Essay on Proteins: Top 8 Essays | Organic Compounds | Cells | Biology

Here is an essay on ‘Proteins’ for class 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Proteins’ especially written for school and college students.

Essay Contents:

1. Essay on the Introduction to Proteins
2. Essay on the Functional Importance of Proteins
3. Essay on the Structure of Proteins
4. Essay on the Properties of Proteins
5. Essay on the Identification of Proteins
6. Essay on the Protein Molecule
7. Essay on the Post-Translational Processing of Proteins
8. Essay on the Molecular Weights of Proteins

Essay # 1. Introduction to Proteins:

Proteins are essential constituents of protoplasm. They differ from carbohydrates and lipids by always con­taining nitrogen and sometimes phosphorus and sulphur. Proteins contain: carbon-54%; hydrogen-7%; nitrogen-16%; oxygen-22%. Some may contain, sulphur-1%; while others, phosphorus-0.6%.

Proteins are among the most abundant organic molecules; in most living systems they make up 50 percent or more of the dry weight. Only plants, with their high cellulose content, are less than half protein. The protein molecule is built up by the union of a large number of amino acids. The amino acids should be considered as the units with which the protein molecule is composed.

It is to be specially noted that the term protein is applied only to the complex protein molecule responding to the characteristic tests of protein, e.g., copper-protein (biuret) test, etc., due to the presence of two or more peptide linkages whereas amino acids do not respond to the characteristic tests of protein; hence they are called one of the non-protein nitrogenous (NPN) constituents.

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Essay # 2. Functional Importance of Proteins:

Protein has got multiple functions of which certain physiological importance’s are described below:

(a) Protein acts as a growth material for the organism,

(b) Structures of living materials are composed of different types of protein molecules,

(c) It also acts as a part of fuel of the organism,

(d) All the pituitary hormones, hypothalamic-releasing factors (R.F.), certain placental hormones, pancreatic hormones, etc., are proteins in nature,

(e) Similarly all enzymes are proteins in nature.

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Essay # 3. Structure of Proteins:

Fibrous Proteins:

In general, fibrous proteins have a regular, repeated sequence of amino acids and so a regular, repetitious structure. An example is collagen, which makes up about one-third of all the protein in vertebrates. The basic collagen molecule is composed of three very long polymers of amino acids-about 1,000 amino acids per chain.

These three polymers, which are made up of repeating groups of amino acids, are held together by hydrogen bonds linking amino acids of different chains in a tight coil. The molecules can coil so tightly because every third amino acid is glycine, the smallest of the amino acids.

Collagen performs many functions in the body. Consider a cow. Tendons, which link muscle to bone, are made up of collagen fibers in parallel bundles; thus arranged, they are very strong but do not stretch. The cow’s hide, by contrast, is made up of collagen fibrils arranged in an interlacing network laid down in sheets.

Even its corneas-the transparent coverings of the eyeballs-are composed of collagen. Boiling in water disperses the polymers of collagen into shorter chains, which we know as gelatin. Other fibrous proteins include elastin, present in the elastic tissue of ligaments, silk, and keratin.

Structural Uses of Globular Proteins:

Some structural proteins are globular. For example, microtubules, which function in a variety of ways inside the cell, are made up of globular proteins. They are long hollow tubes-so long that their entire length can seldom be traced in a single microscopic section.

They apparently act as internal skeletons, stiffening parts of the cell body. They also may serve as tracks along which substances can move inside the cell.

The formation of a new cellulose cell wall in a plant can be predicted by the appearance at the site of large numbers of microtubules; when cellulose fibrils are being laid down outside a plant cell membrane, as a cell wall forms or grows, it is possible to detect microtubules inside the cell aligned in the same direction as the fibrils outside.

Chemical analysis shows that each microtubule consists of a very large number of subunits, each of which is a globular protein made up of two polypeptide chains. The two polypeptides fit together because of their complementary configurations, forming approximately spherical subunits.

The subunits assemble themselves into tubules, adding on length as required. When their job is over, they separate. Assembling requires an input of energy but just how it is triggered is not known.

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Essay # 4. Properties of Proteins:

Proteins are colloidal in nature but many of them can be crystallised. They are generally soluble in water, weak salt solutions, dilute acids and alkalies. Each protein has got a particular isoelectric point at which it is precipitated. These precipitates again dissolve when the reaction of the medium is shifted from isoelectric point or pH.

During precipitation proteins do not undergo any intramolecular change. They simply separate out because the medium is not favourable for solution. Most of the proteins undergo coagulation by heat or acid. Coagulation involves intramolecular change. There is another kind of change which the proteins undergo, called denaturation.

This may be done by many kinds of chemical or physical treatment, such as shaking, change of temperature, change of reaction, addition of neutral salts, etc. The exact nature of change undergone during denaturation is not known. It is probable that the molecular arrangement alters. All proteins show the characteristic properties of colloids.

Proteins not only differ from one another in the number and variety of amino acids they contain but also in chemical structure, physical and physiological properties. There are infinite varieties of proteins, each one differing from the rest in certain characteristics. The proteins of a particular tissue of an animal are quite different from those of the same tissue of another species.

Moreover, in the same species, each type of tissue contains proteins which are distinct from those found in the other tissues. No two proteins are found to be exactly the same in their physiological properties. This is another characteristic of proteins as a class, which are not found with the carbohydrates or fats. Vegetable proteins differ from animal proteins in the fact that the former is generally poorer in essential amino acids.

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Essay # 5. Identification of Proteins:

Proteins can be identified by three groups of tests:

1. Colour reaction,

2. Coagulation reaction, and

3. Precipitation reaction.

1. Colour Reaction:

It depends upon the characteristic radicles in the molecule.

i. Biuret Tests (Piotrowski’s Rose’s):

To 2 to 3 ml of the protein solution in a test tube, when an equal volume of concentrated caustic soda solution and one drop or two drops of 1% copper sulphate solution are added, a distinct violet colour develops. This reaction is due to the presence of peptide linkage in the protein molecule.

ii. Xanthoproteic Reaction:

To 2 to 3 ml of the protein solution when few drops of concentrated nitric acid are added, a white precipitate forms. It is boiled for a minute the precipitate turns yellow and partly dissolves to furnish a yellow solution. It is then cooled and concentrated caustic soda is added till the reaction is alkaline. The yellow colour change to an orange one. This reaction is due to the presence of the phenyl group in the protein molecule.

iii. Millon’s Reaction:

To 5 ml of the protein solution when few drops of Millon’s reagent are added-a white precipitate forms which on heating changes to a brick-red coagulum. This is due to the presence of the tyrosine in the protein molecule.

iv. Molisch’s Reaction:

To 3 or 4 ml of protein solution, three or four drops of an alcoholic solution of thymol are added. The solution is well mixed and to it few drops of concentrated sulphuric acid are added when a distinct purple or violet ring develops at the junction of the two liquids. The reaction is due to the presence of carbohydrate radicles in the protein molecule.

v. Adamkiewicz’s Reaction:

To 3 or 4 ml of protein solution excess of glacial acetic acid is added and heated. It is cooled and few ml of concentrated sulphuric acid are allowed to flow down the side of the inclined test tube, a purple colour develops at the junction of two liquids within a short time. The reaction is due to presence of tryptophan in protein molecule.

vi. Nitroprusside Reaction:

To a solution of protein, add sodium nitroprusside in ammoniacal solution. A reddish colour is produced. This is due to the presence of sulphydryl group in the protein molecule. (Disulphide groups are to be reduced first to sulphydryls by suitable reducing agent to give this reaction.)

2. Coagulation Reaction:

Albumin and globulin solution when heated, fine flocculi appear. They are denatured.

3. Precipitation Reaction:

In a protein solution the proteins may be precipitated by;

(a) Acids, e.g. tannic acid, picric acid, trichloro-acetic acid and phosphotungstic acid,

(b) Salts of heavy metals, and

(c) Alcohol.

Formation of the Protein Molecule:

The synthesis of protein molecule takes place by the union of the -NH₂ group of one amino acid with the -COOH group of another.

For instance:

In this way infinite number of amino acids may join up in straight chains. This junction, CO-NH, through which the ami­no acids become joined together, is called the peptide linkage.

It is obvious that in such a simple chain there will be one free -NH₂ group at one end and another free -COOH group at the other. It sometimes so happens that such a straight chain bends upon itself and the basic radicle at one end combines with the acid radicle on the other by forming the same peptide linkage.

In this way a ring structure is produced. These compounds are called diketopiperazines. The following scheme – shows the formation of diketopiperazine by the condensation of two molecules of glycine.

Amino Acids:

The amino acid is an organic acid in which one or more hydrogen atoms are replaced by -NH, group. Thus it contains at least a free amino group (-NH₂) and a carboxyl group (-COOH). The empirical formula is R-CH.NH₂, COOH. Amino acids are amphoteric in reaction and form salts with both acids and bases.

The amino acids in the body are almost all a-amino acids. They should be regarded as derivatives of saturated fatty acids in which the amino group is attached to that carbon atom which is situated in the a-position, i.e. next to the COOH group. Amino acids are colurless, crystalline substances, soluble in water, easily diffusible and except glycine all are optically active.

Classification of Amino Acids:

Isoelectric pH:

Each of the amino acids has at least one carboxyl group (-COOH) and one amino group (-NH₂). For this reason they are called ampholytes. This characteristic of amino acid is also present in intact protein molecule due to the presence of two terminal free amino acids. However there may be more than one acidic or basic group depending on whether they are dicarboxylic or diamino acids. Due to amphoteric nature, in acid solution protein molecule is positively charged and in alkaline one negatively.

At certain pH the number of positive charge is equal to the number of negative charge and protein remains as zwitterion form. The isoelectric pH or point of a protein is the pH at which the protein does not migrate in an electric field.

At this particular pH the protein molecule does not move either to the positive or negative pole, if placed in an electric field. All proteins, at its isoelectric pH, have got least osmotic pressure, swelling capacity, viscosity, solubility and mobility or migrating power. The proteins are thus precipitated at its isoelectric pH.

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Essay # 6. Protein Molecule:

Some evidence has accumulated to suggest that the amino acids in the protein molecule are arranged in a definite pattern. This is more indicated by the fact that many proteins form well-defined crystals. From X-ray and other observations the existence of two types of proteins is revealed; (he fibrous form with elongated molecules and another rounded form with globular molecule. Keratin is an example of the fibrous type. Keratin may exist in two forms α and β.

By stretching α-keratin, β-keratin is obtained. When the stretch is removed, α-kertatin becomes re­converted into β-keratin. If how­ever the stretched hair is subject­ed to steam, it loses its power of recovery and becomes set per­manently as β-keratin. In this way permanent waving of hair is done. X-ray photographs suggest that the keratin molecule consist of thin bundles of polypeptide chains joined together in a zigzag manner.

Wrinch suggests that the glob­ular type of proteins are also formed in similar lines but vary in detail. She holds that in a folded polypeptide chain a linkage occurs between the NH of one peptide link with the CO of the neighbouring one. Thus a poly­peptide chain of this folded type may be closed into hexagonal loops. Six amino acids in a closed polypeptide chain would give a pattern in which the centre is a hexagon.

Such a molecule is called cyclol. A series of cyclols with 18, 30, 42, 54 etc., amino acids could be formed resulting in a sheet like molecule with repeating pattern. Extension of the sheet may occur in any plain and in this way globular types of protein molecules are formed.

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Essay # 7. Post-Translational Processing of Proteins:

Post-translational modifications are the chemical modifications that most of the proteins which undergo before becoming functional in different body cells. It plays a crucial role in generating the heterogeneity in proteins and also helps in utilizing identical proteins for dif­ferent cellular functions in different cell types. The modifications occurring at the peptide terminus of the amino acid chain play an important role in translocating them across biologi­cal membranes. Translocated proteins carry an N-terminal extension of about twenty amino acids, termed a signal peptide; it binds to a receptor in the membrane as soon as it is synthe­sized and emerges from the ribosome.

The signal peptide is recognized by a multi-protein complex termed the signal recognition particle (SRP). This signal peptide is removed follow­ing passage through the endoplasmic reticulum membrane. These include secretory proteins in prokaryotes and eukaryotes and also proteins that are intended to be incorporated in vari­ous cellular and organelle membranes such as lysosomes, chloroplast, mitochondria and plasma membranes.

Sometimes in eukaryotes different types of functional proteins are pro­duced by proteolytic cleavage at multiple points in the protein chain, in which one gene codes for multiple products. The best studied example is the complex of polypeptide hor­mones produced by the pituitary gland.

The major post translational modifications are:

I. Proteolytic Cleavage:

Most proteins undergo proteolytic cleavage following translation. The simplest form of this is the removal of the initiation methionine. Many proteins are syn­thesized as inactive precursors that are activated under proper physiological conditions by limited proteolysis. Inactive precursor proteins that are activated by removal of polypeptides are termed proproteins. Certain proteins particularly of the enzyme class are synthesized as inactive precursors called zymogens. Zymogens are activated by proteolytic cleavage such as is the situation for several proteins of the blood clotting cascade.

The preproprotein insulin secreted from the pancreas has a prepeptide. After cleavage of the 24 amino acid signal peptide the protein folds into proinsulin, which is further cleaved yielding active insulin, composed of two peptide chains linked together through disulfide bonds.

II. Chemical Modification:

The chemical modification mainly includes methylation, sul­fation, phosphorylation, lipid addition, and glycosylation.

(a) Glycosylation:

Many proteins, particularly in eukaryotic cells, are modified by the addi­tion of carbohydrates, a process called glycosylation. Glycosylation in proteins results in addition of a glycosyl group to asparagine, hydroxylysine, serine, or threonine.

(b) Acylation:

Acylation involves of the addition of an acyl group, usually at the N-terminus of the protein. In most cases the initiator methionine is hydrolyzed and an acetyl group is added to the new N-terminal amino acid. Acetyl-CoA is the acetyl donor for these reactions.

(c) Methylation:

The most common methylations are on the s-amine of lysine residues, oc­curs on nitrogen and oxygen. The activated methyl donor is S-adenosylmethionine (SAM). Methylation of the oxygen of the R-group carboxylates of gutamate and aspartate also takes place and forms methyl esters. Proteins can also be methylated on the thiol R- group of cysteine. Methylation of histones in DNA is an important regulator of chromatin structure and consequently of transcriptional activity.

(d) Phosphorylation:

Post-translational phosphorylation occurs as a mechanism to regulate the biological activity of a protein in animal cells. In animal cells, serine, threonine and tyro­sine are the amino acids subject to phosphorylation. As an example, the activity of numerous growth factor receptors is controlled by tyrosine phosphorylation. Other relevant examples are the phosphorylations that occur in glycogen synthase and glycogen phosphorylase in hepatocytes in response to glucagon release from the pancreas. Phosphorylation of synthase inhibits its activity, whereas, the activity of phosphorylase is increased. These two events lead to increased hepatic glucose delivery to the blood.

(e) Sulfation:

Sulfate modification of proteins occurs at tyrosine residues such as in fibrino­gen and in some secreted proteins (e.g.: gastrin). The universal sulfate donor is 3′- phosphoadenosyl-5′-phosphosulphate (PAPS). Since sulfate is added permanently it is neces­sary for the biological activity and not used as a regulatory modification like that of tyrosine phosphorylation.

(f) Vitamin C-Dependent Modifications:

Modifications of proteins that depend upon vita­min C as a cofactor include proline and lysine hydroxylations and carboxy terminal amidation. The hydroxylating enzymes are identified as prolyl hydroxylase and lysyl hydroxylase. The donor of the amide for C-terminal amidation is glycine. The most important hydroxylated proteins are the collagens. Several peptide hormones such as oxytocin and vasopressin have C-terminal amidation.

(g) Vitamin K-Dependent Modifications:

Vitamin K is a cofactor in the carboxylation of glutamic acid residues. The result of this type of reaction is the formation of a γ- carboxyglutamate (gamma-carboxyglutamate), referred to as a gla residue. The formation of gla residues within several proteins of the blood clotting cascade is critical for their normal function. The presence of gla residues allows the protein to chelate calcium ions and thereby render an altered conformation and, biological activity to the protein. The coumarin-based anticoagulants, warfarin and dicumarol function by inhibiting the carboxylation reaction.

(h) Selenoproteins:

Selenium is a trace element and is found as a component of several pro­karyotic and eukaryotic enzymes that are involved in redox reactions. The selenium in these selenoproteins is incorporated as a unique amino acid, selenocysteine, during translation. A particularly important eukaryotic selenoenzyme is glutathione peroxidase. This enzyme is required during the oxidation of glutathione by hydrogen peroxide (H₂O₂) and organic hydroperoxides.

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Essay # 8. Molecular Weights of Proteins:

Protein molecules are large and have varying molecular weights. Their molecular weights cannot be determined by ordinary methods. The most important method for their determination is based upon the measurement of sedi­mentation rates in the ultracentrifuge. They can also be determined by osmotic pressure measurement, light scatter­ing, etc.

Molecular weights of some of the typical proteins from different sources are as follows: salmine (salmon sperm) – 5,600; cytochrome (heart) -156,000; lactalbumin (milk) -17,400; gliadin (wheat) – 27,400; pepsin (stomach) – 35,500; insulin (pancreas) – 40,900; haemoglobin (human) – 63,000; myogen (muscle) -150,000; thyroglobulin (thy­roid) – 628,000; myosin (muscle) -1,000,000; virus (tomato) – 7,600,000; mosaic virus (tobacco) – 60,000,000.

It has been shown by Svedberg that by a change in the protein concentration or in the pH or in the salt concentration of the medium, proteins in solution, may easily dissociated into smaller component units. If the original condition is resorted the fragments may re-associate and form the original molecule. This shows that proteins of higher molecular weights are built up by conglomeration of smaller units.

Virus Proteins:

These are nucleoproteins of special interest. Several varieties of virus proteins with very high molecular weights have been isolated in crystalline form, from the juice of plants suffering from virus diseases. The virus protein of tobacco mosaic disease has a molecular weight of about 60,000,000, an isoelectric point or pH 3.49 and contains about 5% nucleic acid. It is a highly interesting fact that even minute amount of pure recrystallised virus proteins induces the virus disease when injected into healthy plants.

One millionth of a milligram of tobacco mosaic protein invariably infects a healthy plant and from such infected plants large quantities of the same protein can be recovered. This astonishing property of the virus proteins, in directing the metabolism of the plant cells so that they synthesise more of the same protein, is unique in a chemical compound and is suggestive of the reproduction of a living organism rather than the elaboration of a non-living chemical mol­ecule. The evidence accumulated so far indicates that these giant molecules are purely of chemical nature although we have no absolute proof that they are dead.

Electrophoresis:

The property of migration of protein molecules in an electric field has been utilised by Tiselius for electrophoretic method for the separation of protein molecules. Protein solution in an alkaline buffer has maximum number of negative charges and so the pro­tein molecules move towards the positive pole. Different protein molecules, due to the presence of charged particles in different amounts, move towards the pole at different rates which can be optically measured.

This complicated procedure has been simpli­fied by application on paper. Here a strip of filter paper is moist­ened with suitable buffer and placed in between two glass plates The free ends of the paper dip into two small reservoirs of buffer solution, and either positive or negative electrode is placed in each of them. On passing electric current the protein molecules migrate towards the anode. After definite time interval, the paper is taken put and washed, and a suitable stain is applied to localise the presence of difference in protein molecules (Fig. 2.1).