cross-linked via either covalent or noncovalent bonds to form a network that is capable of entrapping water and other low-molecular-weight substances. Protein gelation refers to transformation of a protein from the “sol” state to a “gel like” state. This transformation is facilitated by heat, enzymes, or divalent cations under appropriate conditions. All these agents induce formation of a network structure.

Binding properties of proteins-

Proteins can hold together a combination of ingredients. When heated, proteins coagulate so that the product is unbroken (ex: cakes, burgers).

Proteins themselves are doorless. However, they can bind Flavour compounds, and thus affect the sensory properties of foods. Several proteins, especially oilseed proteins and whey protein concentrates, carry undesirable flavours, which limits their usefulness in food applications.

Foaming properties of proteins -

Foams consist of an aqueous continuous phase and a gaseous (air) dispersed phase. The foaming property of a protein refers to its ability to form a thin tenacious film at gas-liquid interfaces so that large quantities of gas bubbles can be incorporated and stabilized. Foams are dispersions of gases in liquids. Proteins stabilize by forming flexible, cohesive films around the gas bubbles. During impact, the protein is adsorbed at the interface via hydrophobic areas; this is followed by partial unfolding (surface denaturation).

Foaming properties of proteins are evaluated by foaming capacity& foaming stability.

Functional role in foods                                                                                                 In several processed foods, proteins function as foam forming and foam-stabilizing components, for example in baked goods, sweets, desserts, whipped cream, ice cream, cakes, meringue, bread, souffles, mousses, and marshmallow.