生物化學(xué)Proteinsynth.ppt

1、Proteinsynthesis,Chapter 13,translation,The nucleotide sequence of mRNA is translated into a sequence of amino acids by a genetic code(遺傳密碼) from its 5 end to 3 end.,what the translation needs,mRNA (template) amino acids (materials) ribosome (factory, rRNA and protein) ATP and GTP (energy) tRNA (car

2、rier, interpreter),13.1 The genetic code,The genetic code is the rules that specify how the nucleotide sequence of an mRNA is translated into the amino acid sequence of a polypeptide. The nucleotide sequence is read as triplet codon (三聯(lián)子密碼).,遺傳密碼,mRNA 分子中所儲(chǔ)存的遺傳信息是由其分子的四種堿基以特定順序排列成的三個(gè)一組的三聯(lián)體代表的，每種氨基酸都

3、有其特定的三聯(lián)體密碼。,That means three nucleotides produce one amino acid(43).,Features of genetic code,the genetic code is universal.（通用性） it is almost the same in all organisms. the genetic code is unambiguous.(不重復(fù)) each codon corresponds to only one amino acid.,the genetic code is degenerate(簡并). in most c

4、ases, one amino acid is coded by several codons, which are called synonymous codons(同義密碼子). the genetic code is wobbling(擺動(dòng)) each codon is recognized by a triplet of bases, called an anticodon(反義密碼子) in RNA. Each codon base pairs with its complementary anticodon base. However, the pairing of the thi

5、rd base of a codon is less stringent than for the first two bases.,13.2 Transfer RNA,the secondary structure of tRNA is cloverleaf(三葉草形). It contains five arms: acceptor stem(受體臂), D arm(雙氫尿嘧啶臂), TC arm(假尿嘧啶臂), anticodon arm and extra arm.,the tertiary structure of tRNA is an inverse “L” form.,The f

6、unction of tRNA,Each tRNA becomes covalently bonded to a specific amino acid to form aminoacyl-tRNA (氨酰tRNA) which recognizes the corresponding codon in mRNA and ensures that the correct amino acid is added to the growing polypeptide chain.,13.3 Ribosomes,Ribosomes are large ribonucleoprotein(核蛋白) p

7、articles that contain more RNA than protein and dissociate into large and small subunits.,13.4 Process of translation,During translation the mRNA is read from 5 end to 3 end, and protein is synthesized in an N terminal to C terminal direction.,Principle for DNA replication: base-pairing Principle fo

8、r protein synthesis: anticodon-codon base-pairing （反密碼子密碼子）,Process of translation,amino acid activationsynthesis of aminoacyl-tRNA (氨酰tRNA) initiation of translation elongation of translation termination of translation,Amino acid + tRNA + ATP Aminoacyl-tRNA +AMP + PPi,aminoacyl-tRNA synthetase,(氨酰t

9、RNA合成酶),Each tRNA carries only a single amino acid, and a separate aminoacyl-tRNA synthetase exists for every amino acid.,In prokaryotes the initial amino acid is fMet (甲酰甲硫氨酸)，and the corresponding initial aminocyl-tRNA is fMet-tRNAfMet； In eukaryotes, the initial amino acid is Met，the correspondin

10、g initial aminoacyl-tRNA is Met-tRNAMet.,Initiation step,P位,A 位,The small subunit (30S) binds to the mRNA The initiator tRNA charged with N-formylmethionine (fMet-tRNA) binds to form the 30S initiation complex (30S起始復(fù)合物) The large ribosomal subunit binds to form a 70s initiation complex (70S起始復(fù)合物) w

11、ith the hydrolysis of GTP,Elongation of peptide chain,Aminoacyl-tRNA binding : the corresponding aminacyl-tRNA for the second codon binds to the A site（進(jìn)位）（GTP hydrolysis） Peptide bond formation: the carboxyl end of the amino acid bound to the tRNA in the P site becomes joined by a peptide bond to t

12、he amino group of the amino acid linked to the tRNA in the A site，catalyzed by peptidyl transferase (轉(zhuǎn)肽酶) Translocation (移位): ribosome moves along the mRNA (5 to 3) by three nucleotides so that the dipeptidyl-tRNA in the A site moves to the P site to place the next codon in the A site (GTP hydrolysi

13、s),After translocation, the A site is empty and ready to receive the next aminoacyl-tRNA.,Termination (合成終止),Termination codons appear at the A site (UAG,UAA, UGA) Release factors (終止因子) binds to the ribosome. Peptidyl transferase is transformed to become a hydrolase(水解酶) so that the polypeptide is

14、transferred to a water molecule, effectively cleaving the bond between the polypeptide and tRNA in the P site. Then the polypeptide leaves the ribosome, flollowed by mRNA and free tRNA. The ribosome dissociates into 30S and 50S subunits ready to start translation afresh. (1 ATP),Energy yield,Amino acid activation: 2 ATP Formation o