Составить формулы трипептида гли-ала-сер.

Скорее всего, должно быть так: 

                      O    H             O   H               O

                       ||     |               ||     |                ||

H2N – CH2 – C – N – CH – C – N – CH – C – OH

                                    |                      |

                                   CH3                CH2

                                                           |

                                                          SH