Introduction

The polymers of amino acids are peptides and large peptides are called proteins. Proteins are macromolecules that are constructed by the repetition of one or more structural elements, called monomers. In the case of proteins, the monomers are a group of about 20 amino acids. Proteins have different properties thus the properties of proteins are discussed below.

Proteins are now defined as complex nitrogenous substances which are found in the protoplasm of all animal and plant cells. The term “protein”, derived from the Greek word proteios meaning first, was first of all used by Mulder in 1839 at the suggestion of Berzelius. Mainly proteins are biopolymers containing a large number of amino acids joined to each other by peptide bonds.

These biopolymers have different types of structures. Protein structures are primary, secondary and tertiary structures. Each structure has different properties. The properties of all these structures along with the properties of proteins are described here (1).

Properties of protein 

Some of the properties of proteins are as follows

1. All proteins contain carbon, hydrogen, oxygen, and nitrogen, the presence of nitrogen distinguishing them from carbohydrates and fats.

2. On an average proteins contain 16% nitrogen.

3. Some proteins also have Sulfur and a few proteins have phosphorus and other elements may be present.

4. Protein is colorless and tasteless.

5. Cells contain a very large number of proteins.

6. All proteins typically range in size from a spherical structure of a crystal to a long fibrillar structure.

7. The protein’s molecules are amphoteric type, that is they act as acids and alkalis both like amino acids.

8. According to the net charges, these biomolecules can form a salt with both cations and anions.

9. The molecular weight of proteins varies from about 12,000 Daltons to several million.

10. Proteins are formed by the repetition of amino acids, linked by peptide bonds.

11. Any protein has –NH₂ and –COOH terminal groups. –NH₂ group is known as N-terminal and –COOH group is called C-terminal.

12. When boiled with amino acids they yield amino acids in varying molar ratios.

13. 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 (2) & (4).

Types of proteins

However, there is no satisfactory system of classification that depicts the differences and similarities of proteins. One can do the classification of proteins according to their structure, solubility behavior, or their non-protein moiety. Some of these systems of classification are as follow:

On the basis of shapes and functions

1. Fibrous proteins

These are soluble in concentrated acids and alkalis but insoluble in common solvents. Molecules of fibrous proteins are long and thread-like, and tend to lie side by side to form fibers; in some cases, they are held together at many points by hydrogen bonding. Because of this, the intermolecular forces that must be overcome by a solvent are very strong, and that’s why they are insoluble in water.

Fibrous proteins are highly resistant to digestion by proteolytic enzymes. They function primarily as the chief structural materials of animal tissues, a function to which their insolubility and fiber-forming tendency suit them.

Example

Examples of fibrous proteins are keratin, in the skin, hair, nails, wool, horn, and feathers; collagen, in tendons; myosin, in muscle; fibroin, in silk (3).

2. Globular proteins

Molecules of globular proteins are folded into roughly spherical shapes. The folding takes place in such a way that the lipophilic or hydrophobic parts are turned inward and hydrophilic parts are turned outward. Because of this, globular proteins are soluble in water. Hydrogen bonding is chiefly intramolecular.

These are soluble in water and in dilute acids, alkalis, and salts. These proteins are more highly branched and cross-linked condensation products of basic or acidic amino acids. The polypeptide chains in this type of protein are held together by cross-linked groups or in an aggregate state. Such aggregates may also be folded to three-dimensional structures by weak-non-covalent bonds.

Example

Examples of globular proteins are enzymes, hormones, albumin in eggs, hemoglobin which transports oxygen from lungs to tissues, and fibrinogen, which is converted into insoluble fibrous protein fibrin, and thereby causes clotting of blood (2) & (3).

On the basis of complexity

According to this system of classification, proteins may be divided into three main groups:

1. Simple proteins

It consists of amino acids only or their derivatives, therefore, when hydrolyzed yields only amino acids and their derivatives. They include the following groups.

1. Albumins

These are soluble in water, coagulated by heat, and precipitated by saturated salt solutions like ammonium sulfate. The albumins are usually low or deficient in glycine. Lactalbumin, serum albumin, and egg albumin are examples of albumins.

2. Globulins

Globulins are soluble in dilute salt solutions of strong acids and bases. These are coagulated by heat. They are precipitated by half-saturated solutions with ammonium sulfate. Examples of globulins are tissue globulins, vegetable globulins, etc.

3. Glutelins

These are comparatively rich in arginine, proline, and glutamic acid. Examples of glutelins are glutenin from wheat and oryzenin from rice.

4. Prolamins

Prolamins are soluble in 70 to 90% ethanol. These are insoluble in alcohol, water, and other neutral solvents.

5. Albuminoids or Scleroproteins

These are insoluble in all neutral solvents, in dilute acids and alkalis. Examples of albuminoids are keratin from hair, hoof, etc., and fibroin from silk. Albuminoids are attacked by enzymes.

2. Basic proteins

These proteins are of two types.

1. Histone

Histones are hydrolyzed by pepsin and trypsin. It can be extracted in large amounts from certain glandular tissues, such as thymus and pancreas.  Most histones are combined with nucleic acids, hemoglobin, etc.

2. Protamine

These are more basic than histones. Protamines are soluble in water or in NH₄OH. The proteins are not coagulated by heat. Protamine is mainly found in egg cells. Typical protamines are salmine from salmon sperm, clupeine from herring, and sturine from sturgeon. They contain no Sulphur but have a high nitrogen content due to the presence of large quantities of arginine (1) & (4).

3. Conjugated proteins

These proteins contain some non-protein substance called the prosthetic group. This group may be separated from the protein part by carrying out hydrolysis very carefully. There are few types of conjugated proteins. These are-

1. Nucleoproteins

These proteins are formed by the union of nucleic acid with two molecules of proteins. The nucleoproteins are DNA and RNA.

2. Chromoproteins

Hemoglobin, hemocyanin, cytochrome, flavoproteins, etc. are chromoproteins.

3. Glycoproteins

These are the proteins having carbohydrates prosthetic groups and these on hydrolysis yield amino sugars. It is also known as mucoproteins. Egg albumin is an example of glycoproteins.

4. Phosphoproteins

In these proteins, the prosthetic group possesses phosphoric acid in some form other than in the nucleic acids or in the lipoproteins. Casein is an example of phosphoproteins.

5. Lipoproteins

These are water-soluble proteins in which the prosthetic groups are such as phospholipid and cholesterol.

6. Metalloproteins

They contain a metal which is an integral part of the structure. Metals found in metalloproteins are generally iron, magnesium, copper, and manganese. Examples of metalloproteins are hemoglobin and chlorophyll (1) & (2).

4. Derived proteins

These proteins are not present in nature, produced generally by the hydrolysis of proteins. When proteins are hydrolyzed by acids, alkalis, or enzymes, the degradation products obtained from them are called derived proteins. These proteins are further classified on the basis of progressive cleavage as proteins, primary proteoses, secondary protoses, peptones, polypeptides, simple peptides, and amino acids (1).

Primary protein properties

• Primary protein is in the form of a straight chain of amino acids, which determines the three-dimensional structure and properties.
• These proteins include all the covalent bonds between amino acids and locations of disulfide bonds.
• The primary protein describes the number and order in which amino acids are arranged in the protein molecule.
• The alpha-carbons of adjacent amino acids in primary protein are separated by three covalent bonds.
• These three covalent bonds of primary protein are respectively 0.153 nm, 0.132 nm, and 0.146 nm.
• Three covalent bonds are arranged Cα –C –N –Cα.
• The primary protein describes the further levels of organization of protein molecules.
• N –Cα and Cα –C bonds in primary protein can rotate with bond angles designated φ (phi) and ψ (psi) respectively.
• Limited rotation can occur around two of the three types of bonds in a polypeptide chain of primary protein (1) & (3).

Secondary proteins properties

• Most long polypeptide chains in secondary proteins are folded or coiled in a number of ways and bring about the second level of organization.
• The secondary protein is the conformational relationship of nearest neighbor amino acids with respect to each other.
• Two types of secondary structures in this protein are both particularly stable and commonly encountered alpha-helix and beta-pleated sheets.
• The backbone of the secondary protein is not rigid; it is free to rotate around the alpha carbon.
• Secondary protein in the beta-conformation forms H-bonds between two peptide chains.
• The backbone of the polypeptide chain in secondary protein is extended into a zig-zag rather than helical.
• In beta-conformation, the H-bond of the secondary protein can be either intra-chain or inter-chain between peptide linkages of adjacent polypeptide chains.
• Every secondary protein structure is described completely by the two bond angles φ and ψ that are repeated at each residue (2).

Tertiary protein properties

• Tertiary protein describes the shape of the folding that the polypeptide assumes as it becomes packed into a three-dimensional structure.
• The foldings are made possibly by mutual interactions of the side chains and interactions of the side chains with the solvent.
• These proteins are maintained by different types of bonds, like disulfide bonds, hydrogen bonds, electrostatic bonds, etc.
• In the tertiary proteins, the disulfide bond is covalent and other bonds are non-covalent.
• The two most important shapes taken up by the proteins are globular (spherical shape) and fibrous (rod-type shape).
• In tertiary proteins, ionic bonds are formed when acidic and basic amino acids are ionized and lie close together (4).

Q&A

1. What are two proteins important in foaming properties of eggs?

Ovalbumin, ovomucin, globulins, conalbumin are the proteins important in the foaming properties of eggs. These proteins enable the egg whites to be whipped into foam that can reach six to eight times the amount of uninterrupted, liquid egg whites.

Written By: Manisha Bharati

References

1. B. Powar and G. R. Chatwal. Biochemistry, B. SC (general & honors course) and M. Sc. Himalaya publishing house, Chapter: Peptides and proteins. Page no: 192 to 206.

2. Ajoy Paul. Zoology Honours, Volume- 1, Books & Allied (P) Ltd. Chapter: Proteins. Page no- 771 to 782.

3. Tewari. Advanced Organic Chemistry. Second Edition. Books & Allied (P) Ltd. Chapter: Amino acids, Peptides, Proteins, Enzymes, and Nucleic Acids. Page No: 755 to 759.

4. Chandrasekhar Chakrabarti. Modern approach to a textbook of core Zoology, General & Honours. Nirmala Library, A Publishing House under the Prestigious International Standard Book Number (ISBN) System. Kolkata, (India). Part – II. Chapter- Carbohydrates, protein, and lipid. Page: 2^nd – 25- 21 to 2^nd – 25 – 35.