Biochemistry - The Chemical Reactions of Living Cells Volume 3 - D. Metzler 1980

Metabolism of Nitrogenous Compounds
Synthesis and catabolism of compounds belonging to the glutamic acid family
Transfer of amidino groups and synthesis of creatine

The terminal amidine group of Arginine is transferred intact to A number of Other Compounds via simple substitution reactions. A classic example is The formation of guanidinoacetic acid [Eq. (14-18)].

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As a result of Transmethylation from S-adenosylmethionine [Eq. (14-18), stage b], guanidinoacetic acid is converted into creatine, which plays a particularly crucial role in Muscle tissue. Creatine kinase reversibly transfers a phosphate residue from ATP to creatine to form N-phosphate [Eq. (14-18), stage c]. Creatine phosphate acts as a vital "energy buffer" During Muscle contraction (Box 10-E). The reversible action of creatine kinase ensures the rapid transfer of the phosphate group back to ADP as soon as the latter is generated by ATP Hydrolysis during muscle contraction. The end product of creatine phosphate METABOLISM is creatinine anhydride, which is formed, firstly, from creatine phosphate as shown in Eq. (14-18), stage d, and secondly, directly from creatine. Creatinine excretion in urine remains remarkably constant from day to day for any given individual. Apparently, the amounts of creatinine excreted are directly proportional to a person's muscle mass. Another example of The transfer of arginine amidine groups is The Biosynthesis of streptomycin (Box 12-A).