�。ㄈ�)多肽鏈的延長：
在多肽鏈上每增加一個(gè)氨基酸都需要經(jīng)過進(jìn)位，轉(zhuǎn)肽和移位三個(gè)步驟。
（1)為密碼子所特定的氨基酸t(yī)RNA結(jié)合到核蛋白體的A位，稱為進(jìn)位。氨基酰tRNA在進(jìn)位前需要有三種延長因子的作用，即，熱不穩(wěn)定的EF（Unstable temperature,EF)EF-Tu,熱穩(wěn)定的EF(stable temperature EF,EF-Ts)以及依賴GTP的轉(zhuǎn)位因子。EF-Tu首先與GTP結(jié)合，然后再與氨基酰tRNA結(jié)合成三元復(fù)合物，這樣的三元復(fù)合物才能進(jìn)入A位。此時(shí)GTP水解成GDP，EF-Tu和GDP與結(jié)合在A位上的氨基酰tRNA分離（圖18-12)。
圖18-12 原核生物肽鏈延長因子EFTu與EFTs的作用原理
圖18-13 肽鍵的形成
①核蛋白體“給位”上攜甲酰蛋氨�！』ɑ螂孽�)的tRNA
②核蛋白體“受體”上新進(jìn)入的氨基酰tRNA;
③失去甲酰蛋氨�；ɑ螂孽�)后，即將從核蛋白體脫落的tRNA;
④接受甲酰蛋氨酰基（或肽酰)后已增長一個(gè)氨基酸殘基的肽鍵
�。�2)轉(zhuǎn)肽--肽鍵的形成（peptide bond formation)
在70S起始復(fù)合物形成過程中，核糖核蛋白體的P位上已結(jié)合了起始型甲酰蛋氨酸t(yī)RNA，當(dāng)進(jìn)位后，P位和A位上各結(jié)合了一個(gè)氨基酰tRNA，兩個(gè)氨基酸之間在核糖體轉(zhuǎn)肽酶作用下，P位上的氨基酸提供α-COOH基，與A位上的氨基酸的α-NH2形成肽鍵，從而使P位上的氨基酸連接到A位氨基酸的氨基上，這就是轉(zhuǎn)肽。轉(zhuǎn)肽后，在A位上形成了一個(gè)二肽酰tRNA（圖18-13)。
�。�3)移位（Translocation)
圖18-14 Diagram of the elongation process in protein synthesis following initation.tRNAs are shown as lue lines;AA,aamicoacyl group.The positioning shown of the EF-Tu-GTP on the tRNA and on the ribosome are arbitraty .This diagram does not make evident why the ribosomal enzyme catalysing the peptide bond synthesis is called peptidyl transferase .However,if you do the next round of synthesis yourself(see text),you will see that in all subsequent rounds of synthesis it is a peptide that is transferred to the incoming aminoacyl tRNA-hence the name.
轉(zhuǎn)肽作用發(fā)生后，氨基酸都位于A位，P位上無負(fù)荷氨基酸的tRNA就此脫落，核蛋白體沿著mRNA向3’端方向移動(dòng)一組密碼子，使得原來結(jié)合二肽酰tRNA的A位轉(zhuǎn)變成了P位，而A位空出，可以接受下一個(gè)新的氨基酰tRNA進(jìn)入，移位過程需要EF-2，GTP和Mg2+的參加（圖18-14)。
以后，肽鏈上每增加一個(gè)氨基酸殘基，即重復(fù)上述進(jìn)位，轉(zhuǎn)肽，移位的步驟，直至所需的長度，實(shí)驗(yàn)證明mRNA上的信息閱讀是從5’端向3’端進(jìn)行，而肽鏈的延伸是從氮基端到羧基端。所以多肽鏈合成的方向是N端到C端（圖18-15)。
圖18-15 The elongation phase of protein synthesis on a ribosome.The three-step cycle shown is repeated over and over during the synthesos of a protein chain.An aminoacyl-tRNA moleculie binds to the A-site on the rebosome is step l,a new peptide bond is formed in sted 2,and the ribosome moves a distance of three nucleotides along the mRNA chain in step 3,ejecting an old tRNA molecule and "resetting" the ribosome so that the next aminoacyl-tRNA molecule can bind.As indicated in Figure 6-21,the p-site is drawn on the left side of the ribosome,with the A-site on the right.
圖18-16 The final phase of protein synthesis .The binding of release factor to a stop codon terminates translation.The completed polypeptide is released,and the ribosome dissociates into two separte subunits.
