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					•     C      H          A        P       T         E       R   •       18         •



                   UREA CYCLE
                                          •


•      •     •      •        •       •         •       •   •       •    •         •




    UREA CYCLE
    Function: To provide a route to dispose of the amino groups
       from amino acids during their metabolism.
    Location: Liver, kidney
    Connections: From amino groups of amino acids through gluta-
       mate and glutamate dehydrogenase
       From amino groups of amino acids through aspartate and ar-
       gininosuccinate synthase
       From ammonia through carbamoyl phosphate synthetase
       To urea
    Regulation: Primarily by availability of amino groups and ammonia
    Equation:

    NH 4    CO2     Asp       2ATP ¡
                            NH2C(“O)NH2            fumarate    2ADP         2Pi

    Glu NAD     CO2 Asp 2ATP ¡
      NH2C(“O)NH2   -ketoglutarate fumarate                    2ADP         2Pi



(See Fig. 18-1.)




                                         225
•   226   •                                                                                Basic Concepts in Biochemistry



                                               ketoacid

                                     amino acid
                                                                          Glutamate
                              amino groups into                                      NAD
                                                        glutamate
                              glutamate via
                                                      dehydrogenase                  NADH
                              transamination
                                                      2-ketoglutarate
                                                                               NH4      CO2         2ATP

                                                                                             carbamoyl phosphate
                                                                                                  synthetase
                     O                                                         O




                                                                              —
                     —




          H2N— C — NH2                         NH3                     H2N— C — O — P
                     urea                                              carbamoyl phosphate
                                               —




                                               —
                                               —
                                               —
                                               —
                         NH2              omithine                      NH2
                         —




                                                                        —



                         C — NH2                          citrulline     C—O
                                                                        —
                         —




                                    arginine
                         NH                                             NH
                                                                        —
                         —




                         —
                         —                                               —
                                                                         —
                         —
                         —                                               —
                                                                         —
                                      argininosuccinate
                                                                                              CO2
                                                                               H3N —
                                     NH2                                 ATP             CO2
                                     —




              O2 C                                         CO2
                                     C — NH —                                           aspartate
                                                                                                              ketoacid
                                     —




                                                     CO2                           amino groups into
                      CO2                                                          aspartate via
                                     NH
                fumarate                                                           transamination
                                     —




                                      —
                                      —                                                                       amino acid
                                                                                     Oxaloacetate
                                      —
                                      —


Figure 18-1                 The Urea Cycle

				
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