12
C
Carbon
12.011
Essential: all life
Carbon
Major functions in cells: (1)
-Basis of life: proteins, nucleic acids, lipids. etc.
Environmental and health impacts:
-CO2 fixation (photosynthesis) is key step in global C cycle and can mitigate anthropogenic emissions
Reduce:
-Proteins needed for C fixation are enriched in amino acids with relatively fewer C atoms (2)
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(1) Carbon: An Essential Macronutrient
Carbon is required in all cells and is considered one of the macronutrients that are essential for life. Carbon is required in proteins, nucleic acids, and lipids, which form the basis of all life on earth. Fixation of CO2 into organic molecules, predominantly through the Calvin cycle during photosynthesis, provides the bulk of C that supports life.
(2) C Limitation in Proteomes
Carbon limitation can affect protein composition. A single amino acid change in a protein could add up to 9 carbon atoms (Gly to Trp). In both S. cerevisiae and E. coli, those enzymes specifically involved in processes of C assimilation have, on average, slightly shorter (less C-rich) side chains than the bulk proteome (Bragg et al., 2012).
(3) Degradation of Ribosomes
Elemental-sparing responses are relatively ineffective in the face of the ubiquitous use of C in cell constituents, so when no external sources can be scavenged and cell growth becomes limited for macronutrients, processes of recycling become incredibly important. Ribosomes represent a major fraction of cell mass in rapidly growing bacteria, and studies in E. coli have revealed that ribosomes are degraded in cells presented with C limitation (Kaplan and Apirion, 1975). Degradation appears to occur during the transition to stationary phase and is correlated with the formation of free ribosomal subunits (Zundel, et al., 2009). It is not clear whether degradation of ribosomes serves primarily to liberate nutrients associated with the abundant rRNA (which is ~50% of the ribosome mass), the ribosomal proteins, or both.
(4) Degradation of Abundant Proteins
In addition to ribosomes, other abundant proteins may be targeted for proteolytic destruction upon nutrient limitation. In mammalian cells, protein degradation mediated by the proteasome provides an important source of amino acids for ongoing protein synthesis when amino acid availability becomes limiting (Vabulas and Hartl, 2005).