Proteins are the polymers of aminoacids which constitute the principal structural unit of protoplasmic organelles, enzymes, and other macromolecules of biological importance. It serves as the energy source and source of nitrogen for body of organisms. Proteins in plants are synthesized within the body cells through a genetically controlled mechinary by the virtue of nuclear DNA and Ribosomes. DNA produces messenger RNA by transcription. The sequence of m-RNA translates specific sequence of Amino Acids to produce a polypeptide chain which later undergo post synthetic processing transformed into primary, secondary and tertiary proteins essential for maintenance of life in a cell. DNA acts as the central dogma of protein synthesis and which especially controlled by genes present on DNA.
In a protein aminoacids link together by polypeptide bond in which the carboxyl group of one amino group of the other and so on. Ribosomal enzymes help in establishment of peptide bond and acts as the site for protein synthesis. The sequence of twenty different aminoacids on a peptide chain is determined by the sequence of triplet bases on the m-RNA. Mechanism of Protein synthesis involves following steps.
For protein synthesis three types of RNAs are involved to carry genetic information’s from nucleus to the site for protein synthesis. These are messenger RNA (m-RNA), transfer RNA (t-RNA) and ribosomal RNA (r-RNA).
Transcription is the process of synthesis of m-RNA from Nuclear DNA which takes place in presence of DNA-Directed-RNA polymerase enzyme. M-RNA molecule is transcribed from DNA molecule in from of templet binding. This process of transcription takes place through following steps.
i) First the parent DNA strands uncoil in presence of RNA polymerase enzyme.
ii) Only one of strand called sense strand take part in templet formation.
iii) On the sense strand a new RNA strands is formed as replica in which Uracil residues are inserted in the position of specified adenine in place of thymine; thus adenine and uracil forms complementary base pairing Guanine and Cytosine residue in DNA specify cytosine and gwanine respectively in new RNA starnd.
iv) During transcription the new RNA strand base, pairs temporarily with the templet DNA strand to form a short length of hybrid strand of DNA-RNA double helix. Then the RNA “Peels off” shortly after its formation.
v) RNA polymerase is a complex enzyme containing five polynucleotide subunits as helonzyme. During transcription this helonzyme binds to a specific site in the DNA called promoters site which consist of a short sequence recognized by the RNA polymerase enzyme.
vi) RNA polymerase correctly positioned at the promoters site of DNA from where the DNA splits and transcription initiates.
vii) The core enzyme signals specific sequence to terminalise the RNA elongation.
In higher plants cells (Eukaryotic) there are three RNA Polymerase enzymes involve in transcription.
RNA- Polymerase – I – It cause formation of r-RNA
RNA- Polymerase –II – Help in transcription of m-RNA
RNA- Polymerase –III – Transcribe t-RNAs.
After transcription RNA Polymerase also help in post transcriptional processing of different RNA. In Eukaryotic plant cell m-RNA undergo special post transcription modification to yield longer and shorter precursors from which t-RNA and r-RNA are transcribed respectively.
It is experimentally found that in Prokaryotic plant cell m-RNAs are processed from heterogeneous nuclear RNA initially transcribed from DNA.
Genetic message for arranging amino acids in specific sequence are carried by m-RNA in terms of triplet base cadons in polypeptide chain.
(II) TRASNLATION: –
Translation is the central process of protein synthesis by which polypeptide chain is formed with sequential arrangement of amino acid. It takes place in the Ribosome which involves hundreds of enzyme complex and RNAs. The process of translation refers to the transformation of codes into sequence of amino acids. It involves following steps.
i) Activation of Amino Acids:
This process takes place in cytosol (cytoplasm). Amino acids which are preserved in cytosol are in inactive state. These are activated at the expense of energy from ATP in presence of aminoacyl RNA synthetase enzyme. Amino acids bound to the enzyme forming a highly reactive amino-acid-adenylate complex with release to pyrophosphate. This reaction is catalyzed by Mg++ .
AA + ATP + E à E-AA ~ AMP +PP
(Aminoacid) (Enzyme) (Active complex)
ii) Attachment of activated Amioacid to the t-RNA:- The enzyme bound activated amino acid adenylate attach with respective t-RNA molecule. For each aminoacid there is a specific.
t-RNA having anticodon complementary to the codon of m-RNA. Amino acid attach to the CCA end of t-RNA. This reaction is catalysed by aminocyl t-RNA synthetase enzyme.
iii) Initiation of the peptide chain:
For initiation of polypeptide chain necessary components includes m-RNA, Methionly- t-RNA complex, 40-S Ribosomal segment 605-Ribosomal subunit, GTP, MG++ and initiation factors IF-1, IF-2, IF-3 and imitating condon “AUG”.
During initiation the messenger RNA bearing the codes for the polypeptide to be made is bound to the 40s sub unit of ribosome (30 in Bacterai). Followed by this the initiating amino acid metheonine (F-Methonine in Bacteria) binds to its t-RNA to form initiation complex. The t-RNA of the initiating amino acid base-pairs with a specific nucleotide triplet or codon on the m-RNA that signals the begging of the polypeptide chain.
This process requires guanosine triphosphate (GTP) and is promoted by three specific Cytosolic Proteins called initiation factors (IF-1, IF-2, IF-3). Francis Crick opined t-RNA functions as adopt or, so that one part of t-RNA molecule can bind a specific amino acid and other end recognize a short nucleotide sequence in the messenger RNA coding for that amino acid.
At one binding site t-RNA molecule bears corresponding anticondon UAC covalently attach to the complementary codon AUG on the m-RNA at the “P” side of Ribosome. It results the formation of an initiation complex with both subunits of Ribosome. But all other incoming aminoacyl t-RNA complex binds at ‘A’ side of Ribosome.
iv) Elongation of Polypeptide Chain:
After formation of imitating complex at ‘P’ site the ‘A’ site is free to accept a specific charged t-RNA. Then in presence of enzyme peptide synthatase a co-valent peptide bond is formed between methyonine and the second amino acid. This is regulated by some elongation factors. Energy required for binding each aminoacyl-t-RNA and for the movement of the ribosome along the messenger RNA by one condon comes from the hydrolysis of two molecules of GTP for each residue added to the growing polypeptide. After formation of a dipeptide the t-RNA of P-site after donating amino acid is removed from the ribosome. Then both m-RNA and ribsome move in opposite direction a step further so that the third codon of m-RNA occupies the A-site and the amino acid of the A-site shifted to P-site. The first codon of m-RNA whose translation in completed gets out of the ribosome. Now enzyme peptide synthetase help in further establishment of second peptide bond between second and third amino acid.
In this way M-RNA is translated on e condon after another beginning with 5 end to 3 end. Polypeptide chain keep on growing causing elongation of chain by addition of amino acid one by one is specific sequence.
v) Chain termination
There are specific termination condons present in the sequence of condon along m-RNA specified by DNA. These condones are UUA, UAG and UGA. These condones assign no amino acids. When one of these condon appear on the m-RNA a gap in peptide chain appear and the chain is terminated. These condons are called terminator or nonsense condons. After the chain is terminated it released from Ribosome which promoted by releasing factors, i.e. Proteins R1. R2 and S.
Now two sub units of Ribosome dissociate to again engaged in formation of a new peptide.
vi) Post-translation Processing:
In order to achieve its native biologically active from the polypeptides undergo folding into its proper three dimensional conformation. Before or after folding new polypeptides undergo processing by enzymatic action.
These post translation modifications involves:-
Modification of amino terminal and carboxyl terminals
Loss of signaling sequence by peptidase activity
Phosphorylation of hydroxyl amino acids by ATP.
Carboxylation of some amino residues.
Methylation of R-group
Attachment of carbohydrate side chains
Addition of prosthetic group.
Formation of disulphide cross links between the overlapping folds.
By above process a finished protein is produced and are directed to their destinations in cell.