Structure of Proteins - ( In Detail )

SaQLaiN HaShMi 12:13 AM

STRUCTURE OF PROTEINS:

Definition:

Proteins are polymers of amino acids, the compounds containing carbon, nitrogen, oxygen and hydrogen.

The number of amino acids varies from a few to 3000 or even more in different proteins.

Abundance:

Proteins are the most abundant organic compounds in cells. They are over 50% of total dry weight of the cell.

Amino Acids:

About 170 types of amino acids are present in cells and tissues.

Of these, about 25 types are involved in the formation of proteins. However, most of the proteins are made of 20 types of amino acids.

Structure of Amino Acid:

All the amino acids have an amino group (-NH₂) and a carboxyl group (-COOH) attached to the same carbon (alpha carbon). The general formula of a amino acid is a follows:

The amino acids differ due to the type of R group. For example, when R is H, it forms glycine. Similarly, when R is CH₃, it forms alanine.

Formation of Protein Molecule by Peptide Linkage:

Amino acids are linked to form polypeptide. The polypeptides may assemble to form proteins.

The amino group of one amino acid reacts with the carboxyl group of another amino acid and a water molecule is released. Here OH is released from carboxyl group of one amino acid while H is released from amino group of next amino acid.

The linkage between C of carboxyl group of one amino acid and N of amino group of next amino acid is called peptide bond

Example:

Glycine and alanine combine to form glycylalanine.

Glycylalanine has two amino acids and is called dipeptide.

A dipeptide has an amino group at one end and a carboxyl group at the other end.

So both reactive sites are available for the formation of further peptide bonds. As a result tripeptides, tetrapeptides, pentapeptides and polypeptides etc can be produced.

Structure of Proteins:

Each protein has specific properties which are due to:

(i) Number of amino acids,

(ii) Kinds of amino acids.

(iii) Specific sequence of amino acids and.

(iv) The shape of protein molecule.

As a result protein structure is formed which is organized at four levels.

(1) Primary Structure of Protein:

It includes the number, kind and sequence of amino acids in a protein molecule.

Size of protein Molecule:

The size of a protein molecule is determined by the type of amino acids and the number of amino acids in that protein molecule.

Sequence of Amino Acids:

There are over 10,000 proteins in the human body. These are formed by the specific arrangements of 20 types of amino acids.

The sequence of amino acids is determined by the order of nucleotides in the DNA.

(2) Secondary Structure:

It is the coiling of primary polypeptide chains.

The chains in a protein molecule are not flat. They usually coil into α helix, or into β-pleated sheet.

(i) α-Helix:

It is most common structure in which the basic polypeptide chain is spirally arranged.

It is very uniform geometric structure with 3.6 amino acids in each turn of the helix.

The helical structure is kept by the formation of hydrogen bonds among amino acid molecules in spiral turns.

(ii) β-Pleated Sheets:

These are formed by the fold backs of the polypeptide (e.g. silk).

(3) Tertiary Structure:

In the tertiary structure, a polypeptide chain bends and folds upon itself forming a globular shape.

Structural Maintenance:

It is maintained by three types of bonds:

(a) Ionic bonds, (b) Hydrogen bonds and (c) Disulfide (-S-S-) bonds

Stable Configuration:

In aqueous environment (aqueous solution) the most stable tertiary structure is that in which hydrophobic amino acids are hidden inside while the hydrophilic amino acids are on the surface of the molecule.

(4) Quaternary Structure:

Quaternary proteins are polymers of several tertiary structures.

In quaternary structure, the highly complex polypeptide tertiary chains are aggregated and held together by:

(i) Hydrophobic interactions.

(ii) Hydrogen bonds and,

(iii) Ionic bonds.

Haemoglobin, the oxygen carrying protein of red blood cells show quaternary structure.

Example:

(i) Structure of Insulin:

F. Sanger was the first scientist who determined the sequence of amino acids in a protein molecule.

After ten years of work, he concluded that insulin is composed of 51 amino acids in two chains. One with 21 amino acids and the other with 30 amino acids. Both the chains are linked by disulphide bonds.

(ii) Structure of Haemoglobin:

Haemoglobin is composed of four chains, two alpha and two beta chains.

Each alpha chain contains 141 amino acids, while each beta chain contains 146 amino acids.

Abnormal Sequence of Amino Acids:

The arrangement of amino acids in a protein molecule determines its function.

If any amino acid is not at its normal place, the protein fails to carry on its normal function.

Example:

The best example is the sickle cell haemoglobin of human beings.

In this case only one amino acid (Glutamic acid) in each beta chain out of the 574 amino acids does not occupy the normal place in the proteins. Actually glutamic acid is replaced by valine, Therefore hemoglobin fails to carry sufficient oxygen. The result is the death of the patient.

Structure of Proteins - ( In Detail )

Reviewed by SaQLaiN HaShMi on 12:13 AM Rating: 5

The Evolution Of Seed Habit

SaQLaiN HaShMi 7:31 PM

EVOLUTION OF SEED HABIT:

Dominancy of Seed Plants Over the Non-seed Plants:

A review of the kingdom Plantae indicates that the seed-pants (spermatophytes) predominate over non-seed vascular plants.

Development of Seed Habits:

One of the most significant events in the history of land plants was the development of seed habit.

Change in Reproductive Structure:

It was an important change in the reproductive system of the vascular plants which occurred approximately 390 million years ago.

First Seed Appear in Late Devonian:

First complete seeds appeared approximately 365 million years ago during late Devonian times.

Seed is Fertilized Ovule:

Technically a seed may be defined as a fertilized ovule.

Ovule is Immature Seed:

Integument Indeshiscent:

An ovule is an integumented indehiscent (not discharge or ripe) megasporangium.

Integuments are specialized protective coverings around megasporangium which vary in number.

Spermatophytes:

All seed-producing plants are called spermatophytes.

Various steps involved in the evolution of seed habit are as follows.

Steps Involved in Evolution of Seed Habit:

Summary:

(i) Evolution of Heterospory.

(ii) Retention and germination of megaspore within the megasporangium.

(iii) Development of protective layers around megasporangium.

(iv) Reduction to a single functional megaspore per sporangium.

(v) Development of an embryo sac within the sporangium.

(vi) Modification or distal end of megasporangium for pollen capture.

In Detail:

(i) Evolution of Heterospory:

Primitive vascular land plants produced one kind of spores, a condition called homospory.

All groups of land plants up to pteridophytes are homosporous.

During the early phase of evolution, some plant groups started producing two different types of spores.

(a) Smaller ones called microspores and
(b) Larger ones are known as megaspores.

The microspores produced inside microsporangia germinated to form male gametophyte or microgametophyte.

The megaspores germinated to form female gametophyte or megagametophyte.

(ii) Retention and Germination of Megaspore within the Megasporangium:

During the usual reproductive cycle in the heterosporous vascular land plants, the megaspores used to be shed and dispersed soon after their formation in order to germinate into the female gametophyte.

However, in some plants (e.g. Selaginella) the megaspore was not allowed to escape from megasporangium immediately after its formation.

In other, the megaspore(s) was/were permanently retained within the megasporangium.

Here, within the confines of the megasporangium wall, the megaspore germinated to form egg containing female gametophyte.

(iii) Development of Protective Layers Around Megasporangium.

Some branch like structures of sporophyte surrounding the megasporangium fused around to megasporangium to form protective envelope or integument.

The megasporangium tightly locked by integuments becomes totally indehiscent. This important change led to the evolution and formation of the ovule, which is nothing but an integumented indehiscent megasporangium.

In this way, more protection was accorded to the egg containing apparatus in terrestrial environment.

(iv) Reduction to a Single Functional Megaspore per Sporangium.

Each megaspore mother cell within a megasporangium used to produce four functional megaspores.

These megaspores germinated to produce four variable female gametophytes.

There was a competition for space and food among the four gametophytes.

Soon the early vascular plants adopted a new strategy i.e., only one megaspore was selected for further development into a healthy female gametophyte while the remaining three were aborted.

(v) Development of an Embryo Sac within the Sporangium.

The single healthy megaspore retained with the megasporangium germinated to form an egg containing female gametophyte called an embryo sac.

(vi) Modification or Distal end of Megasporangium for Pollen Capture.

When most of the structural and functional changes leading to the development of seed habit were completed.

Another important modification took place in the megasporangium which was now integumented, indehiscent and permanently attached to the sporophyte.

The distal end of the megasporangium became modified for capturing pollen (microspore containing male gametophyte).

Pollen after being trapped in the distal cavity of the megasporangium produced pollen tube which carried male gametes deep into the embryo sac to fertilize the egg, forming a zygote, which forms an embryo.