Joined: May 2002
Might as well add these as I'm discussing them over at EvC:
On the evolution of PCP degradation:
Evolution of a metabolic pathway for degradation of a toxic xenobiotic: the patchwork approach.
Trends Biochem Sci 2000 Jun;25(6):261-5
Dept of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental Studies, University of Colorado at Boulder, Boulder, CO 80309, USA. firstname.lastname@example.org
The pathway for degradation of the xenobiotic pesticide pentachlorophenol in Sphingomonas chlorophenolica probably evolved in the past few decades by the recruitment of enzymes from two other catabolic pathways. The first and third enzymes in the pathway, pentachlorophenol hydroxylase and 2,6-dichlorohydroquinone dioxygenase, may have originated from enzymes in a pathway for degradation of a naturally occurring chlorinated phenol. The second enzyme, a reductive dehalogenase, may have evolved from a maleylacetoacetate isomerase normally involved in degradation of tyrosine. This apparently recently assembled pathway does not function very well: pentachlorophenol hydroxylase is quite slow, and tetrachlorohydroquinone dehalogenase is subject to severe substrate inhibition.
[On the key step of the origin of PcpC, the central step in the origin of PCP degradation]
Recruitment of a double bond isomerase to serve as a reductive dehalogenase during biodegradation of pentachlorophenol.
Biochemistry 2000 May 9;39(18):5303-11
Anandarajah K, Kiefer PM Jr, Donohoe BS, Copley SD.
Department of Molecular, Cellular and Developmental Biology and Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Campus Box 216, Boulder, Colorado 80309-0216, USA.
Tetrachlorohydroquinone dehalogenase catalyzes the replacement of chlorine atoms on tetrachlorohydroquinone and trichlorohydroquinone with hydrogen atoms during the biodegradation of pentachlorophenol by Sphingomonas chlorophenolica. The sequence of the active site region of tetrachlorohydroquinone dehalogenase is very similar to those of the corresponding regions of maleylacetoacetate isomerases, enzymes that catalyze the glutathione-dependent isomerization of a cis double bond in maleylacetoacetate to the trans configuration during the catabolism of phenylalanine and tyrosine. Furthermore, tetrachlorohydroquinone dehalogenase catalyzes the isomerization of maleylacetone (an analogue of maleylacetoacetate) at a rate nearly comparable to that of a bona fide bacterial maleylacetoacetate isomerase. Since maleylacetoacetate isomerase is involved in a common and presumably ancient pathway for catabolism of tyrosine, while tetrachlorohydroquinone dehalogenase catalyzes a more specialized reaction, it is likely that tetrachlorohydroquinone dehalogenase arose from a maleylacetoacetate isomerase. The substrates and overall transformations involved in the dehalogenation and isomerization reactions are strikingly different. This enzyme provides a remarkable example of Nature's ability to recruit an enzyme with a useful structural scaffold and elaborate upon its basic catalytic capabilities to generate a catalyst for a newly needed reaction.
[atrazine degradation, a similar case]
Melamine deaminase and atrazine chlorohydrolase: 98 percent identical but functionally different.
J Bacteriol 2001 Apr;183(8):2405-10
Seffernick JL, de Souza ML, Sadowsky MJ, Wackett LP.
Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, St. Paul, Minnesota 55108, USA.
The gene encoding melamine deaminase (TriA) from Pseudomonas sp. strain NRRL B-12227 was identified, cloned into Escherichia coli, sequenced, and expressed for in vitro study of enzyme activity. Melamine deaminase displaced two of the three amino groups from melamine, producing ammeline and ammelide as sequential products. The first deamination reaction occurred more than 10 times faster than the second. Ammelide did not inhibit the first or second deamination reaction, suggesting that the lower rate of ammeline hydrolysis was due to differential substrate turnover rather than product inhibition. Remarkably, melamine deaminase is 98% identical to the enzyme atrazine chlorohydrolase (AtzA) from Pseudomonas sp. strain ADP. Each enzyme consists of 475 amino acids and differs by only 9 amino acids. AtzA was shown to exclusively catalyze dehalogenation of halo-substituted triazine ring compounds and had no activity with melamine and ammeline. Similarly, melamine deaminase had no detectable activity with the halo-triazine substrates. Melamine deaminase was active in deamination of a substrate that was structurally identical to atrazine, except for the substitution of an amino group for the chlorine atom. Moreover, melamine deaminase and AtzA are found in bacteria that grow on melamine and atrazine compounds, respectively. These data strongly suggest that the 9 amino acid differences between melamine deaminase and AtzA represent a short evolutionary pathway connecting enzymes catalyzing physiologically relevant deamination and dehalogenation reactions, respectively.
Edited by niiicholas on Dec. 01 2002,20:39