Biochem lecture 35- Translation

Steps involved in protein biosynthesis

-Amino acid is activated

-polypeptide chains are formed

Activated

Process by which an amino acid is covalently bonded to tRNA, forming an aminoacyl-tRNA

Chain initiation

Binding of the first aminoacyl-tRNA to the start site of the ribosome

Chain elongation

Formation of peptide bonds between successive amino acid residues (three binding sites for tRNA are present on the 50S subunit of the 70S ribosome)

Chain termination

Release of a newly formed protein from the ribosome

Transfer RNA (tRNA)

-Single stranded polynucleotide chain between 73 and 94 nucleotide residues long

-where interchain hydrogen bonding occurs

Aminoacyl-tRNA

Carries an animo acid at its 3’ end (also at 2’end)

inosine (I)

A nucleoside formed when adenosine (A) is deaminated is found as an anticodon

Ribosomal RNA, rRNA

-Found in ribosomes, the site of protein synthesis

-Total weight of the ribosome: 60%-65% rRNA and 35% to 40% protein

What do prokaryotes and eukaryotes ribosomes consist of

-One large and one small subunit

-prokaryotic ribosome (70S) composition

-eukaryotic ribosome (80S) composition

Triplet code

Sequence of three bases (codons) are needed to specify one animo acid

Nonoverlapping

No bases are shared between consecutive codons and ribosome moves along the mRNA three bases at a time rather than one or two at a time

Comma-less; continuous

No intervening bases between codons exist

Nature of the genetic code

-64 possible triplets of the 4 bases occur in RNA

-all 64 codons have assigned meanings

-degenerate code

-universality of the code

What are the 64 codons made up of

61 codons code for amino acids and the other 3 (UAA, UAG, and UGA) serve as termination signals

Condon-anticodon pairing

A codon can form base pairs with a complementary anticodon of a tRNA when an amino acid is incorporated during protein synthesis. Some tRNAs bond to one codon exclusively but many tRNAs can recognize more than one codon

Anticodons

Sequence of three bases in tRNA that hydrogen bonds with the mRNA triplet that specifies a given amino acid

Amino acid activation

-specificity of the enzyme contributes to the accuracy of the translation process

-primary function is to assure that the right amino acid pairs up with the right tRNA

-both are catalyzed by aminoacyl-tRNA synthetase

What is step 1 of the formation of aminoacyl-tRNA?

Amino acid forms a covalent bond to an adenine nucleotide, producing an aminoacyl-AMP ( free energy hydrolysis of ATP provides energy for bond formation)

What is step 2 of the formation of aminoacyl-tRNA?

Ester linkage is formed between the amino acids and either the 3’hydroxyl or the 2’hydroxyl of the ribose at the 3’ end of the tRNA

Second genetic code

-extra level of proofreading by aminoacyl-tRNA synthetases

-two-stage activation allows selectivity at two levels (amino acid and tRNA)

Chain initiation

Synthesis of polypeptide chain starts at the N-terminal end (grows from N-terminus to C-terminus)

Shine-Dalgarno Sequence

-Purine-rich leader sequence that precedes the start signal

-lies about 10 nucleotides upstream of the AUG start codon

-acts as a ribosomal binding site

-binds to a pyrimidine-rich sequence on the 16S ribosomal RNA part of the 30S subunit (aligns it for proper translation)

Initiation complex

3’-UAC-5’ triplet on tRNA^fmet recognizes the AUG triplet (start signal) when it occurs at the beginning of the mRNA sequence that directs polypeptide synthesis

What are the components that enter the initiation complex?

-initiation codon (AUG) of mRNA

-30S and 50S ribosomal subunits

-fmet-tRNA^fmet

-GTP

-three protein initiation factors (IF-1, IF-2 and IF-3)

P (peptidyl) site

Binds the tRNA that carries a peptide chain

A (aminoacyl) site

Binds an incoming aminoacyl-tRNA which has to be added to the growing peptide chain

E (exit) site

Carries an uncharged tRNA that is about to be released from the ribosome

What is step 2 of chain elongation?

Requires GTP and three protein elongation factors, GTP is hydrolyzed and EF-Tu-GTP is regenerated

What is step 3 of chain elongation?

Peptide bonds are formed in a reaction catalyzed by a peptidyl transferase