Amino acid synthesis
The regulation of the synthesis of glutamate from α-ketoglutarate is subject to regulatory control of the Citric Acid Cycle as well as mass action dependent on the concentrations of reactants involved due to the reversible nature of the transamination and glutamate dehydrogenase reactions.
[2] The confirmation of the enzyme plays a role in regulation depending on if GS is in the taut or relaxed form.
The taut form of GS is fully active but, the removal of manganese converts the enzyme to the relaxed state.
The regulation of proline biosynthesis can depend on the initial controlling step through negative feedback inhibition.
[4] In E. coli, proline allosterically inhibits Glutamate 5-kinase which catalyzes the reaction from L-glutamate to an unstable intermediate L-γ-Glutamyl phosphate.
[4] Arginine synthesis also utilizes negative feedback as well as repression through a repressor encoded by the gene argR.
This process is mediated by a phenylalanine (PheA) or tyrosine (TyrA) specific chorismate mutase-prephenate dehydrogenase.
The oxaloacetate/aspartate family of amino acids is composed of lysine, asparagine, methionine, threonine, and isoleucine.
The enzyme aspartokinase, which catalyzes the phosphorylation of aspartate and initiates its conversion into other amino acids, can be broken up into 3 isozymes, AK-I, II and III.
As a sidenote, AK-III catalyzes the phosphorylation of aspartic acid that is the committed step in this biosynthetic pathway.
The initial two stages of the DAP pathway are catalyzed by aspartokinase and aspartate semialdehyde dehydrogenase.
Transcription of aspartokinase genes is regulated by concentrations of the subsequently produced amino acids, lysine, threonine, and methionine.
Finally, DAP decarboxylase LysA mediates the last step of the lysine synthesis and is common for all studied bacterial species.
Glutamine donates an ammonium group, which reacts with β-aspartyl-AMP to form asparagine and free AMP.
This reaction occurs at a key branch point in the pathway, with the substrate homoserine serving as the precursor for the biosynthesis of lysine, methionine, threonin and isoleucine.
The His operon operates under a system of coordinated regulation where all the gene products will be repressed or depressed equally.
In this system the full leader sequence has 4 blocks of complementary strands that can form hairpin loops structures.
When histidine charged tRNA levels are low in the cell the ribosome will stall at the string of His residues in block 1.
However, when histidine charged tRNA levels are high the ribosome will not stall at block 1, this will not allow strands 2 and 3 to form a hairpin.
The regulation of glyA is complex and is known to incorporate serine, glycine, methionine, purines, thymine, and folates, The full mechanism has yet to be elucidated.
[15][16] On the other hand, PurR, a protein which plays a role in purine synthesis and S-adeno-sylmethionine are known to down regulate glyA.
PurR binds directly to the control region of glyA and effectively turns the gene off so that glycine will not be produced by the bacterium.
In the absence of the inducer, NAS, CysB will bind the DNA and cover many of the accessory half sites.
This conformational change allows CysB to bind properly to all the half sites and causes the recruitment of the RNA polymerase.
Reactions beginning with either one or two molecules of pyruvate lead to the synthesis of alanine, valine, and leucine.
The only definite method is the bacterium's ability to repress Transaminase C activity by either valine or leucine (see ilvEDA operon).
It begins with the condensation of two equivalents of pyruvate catalyzed by acetohydroxy acid synthase yielding α-acetolactate.
The second step involves the NADPH+-dependent reduction of α-acetolactate and migration of methyl groups to produce α, β-dihydroxyisovalerate.
Valine biosynthesis is subject to feedback inhibition in the production of acetohydroxy acid synthase.
When one of these amino acids is limited, the gene furthest from the amino-acid binding site of this operon can be transcribed.
