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tion as "probably negligible" and did not include the energy costs of posttranslational modifications. Today, however, it is known that these costs are not negligible. Using another approach relating protein (ATP) utilization to total ATP synthesis, Reeds et al. (1985) came to roughly similar conclusions but, after addressing the potential additional costs of all the cellular regulatory reactions, estimated that these control reactions and other, unaccounted-for, costs would increase the true energy cost of protein metabolism significantly.

Young and Yu (1996) have listed some of these energy-dependent processes (Table 5-2). In addition to the costs of protein synthesis, there is an energy-dependent cost of protein degradation via the ubiquitin pathway.

TABLE 5-1 Some Functions of Amino Acids and Their Products

Function

Example

Substrates for protein synthesis

Those amino acids for which there is a codon

Regulators of protein turnover

Leucine, arginine?

Regulators of enzyme activity

Arginine and N-acetylglutamate synthetase Phenylalanine and phenylalanine hydroxylase activation

Precursor of signal transducer

Arginine and nitric oxide

Methylation reactions

Methionine

Neurotransmitter

Tryptophan, glutamate

Ion fluxes

Taurine, glutamate

Physiologic molecular precursors

Arginine, glutamine, purines

Nitrogen transporters

Alanine, glutamine

Circulating transporters

Ceruloplasmin; apolipoproteins Vitamin and hormone binding proteins

Messengers/Signals

Insulin, growth factors

Movement

Actin, kinesin

Immunity

Antibodies, interleukins

Growth, differentiation, gene expression

Growth factors, transcription factors

 

SOURCE: Adapted from Young and Yu, 1996.



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