Introduction

Simple protein is a type of protein it is a biomacromolecule. Biomacromolecules are large in size, with a higher molecular weight of 10000 Daltons and above. There are different types of protein. And each protein has a specific structure and some functionality. Below is a discussion of simple proteins structure and function.

Protein is the most important and abundant intracellular organic biomolecule. These are polypeptides having chains of amino acids arranged linearly that are linked by peptide bonds. Thus, each protein is a polymer of amino acids. These biomolecules can be separated from each other mainly by their amino acid sequences.

A large number of functions inside the organism, such as the catalyst for metabolic reactions, DNA replication, response to stimuli, transport of various molecules from one place to another, etc. are performed by proteins. They are classified on the basis of shape, chemical composition, and function. According to the structure, there are three types of proteins. Simple protein is one of them.

Simple protein

Proteins that are not attached to any other elements are called simple proteins. Simple proteins consist of only amino acid subunits joined together by a bond. This bond is known as a peptide bond. When simple proteins are broken down by enzymes, they produce only amino acids. The protein that falls into the simple protein category is the simplest of all proteins.

Structure

Simple proteins are made up of only amino acids. The amino acids are small organic molecules. It is made up of an alpha-carbon atom linked to an amino group, a carboxyl group, a hydrogen atom, and a side chain. These amino acids are the building blocks of simple proteins. They are peptide chains having amino acids but no other non-protein components. Simple protein has a set of amino acids in a particular order.

Types

Simple proteins are classified into different types. These are such proteins that upon hydrolysis yield amino acids or their derivatives. Some types of simple proteins and their structure are described below.

1. Globulins

Globulins are mainly blood proteins. The normal amount of globulin in human blood is about 2.5 to 3.5 g/dL.
It is insoluble in pure water.
Globulins are a type of globular protein.
It has a higher molecular weight than albumins.
These are coagulated by heat and acids.
They are soluble in dilute salt solutions of strong acids and bases.
Globulins are precipitated by half-saturated solutions like MgSO₄ AND (NH₄)₂SO₄.

Structure

Globulins are formed by amino acids. There are three types of globulins namely alpha 1 globulin, alpha 2 globulin, beta globulin, and gamma globulin. Gamma globulins are known as antibodies. Its molecular weight is about 100 to 1000 kDa.

These types of proteins have the highest protein fraction. It has amounts of aromatic and sulfur amino acids, glutamic and aspartic acids, threonine, and histidine. Each subunit of globulin consists of an acidic subunit of 40000 Da and a basic subunit of 20000 Da.

Example

It includes serum globulin, ovoglobulin (egg yolk), crystalline (lens of the eye), fibrinogen (plasma), edestin (cotton), legumin (peas), myosin (muscle).

2. Glutelins

Glutelins are soluble in dilute acids and alkalis.
These are insoluble in neutral salts.
It is coagulated by heat.
Most grass species typically have both high molecular weight and low molecular weight.
They are also soluble in detergents, chaotropic agents, or reducing agents.
Glutelins cross-link themselves and other proteins during baking through disulfide bonds.

Structure

Glutelins are comparatively rich in arginine, proline, and glutamic acid. It is a class of propain prolamin proteins. These proteins formed a major component of the protein compound collectively known as gluten. They are found in wheat, barley, and rye. Glutelins are rich in hydrophobic amino acids, containing phenylalanine, valine, tyrosine, proline, and leucine amino acids.

Example

It occurs only in plants. Examples of glutelins are glutenin from wheat and oryzenin from rice.

3. Prolamins

Prolamins are soluble in 70 to 90% ethanol.
These are a type of plant storage proteins.
They are found in plants, mainly in the seeds of cereal grains like wheat, barley, oats, corn, etc.
These are insoluble in water and other neutral solvents.
According to the molecular weight, there are four types of maize and sorghum prolamins. These are alpha (α), beta (β), gamma (γ), and delta (δ).

Structure

Prolamins consist mainly of proline and glutamine amino acids. These two amino acids are present in large numbers in prolamin proteins. In addition to these two amino acids, there are also some amino acids present in these types of proteins but their amount is small. They are arginine, lysine, and histidine.

These proteins are classified into sulfur-poor, sulfur-rich, and high-molecular-weight proteins. The sulfur-rich and high-molecular-weight prolamin proteins have six to eight amino acids.

Example

Examples of prolamins are zein from maize, gliadin from wheat, and hordein from barley.

4. Albumins

Albumins are a type of globular protein.
They are soluble in water.
These proteins are precipitated by saturated salt solutions like ammonium sulfate.
Albumins are coagulated by heat.
They are mainly found in blood plasma.
One of the proteins made in the liver is albumin.
It is not glycosylated, so it is different from other blood proteins.
Albumins are the amphoteric types which means they are able to react both as a base and as an acid.
They occur in egg white, blood, milk, and plants.

Structure

Albumins are simple proteins, so they only consist of amino acids. They are made of a single polypeptide chain. The molecular weight of albumin is about 65 kDa. There are three homologous domains in these proteins. The main function of these domains is to form heart-shaped proteins.

The amino residues that are present in large amounts in albumins are glutamic acid, aspartic acids, lysine, and arginine. And the albumins are usually low or deficient in glycine amino residues.

Example

Egg albumin, serum albumin, lactalbumin (milk), myoalbumin (muscle), and leucosin (wheat).

5. Albuminoids or Scleroproteins

Scleroproteins are insoluble in water, neutral solvents, dilute acids, and alkalis.
It is one of the three main classifications of protein structure.
They form a long protein filament.
Are also known as fibrous proteins.
Animals have high levels of this protein in their bones, hair, nails, skin, and connective tissue.
Scleroproteins are attacked by enzymes.
Sometimes these proteins are dissolved by a mixture of acids such as citric and acetic acid.
Collagens and keratins are the two important classes of scleroproteins.

Structure

They are made of many superfamilies such as collagen, keratin, elastin, and fibrin. Glycine, proline, and hydroxyproline amino acids are present in these types of proteins. There are two kinds of scleroproteins. These are alpha-keratin and collagen. Keratin and collagen both form long fibers that serve a structural role in the body.

Example

Keratin (hoof, nails, and horns), elastin (cartilage), collagen (fibrous tissue), Ossein (bones and teeth).

Basic proteins

Are classified into two types.

1. Protamine

Protamines are the smallest, arginine-rich nuclear protein.
They have a similar structure with a relatively low molecular weight.
Protamines are soluble in water or in NH₄
These proteins are not coagulated by heat.
It is a positively charged protein molecule that binds the negatively charged heparin.
They are hydrolyzed by enzymes like trypsin, and papain not by pepsin.
Protamines are basically found in egg cells.

Structure

Protamine contains 28 amino acid residues with 19 arginine. They contain no Sulphur but have a high nitrogen content due to the presence of large quantities of arginine. The amount of nitrogen in protamine is about 25 to 30%. Tyrosine and tryptophan are absent in these proteins. Its molecular weight is 5.5 to 13.0 kDa. The amount of arginine in protamine is 67%.

Example

They are found to bind with DNA in the sperm of fishes, such as salmon, sturgeon, etc. Salmon fish contain a protein called saline.

2. Histone

Histones are water-soluble proteins.
They are insoluble in dilute ammonia.
Are not coagulated by heat.
Histones are readily hydrolyzed by pepsin and trypsin.
It is the main protein of chromatin.
They are more common in the nucleus and with nucleic acids.
Are precipitated by (NH₄)₂SO₄.
It is a family of highly alkaline proteins present in the nucleus of eukaryotic cells.

Structure

Histone is made of basically positively charged amino acids like lysine and arginine. They are rich in histidine also, but deficient in tryptophan and contain little amount of cystine or methionine. There are 5 types of histones, such as H2A, H2B, H3, H4, and H1 linker histone.

Example

Histones can be extracted in large amounts from certain glandular tissues, such as the thymus and pancreas. Most histones are combined with nucleic acids, hemoglobin, etc. In eukaryotes, the DNA of chromosomes is associated with histones to form nucleoprotein.

Function or uses

Globulins carry lipids, hormones, and cholesterol through the bloodstream. They also help to fight infection and play an important role in liver and blood clotting.
Glutelin is a source of energy for immune cells and the intestines. It plays a vital role in the intestines. These proteins help in the growth of intestinal cells.
Prolamins play a role in maintaining the immune system.
The primary role of albumin protein is to protect the osmotic pressure of the blood vessels, nourish the tissues, and transport vitamins, drugs, calcium, hormones, etc. to the body.
Scleroproteins protect the connective tissue, tendons, bones, and muscle fiber. These proteins form a protective barrier. The barrier is waterproof.
Protamine binds to DNA and plays a vital role in the formation of chromatin.
Histone helps in the formation of the structure of DNA. They play a role in packaging DNA into nucleosomes. These proteins also help to shape the chromosomes and regulate gene activities.

Q&A

1. What are the simple units that makeup proteins?

Amino acids are simple units. Proteins are made of hundreds or thousands of amino acids. There are twenty different types of amino acids. These different types of amino acids combine together to form substances. This substance is known as protein.

2. What are simple proteins?

Proteins are made up of amino acids only and have no non-protein substances.

3. Why does facilitated diffusion require membrane transport proteins while simple diffusion does not?

In facilitated diffusion, the molecules spread across the plasma membrane with the help of membrane transport proteins such as channel protein, carrier proteins, etc. Facilitated transport proteins protect these molecules from the hydrophobic core of the membrane, providing a route through which they can pass.

4. What are simple units that make up proteins?

Amino acids are the simple unit that makes up protein. Twenty types of amino acids are attached together to form proteins.

5. What are two simple proteins that form gluten?

Gliadin and glutenin.

References

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

2. 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.

3. https://www.google.co.in/books/edition/Biochemistry_5th_Edition_Updated_and_Rev/P1jwDwAAQBAJ?hl=en&gbpv=1&dq=u+satyanarayana+biochemistry&printsec=frontcover

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 – 25-35.