H protein synthesis (thuyết trình)

Synthesis of aminoacyl-tRNA is crucially important for two reasons:• First :each amino acid must be covalently linked to a tRNA molecule in order to take part in protein synthesis,which

• H1 The genetic code

• The genetic code is a triplet code

• The genetic code is degenerate

• Universality of the genetic code

• Reading frames

• Open reading frames

H1 The genetic code

Triplet code

• The nucleotide sequence of the

mRNA is colinear with the amino acid sequence of the polypeptide in

encodes The relationship between

them is called the genetic code

• 5’ -3’ N terminal -C terminal

• The three nucleotides in group called codons

• 4 3 =64 codons

• Initiation codon (AUG)

• stop codon ( UAA, UAG,UGA)

Triplet code

The genetic code is degenerate

• 64 codons ,but only 20 amino

acids,so that ,a single amino acid is coded for by several different

codons, that is degenerate

• Synonyms:different codons specify the same amino acid

• Codon and anticodon: a triplet of bases in a

specific tRNA molecule.each base is the codon base –pairs with its complementary base in the anticodon.

• Wobble base-pairing

Degeneracy of the genetic code

Wobble rules

Universality of the genetic code

• All living organism used the

Different code

Reading frames

Open reading frames

• An open reading frame(ORE) is a run of codons that starts with ATG and ends with atermination codon, TGA TAA or TAG.Coding regions of genes contain relatively long OPFs unlike noncoding DNA where ORFs are comparatively short

• Translation relies upon

aminoacyl-tRNA that carry specific amino acids and recognize the

corresponding codons in mRNA

by anticodon-codon base-pairing

Synthesis of tRNA(amino acid activation)

Synthesis of aminoacyl-tRNA is crucially important for two reasons:

• First :each amino acid must be

covalently linked to a tRNA molecule in order to take part in protein

synthesis,which depend upon the

adaptor function of tRNA

• Second :the covalent bond is a

high energy bond that enables the amino acid to react with the end of the growing polypeptide chain

The synthesis reaction occurs in

two steps

• The first step is the reaction of amino acid and ATP to form an aminoacyl-AMP

• The second step is the aminoacyl

group of aminoacyl-AMP is

transferred to the 3’end of the Trna

molecule to form aminoacyl-tRNA

Step one

Step two :

Aminoacyl-AMP+tRNA -aminoacyl-tRNA +AMP

Initiation of protein synthesis

fMet-tRNA f Met

Elongaion

Protein synthesis in E.coli

• Each ribosome has three

binding sites for tRNAs; an A site where the incoming aminoacyl-

tRNA binds, a P site where the

tRNA linked to the growing

polypeptide chain is bound, and

an E site which binds tRNA prior

to its release from the ribosome

• Translation in prokaryotes begins by

the formation of a 30s initiation

complex between the 30s ribosomal

subunit, mRNA, initiation factors and fMet tRNA f met The 30s subunit

binds to the Shine-Dalgarno sequence which lies 5' to the AUG Start codon

and is complementary to the 16s rRNA

of the small ribosomal subunit

• The ribosome then moves in a 3'

direction along the mRNA until it

encounters the AUG codon The 50s ribosomal subunit now binds to the

30s initiation complex to form the 70s intiation complex In this complex,the anticodon of form the 70s intiation

complex.In this complex, the

anticodon of the fMet tRNA f met is

base-paired to the AUG initiation

codon (start codon) in the P site

INITIATION

ELONGATION

Elongation Factor Tu(EF-Tu)

TERMINATION

• Initiation

• Elongation

• Termination

H3 Translation in eukaryotes

• Overview

• Secretory protein

• Plasma membrane proteins

• Proteins of the endoplasmic reticulum

• Lysosomal proteins

• Mitochondrial and chloroplast proteins

• Nuclear proteins

H4 Protein targeting

• Both in prokaryotes and

eukaryotes, newly synthesized

proteins must be delivered to

specific subcellular location or

exported from the cell for correct for activity This phenomenon is called protein targeting

Secretory proteins

• Secretory proteins have an N-terminal

signal peptide which targets the protein to

be synthesized on the rough endoplasmic reticulum

• During synthesis it is translocated through the RER membrane into the lumen

• Vesicles then bud off from the RER and carry the protein to the Golgi

complex,where it becomes glycosylated.

• Others vesicles then carry it to the

plasma membrane.

• Fusion of these transport vesicles with the plasma membrane then releases the protein to the cell exterior.

Plasma membrane proteins

• The orientation of the protein in the membrane is determined

by topogenic sequences within the polypeptide chain 。 Type proteins have a cleaved N-terminal signal sequence and a

Ⅰ

hydrophobic stop-transfer sequence ， Type have an Ⅱ

uncleaved N-terminal signal sequence that doubles as the

membrane-anchoring sequence ， and Type have multiple Ⅲ signal sequences and stop-transfer sequences 。

Proteins of the endoplasmic reticulum

• Proteins destined for the ER have an

N-terminal signal peptide ， are

synthesized on the RER ， are

translocated into the RER lumen and

transported by vesicles to the Golgi 。

Once there ， a C-terminal amino acid sequence （ KDEL ） is recognized by

a Golgi receptor protein that causes other vesicles to return the protein to the ER

Lysosomal proteins

• Lysosomal proteins are targeted to

the lysosomes via the addition of a

mannose 6-phosphate signal that is addad in the cis-compartment of the Golgi and is recognized by a receptor protein in the trans-compartment of

the Golgi 。

Mitochondrial and chloroplast

proteins

• Most mitochondria and chloroplast

protein are made on free cytosolic

ribosomes ， released into the

mitochondrial matrix requires a

matrix-targeting sequence and

occurs at sites where the outer and

inner mitochondrial membranes

come into contact 。

• Three types of protein glycosylation

• Synthesis of O-linked oligosaccharides

• Synthesis of N-linked oligosaccharides

H5 protein glycosylation

Three types of protein

glycosylation

• Many protein synthesized by ribosomes

of the RER contain short chains of

and are called glycoproteins The

oligosaccharides are of two main types

； O-linked （ to the Ohside chain of

Synthesis of O-linked oligosaccharides

• O-linked oligosaccharides are

Synthesized by the sequential

addition of monosaccharides to the

protein as it passes through the Golgi complex

Synthesis of N-linked

oligosaccharides

• All N-linked oligosaccharides have a

common pentasaccharide core structure

of three mannose residues and two

This is then transferred to the protein

and subsequently trimmed during

passage of the protein through the RER and Golgi complex, Additional

monosaccharides are added in the

Golgi to produce either a high mannose type oligosaccharide or a complex type oligosaccharide