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1、Chapter 8 Translation,Where? Ribosomes Direction on mRNA: 5-to- 3 direction Direction of protein synthesis:N-terminal to C-terminal,Translation :messenger RNA (mRNA) is decoded to produce a specific polypeptide according to the rules specified by the genetic code.,Amino acids are joined together in
2、proteins by peptide bonds. A peptide bond forms between the carboxyl group of one amino acid and the amino group of the adjacent amino acid.,Section 1 An Introduction,1. Genetic codetripcode1967年，科学家们破译了全部密码子。,遗传密码的简并性: 多种密码子编码一个氨基酸的现象。 degeneracy：The presence in the genetic code of more than one co
3、don for the same amino acid. (The extra codons mean the code is more resilient- small errors can still result in the same protein being produced. ?) 遗传密码的通用性universal ：生物界从低等到高等，都使用几乎完全相同的一套密码子。但真核生物线粒体的密码子有若干处不同于通用密码子。protein synthesis in mitochondria relies on a genetic code that varies from the c
4、anonical code.,2、tRNA是氨基酸的运载工具,tRNA secondary and tertiary structure,Wobble of the anticodon 摆动假说,The base at the 5-end of the anticodon is not as constrained as the other two bases,Definition of wobble,According to the wobble hypothesis only the first two positions of a triplet codon on mRNA have a
5、 precise pairing with the bases of the tRNA anticodon. The pairing of the third position base of the codon with the first position base of an anticodon may be ambiguous, and varies according to the nucleotide present in this position. For example, G-U ,U-G,I- U,I-C,etc. Thus a single tRNA type is ab
6、le to recognize two or more codons differing only in the third base. For example, inosine in position 1 in the anticodon can base pair with A, u or C in position 3 in the mRNA codon, so that for example UCU, UCC, UCA all code for serine using an inosine anticodon.,副密码子：tRNA分子上决定其携带AA的区域被称为paracodon
7、。 Factors that determine the specificity of tRNA synthetases: Specific anticodon in tRNA Other recognition regions: the base next to the CCA in the acceptor stem correlates with the nature of the cognate amino acid of that tRNA Single base pair in acceptor stem 3D structure of tRNA synthetases 同功tRN
8、A( isoaccepting tRNAs): 携带AA相同而反密码子不同的一组tRNA称为同功tRNA. 一组同功tRNA由同一种氨酰基-tRNA合成酶催化而携带AA。,Encyclopedia of Genetics, Genomics, Proteomics and Informatics 2008, p 1036 Isoacceptor tRNAsThis group of different tRNAs accepts the same amino acid. The identity of a specific isoaccepting tRNA is determined by
9、particular nucleotide sites within the group. The anticodon may serve as an identity site, however the six serine tRNAs do not share a common anticodon base. The nucleotide 73 is a discriminator base and the first base pairs within the acceptor stem. The long variable arm of the tRNASer, the extra G
10、1:C73 bp in tRNAHis, and the G3:U70 bp in tRNAAla can characterize E. coli tRNAs. In other species identity may be differently determined. The anticodon-flanking nucleosides may play an important discriminating role. Trna, aminoacyl-tRNA synthetase, wobble, protein synthesis, transcript; Heyman T et
11、 al 1994 FEBS Lett 34723:143; Chaley MB et al 1999 J Mol Evol 482:168; Crain PF et al 2002 RNA 8:752; Agris P 2004 Nucleic Acids Res. 32:223.,3、Aminoacyl-tRNA synthetase 氨酰基-tRNA合成酶,（一）合成原料氨基酸由mRNA编码的氨基酸共有20种，只有这些氨基酸能够作为蛋白质生物合成的直接原料。某些蛋白质分子还含有羟脯氨酸、羟赖氨酸、-羧基谷氨酸等，这些特殊氨基酸是在肽链合成后的加工修饰过程中形成的.(二) 氨酰基-tRNA合
12、成酶(ARS)catalyzes the bonding between specific tRNAs and the amino acids.,(1) AA activation（氨基酸活化） AA + ATP + E* Aminoacyl -AMP-E + PPi E: aminonacyl-tRNA synthetase (ARS ) (2) The appropriate tRNA displaces the AMP Aminoacyl-AMP-E + tRNA AA - tRNA + AMP +E,E AA + tRNA + ATP aminonacyl-tRNA + AMP +pp
13、i,4、蛋白质合成的机器核糖体 Ribosome,E.coli 70S核糖体由RNA与蛋白质组成。含两个亚基（50S和30S）。大亚基约为小亚基的两倍。小亚基有21种蛋白质 ( S1S21 )和16S RNA； 大亚基有31种蛋白质(L1L31 )和23S、5S RNA。16S 结构保守，3 端一段ACCUCCUUA的保守序列，（反SD序列，the anti-Shine-Dalgarno sequence），与mRNA 5 端翻译起始区富含嘌呤的序列互补，故与蛋白质的合成启始有关。在邻近3 端还有一段与23S rRNA互补的序列，在30S与50S亚基的结合中起作用。,Definition of
14、 Shine-Dalgarno (S-D) sequence,An eight nucleotide concensus sequence 5 UAAGGAGG 3 found in bacterial mRNAs five to ten bases before the translation initiation codon (AUG). It is thought to be involved in initiation of translation by helping the mRNA bind to the ribosome (16S rRNA), thus it can be c
15、alled the ribosomal binding site .,1.mRNA结合位点 位于30S亚基的头部。16SrRNA3端的富含嘧啶区与mRNA上AUG之前的富含嘌吟区发生碱基配对，是30S亚基与mRNA的初始结合不可缺少的。 2.P site 肽基-tRNA结合位点，主要在小亚 3.A site 氨酰基-tRNA结合位点,主要在大亚基 4.肽基转移酶(peptidyl transferase)活性位点 位于P位和A位的连接处，靠近tRNA的接受臂。在 50S亚基上，位于P、A之间.该活性既需要rRNA,也需要50s亚基蛋白质。催化活性可能与50s亚基的RNA有关。,peptidyl
16、 transferase,An enzyme that catalyzes the addition of amino acid residues to the growing polypeptide chain in protein synthesis by means of peptide bonds.It is the primary enzymatic function of the ribosome. Integral enzymic activity of the large subunit of a ribosome. It is a ribozyme and the catalytic function resides in the 23S ribosomal RNA.,5、蛋白质因子 initiation factors, elongation factors, release factors 起始因子（3）、 延伸因子（ 3 ）和释放因子（3） （结合过程讲述）,
