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TABLE 5-2 Some Energy-Dependent Processes Associated with Protein Metabolism

1. Protein Turnover

Formation of initiation complexes; peptide bond synthesis

Protein breakdown via ubiquitin-proteosome pathway

2. RNA Turnover

Ribosomal RNA; transfer RNA; pre-messenger RNA splicing; and messenger RNA

3. Amino acid transport

4. Regulatory processes

Reversible phosphorylation, GTP-GDP exchange proteins,

Ion pumps and channels, second messengers

5. Nitrogen metabolism

Glutamine/glutamate cycle

Glucose/alanine cycle

Urea synthesis

 

SOURCE: Adapted from Young and Yu, 1996.

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.

ENERGY COSTS OF PROTEIN SYNTHESIS AND REGULATION

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



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