Biology Reference
In-Depth Information
5.1.3.1 Glutamate tRNA Ligase
A query for Glutamate t-RNA Ligase addressed to the SwissProt and PIR protein
databases via the Biology Workbench, yielded 38 distinct sequences which are
depicted on the LPBP website at http://www.vlpbp.org/greening/xv/ , Sequenced
Enzymes/Glu-tRNA Ligase, as well as in Appendix I .
5.1.3.2 Glutamate tRNA Oxido Reductase
A query for Glutamate t-RNA (Oxido) Reductases addressed to the PIR, PRODOM,
SwissProt, and TREMBL protein databases via the Biology Workbench, yielded
many distinct sequences which are depicted on the LPBP website at http://www.
vlpbp.org/greening/xv/ , Sequenced Enzymes/Glu-tRNA Reductase, as well as in
Appendix I .
5.1.3.3 Glutamate Semialdehyde Aminotransferase
A query for GSA-Aminotransferase addressed to the PIR, PRODOM, SwissProt,
and TREMBL protein databases via the Biology Workbench, yielded 28 unique
sequences which are depicted on the LPBP website at http://www.vlpbp.
org/greening/xv/ , Sequenced Enzymes/GSA-Aminotransferase”, as well as in
Appendix I .
5.2 Biosynthesis of Porphobilinogen (PBG)
PBG is the precursor of uroporphyrinogen III (Urogen III) that is the precursor of all
intermediates of the heme and Chl biosynthetic pathways (Fig. 5.6 ). It is formed
from two molecules of ALA; in the process two molecules of water are liberated
(Fig. 5.7 ). The dimerization reaction is catalyzed by ALA dehydratase also known
as PBG synthase (Gibson et al. 1955 ; Schmid and Shemin 1955 ). The enzyme
binding sites of the two ALA substrates have been designated the A and P sites. The
A site gives rise to the acetic side chain, while the P site gives rise to the propionic
side chain of PBG. The first substrate binds to the P site where it forms an Schiff
base with the enzyme. The second ALA molecule interacts with the A site (Jordan
and Seehra 1980 ) to form an enzyme-two ALA substrate complex. The precise
mechanism by which the 5-membered PBG ring is formed from the enzyme-two
substrate complex is still uncertain.
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