The primary structure of a protein, or its amino acid sequence, is the product of mRNA translation and is synthesized in the cytoplasm by ribosomes.
The aa pool inside a cell can be used for various metabolic purposes, including:
synthesis of body proteins (e.g. hemoglobin, enzymes, hormones)
synthesis of non-protein, nitrogen containing compounds (e.g. serotonin)
gluconeogenesis
fat production
energy production (branched chain aa leucine, isoleucine and valine)