Rattus norvegicus Bifunctional
Enzyme and Coactivator Protein, DCoH
Kristen Edgeworth '20 and Natalie Bresnahan '20
Contents:
I. Introduction
Model View:
The dimerization cofactor of hepatocyte nuclear factor 1, or
DCoH, is a bifunctional protein that functions as both a
transcriptional coactivator and a metabolic enzyme. Here we are looking at DCoH
purified from rat liver. It is suspected to be involved in
stimulating the expression of homeodomain-containing, hepatocyte
nuclear factor 1 controlled genes[1]. HNF-1 genes are involved in growth and development of the liver[3]. It is possible that it does this
by enhancing dimerization and stability of HNF-1 proteins. DCoH may
also bind products of target genes or different components of the transcription complex[1].
The mechanism of transcriptional coactivation by DCoH is
much less understood than its metabolic enzyme function. DCoH was
originally only known for its enzymatic function as it was shown to
catalyze the dehydration of a 4a-carbinolamine form of the biopterin
cofactor utilized by the aromatic amino acid hydroxylases and NO
synthase. Its Pterin 4a-carbinolamine dehydratase (PCD) activity of
DCoH with the enzyme dihydropteridine reductase regenerates the
active reduced form of the cofactor. PCD dehydrates spontaneously in
aqueous solution, but at rates too slow to support the maximum
turnover rate of phenylalanine hydroxylase. In the cell,
DCoH-catalysed dehydration reaction appears to prevent the formation
of 7-substituted pterins which are caused by spontaneous
rearrangements of pterin-4a-carbinolamine. Often large amounts of
7-substituted pterins are excreted in patients with mutated DCoH
genes, so the function of DCoH in blocking the pterin cofactor from
rearranging is potentially just as important as its function
stimulating aromatic amino acid hydroxylase enzymes[1]. .
Figure 1. The metabolic cycle that includes DCoH.
(Cronk and Enrizzi, 1996)
II. General Structure
DCoH is a
containing four identical monomers A,
B, C,
and D. Each monomer consists
of an antiparallel
surrounded by three
These monomers interact with one another along two different
interfaces. The
is made of antiparallel H2
helices and S3 strands
from neighboring monomers. The interactions between the two subunits
creates a
a continuous eight-stranded antiparallel
Beta-sheet. This saddle houses a
that contains an aromatic arch, including residues Phe
43, Phe 47, His
63, Trp 66, and His
80. The other type of interface, the
consists of interactions between the
of all four monomers[1].
III. Variations Between Molecules
DCoH crystals have two independent tetramers that differ
structurally from one another due to crystal packing forces. Monomer
conformational differences occur mostly in the chain termini and
of the saddle. The stirrup is
composed of loops made from residues 28-32 that connect beta-strands
1 and 2. The
cause the saddles to vary in width from 24 A to 27 A.[1].
Along with the varying widths of the saddles, The
of Arg 21, Phe
35, Gln 37, His
39, His 74, and His
80 all adopt different rotamers in the eight different
monomers. Four of the side chains can even be found in the saddle
which suggests that the saddle actually has an element of
plasticity. One major difference between the crystal contacts of
the two different tetramers is that one tetramer packs an
into the saddle of a neighboring molecule[1].
IV. Structure of the 7,8-BH2 complex
7,8-dihydrobiopterin (7,8-BH2) is a ligand that binds
four clefts(the proposed active sites) of DCoH. Residues that
are found in both monomers of the saddle-forming dimer can be
found in the clefts. These clefts border the
The active site is lined by the
His 62, His
63, and Pro 64.
Asp 61 and loops containing
Leu 78-Gly 81 and Val
69' -Tyr 70' of the adjacent monomer are also found in
the cleft. In order to form the active site, Asp 61, Ala 83, and
Tyr 70’ must be in a left-handed helical conformation. Within
the arch of aromatic residues that form the hydrophobic core,
the 7,8-BH2 molecule binds the active site[1].
VIII. References
[1] Cronk, J., Endrizzi, J., and Alber, T. 1996. High-resolution structures of the bifunctional enzyme and its transcriptional coactivator DCoH and its complex with a product analogue.Protein Science. 5:1963-1972.
[2] Endrizzi, J., Cronk, J., Wang, W.,
Crabtree, G., and Alber, T. 1995. Crystal Structure of DCoH, a
Bifunctional, Protein-Binding Transcriptional Coactivator. Science268:
556-559.
[3] Frank J. Gonzalez. 2008. Regulation of Hepatocyte Nuclear Factor 4 alpha-mediated Transcription. Drug Metab. Pharmacokinet (2008). 23: 2–7.
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