Similarly, there are a large number of energy-dependent processes involved in the regulation of protein metabolism, as expanded on below, and there are other assorted costs attached to the body's nitrogen metabolism cycles. The latter cycles are addressed by others in this volume.

Of first concern are the energy costs of cell replication. Before the cost of protein synthesis itself can be calculated, one must recognize that the proteolytic degradation of cyclins regulates the movement from one phase of the cell cycle to the next through energy-dependent ubiquitin pathways (King et al., 1996; Martin-Castellanos and Moreno, 1997). Of second concern is that mitotic events, including spindle microtubular growth, and spindle elongation during anaphase require hydrolysis of GTP or ATP.

Protein synthesis itself has a higher cost than previously estimated. While transcription of the amine acid mRNA codon requires six ATP per amine acid and activation to amine acyl-tRNA requires another two ATP, there is