Protein Metabolism and Biosynthesis of Proteins


Protein Metabolism and Biosynthesis of Proteins

1) Transamination: Enzymatic transfer of an amino group from α amino acid to an (α) Keto amino acid.

2) Deamination: The enzymatic removal of amino groups from biomolecules such as amino acids or nucleotides.

3) Decarboxylation: Removal of CO2 from the carboxylic group and convert the amino acid to its corresponding amines.

Biosynthesis of Proteins:

Three steps are involved in biosynthesis of protein

1) Replication: Flow of genetic information from DNA, Synthesis of DNA
2) Transcription – Information contained in DNA is copied by base pairing to form complementary ribo-nucleotides form RN, Synthesis of RNA
3) Translation: Information contained on mRNA directs the ordered polymerization of specific amino acids to form protein.
Thus DNA makes RNA and RNA makes protein.

1) Replication and DNA Biosynthesis:

Free deoxyribo-nucleotides are assembled linearly to form an identical sequence or replication of original DNA structure for hereditary transmission.

a) Initiation: Unwinding proteins are essential for initiation and continuation of replication which separate the DNA strand for enzyme polymerase to function.
b) Elongation:
Deoxyribo-nucleotide is properly positioned elongation will continue until 500 to 1000 deoxyribo-nucleotide residues are added to form daughter strands and RNA-DNA fragment (called Okazaki fragments).
c) Termination: As 3 OH terminus approach 5′ PPP terminus 3 events occur excision of RNA, filling of gaps with deoxyribo-nucleotide (done by DNA polymerase) and fusion of DNA fragments i.e. (3’OH terminus with 5′ PPP terminus by enzyme DNA ligase.

2) Transcription and RNA Biosynthesis:

Process of information flow from DNA to RNA, DNA base pair with r-RNA, t-RNA, m-RNA form DNA-RNA hybrid (Okazaki  fragment), occur only when region on DNA are complementary to reaction in RNA e.g. A-T-T-C-C in the DNA pairs with U-A-A-C-G in RNA. The RNA formed has a composition and sequence complimentary to that of DNA. RNA synthesis is copying reaction, reaction similar to DNA synthesis and proceeds by base pairing A to T, G to C and U to A.

The synthesized RNA undergoes modifications in cytoplasm of prokaryotic cell to reduce the length of RNA to form ribosomal, messenger and transfer RNAs.

3) Translation and Protein Biosynthesis:

a) Activation: Activation of amino acids by activating enzymes making use of ATP energy to form amino acyl t-RNA. Each amino acid has specific activation enzyme and t-RNA (to carry it at site of synthesis). (Code sequence of nitrogenous base in the DNA molecule constitutes the code which determines the order in which amino acids are joined to form the protein molecule, sequence of base of mRNA. Amino acid code 3 adjacent nucleotide residues on mRNA out of 64 triplet codon 61 codons encode amino acids any are tenninating codons. Ribosome’s site of translation is ribo-nucleoprotein r-RNA serves as structural polymer holding multi-protein particle in compact configuration).

b) Initiation: Binding of 30S subunit of ribosome to m-RNA in presence of IF3 (initiation factor), next bindings of t-MET-t-RNA to 305-mRNA 1F3 complex of 30S-mRNA. t-MET-t-RNA-GTP complex and release of 1F3 next GTP hydrolyzed and 508 subunit of ribosome combine with complex to form 70S complex containing t-MET t-RNA on P site, codon AUG on m-RNA.
c) Elongation:

Stage 1) New acyl-t-RNA bound to site A
Stage 2) Formation of peptide bond-peptide moiety of t-RNA on P site
Stage 3) Translocation process

 Shift of new peptide t-RNA from A site to P site. Shift of ribose to next carbon on mRNA.

d) Termination: 2 event occur

i) The recognition of termination signal in the m-RNA, Terminating codon VAA, UAG and UGA (nonsense codons).
ii) The hydrolysis of peptide t-RNA linkage to release protein. 70S ribosome dissociate from m-RNA into 30S and 50S subunit to enter the protein synthesis.

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