DB code: D00265

RLCP classification 6.30.100000.5310 : Double-bonded atom exchange
8.211.591500.5520 : Isomerization
5.30.1645000.5521 : Elimination
4.504.771000.5521 : Addition
8.211.591500.5521 : Isomerization
CATH domain 3.40.640.10 : Aspartate Aminotransferase; domain 2 Catalytic domain
3.90.1150.10 : Aspartate Aminotransferase, domain 1
E.C. 2.5.1.48
CSA 1cs1 1qgn
M-CSA 1cs1 1qgn
MACiE

CATH domain Related DB codes (homologues)
3.40.640.10 : Aspartate Aminotransferase; domain 2 D00085 D00092 D00101 D00102 D00103 D00104 D00107 D00108 D00109 D00255 D00257 D00258 D00269 D00515 M00031 D00279
3.90.1150.10 : Aspartate Aminotransferase, domain 1 D00085 D00092 D00101 D00102 D00103 D00104 D00107 D00108 D00109 D00255 D00257 D00258 D00269 D00515 M00031 D00279

Uniprot Enzyme Name
UniprotKB Protein name Synonyms RefSeq Pfam
P00935 Cystathionine gamma-synthase
CGS
EC 2.5.1.48
O-succinylhomoserine (thiol)-lyase
NP_418374.1 (Protein)
NC_000913.2 (DNA/RNA sequence)
YP_491512.1 (Protein)
NC_007779.1 (DNA/RNA sequence)
PF01053 (Cys_Met_Meta_PP)
[Graphical View]
Q9ZPL5
Cystathionine gamma-synthase
PF01053 (Cys_Met_Meta_PP)
[Graphical View]

KEGG enzyme name
cystathionine gamma-synthase
O-succinyl-L-homoserine succinate-lyase (adding cysteine)
O-succinylhomoserine (thiol)-lyase
homoserine O-transsuccinylase
O-succinylhomoserine synthase
O-succinylhomoserine synthetase
cystathionine synthase
cystathionine synthetase
homoserine transsuccinylase
4-O-succinyl-L-homoserine:L-cysteineS-(3-amino-3-carboxypropyl)transferase

UniprotKB: Accession Number Entry name Activity Subunit Subcellular location Cofactor
P00935 METB_ECOLI O(4)-succinyl-L-homoserine + L-cysteine = L- cystathionine + succinate. Homotetramer. Cytoplasm. Pyridoxal phosphate.
Q9ZPL5 Q9ZPL5_TOBAC

KEGG Pathways
Map code Pathways E.C.
MAP00271 Methionine metabolism
MAP00272 Cysteine metabolism
MAP00450 Selenoamino acid metabolism
MAP00920 Sulfur metabolism

Compound table
Cofactors Substrates Products Intermediates
KEGG-id C00018 C01118 C00097 C00542 C00042
E.C.
Compound Pyridoxal phosphate O-Succinyl-L-homoserine L-Cysteine Cystathionine Succinate Quinonoid intermediate
Type aromatic ring (with nitrogen atoms),phosphate group/phosphate ion amino acids,carbohydrate,carboxyl group amino acids,sulfhydryl group amino acids,sulfide group carboxyl group
ChEBI 18405
 18405
 		16160
 16160
 		17561
 35235
 17561
 35235
 		17755
 17755
 		15741
 15741
PubChem 1051
 1051
 		439406
 439406
 		5862
 6419722
 5862
 6419722
 		4083111
 834
 4083111
 834
 		1110
 21952380
 1110
 21952380
1cs1A01 Bound:LLP Unbound Unbound Unbound Unbound Unbound
1cs1B01 Bound:LLP Unbound Unbound Unbound Unbound Unbound
1cs1C01 Bound:LLP Unbound Unbound Unbound Unbound Unbound
1cs1D01 Bound:LLP Unbound Unbound Unbound Analogue:DHD Unbound
1qgnA01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1qgnB01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1qgnC01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1qgnD01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1qgnE01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1qgnF01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1qgnG01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1qgnH01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i41A01 Analogue:HEN Unbound Unbound Unbound Unbound Intermediate-analogue:HEN
1i41B01 Analogue:HEN Unbound Unbound Unbound Unbound Intermediate-analogue:HEN
1i41C01 Analogue:HEN Unbound Unbound Unbound Unbound Intermediate-analogue:HEN
1i41D01 Analogue:HEN Unbound Unbound Unbound Unbound Intermediate-analogue:HEN
1i41E01 Analogue:HEN Unbound Unbound Unbound Unbound Intermediate-analogue:HEN
1i41F01 Analogue:HEN Unbound Unbound Unbound Unbound Intermediate-analogue:HEN
1i41G01 Analogue:HEN Unbound Unbound Unbound Unbound Intermediate-analogue:HEN
1i41H01 Analogue:HEN Unbound Unbound Unbound Unbound Intermediate-analogue:HEN
1i41I01 Analogue:HEN Unbound Unbound Unbound Unbound Intermediate-analogue:HEN
1i41J01 Analogue:HEN Unbound Unbound Unbound Unbound Intermediate-analogue:HEN
1i41K01 Analogue:HEN Unbound Unbound Unbound Unbound Intermediate-analogue:HEN
1i41L01 Analogue:HEN Unbound Unbound Unbound Unbound Intermediate-analogue:HEN
1i43A01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i43B01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i43C01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i43D01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i43E01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i43F01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i43G01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i43H01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i43I01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i43J01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i43K01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i43L01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i48A01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i48B01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i48C01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i48D01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i48E01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i48F01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i48G01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i48H01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i48I01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i48J01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i48K01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1i48L01 Bound:PLP Unbound Unbound Unbound Unbound Unbound
1cs1A02 Unbound Unbound Unbound Unbound Unbound Unbound
1cs1B02 Unbound Unbound Unbound Unbound Unbound Unbound
1cs1C02 Unbound Unbound Unbound Unbound Unbound Unbound
1cs1D02 Unbound Unbound Unbound Unbound Unbound Unbound
1qgnA02 Unbound Unbound Unbound Unbound Unbound Unbound
1qgnB02 Unbound Unbound Unbound Unbound Unbound Unbound
1qgnC02 Unbound Unbound Unbound Unbound Unbound Unbound
1qgnD02 Unbound Unbound Unbound Unbound Unbound Unbound
1qgnE02 Unbound Unbound Unbound Unbound Unbound Unbound
1qgnF02 Unbound Unbound Unbound Unbound Unbound Unbound
1qgnG02 Unbound Unbound Unbound Unbound Unbound Unbound
1qgnH02 Unbound Unbound Unbound Unbound Unbound Unbound
1i41A02 Unbound Unbound Unbound Unbound Unbound Unbound
1i41B02 Unbound Unbound Unbound Unbound Unbound Unbound
1i41C02 Unbound Unbound Unbound Unbound Unbound Unbound
1i41D02 Unbound Unbound Unbound Unbound Unbound Unbound
1i41E02 Unbound Unbound Unbound Unbound Unbound Unbound
1i41F02 Unbound Unbound Unbound Unbound Unbound Unbound
1i41G02 Unbound Unbound Unbound Unbound Unbound Unbound
1i41H02 Unbound Unbound Unbound Unbound Unbound Unbound
1i41I02 Unbound Unbound Unbound Unbound Unbound Unbound
1i41J02 Unbound Unbound Unbound Unbound Unbound Unbound
1i41K02 Unbound Unbound Unbound Unbound Unbound Unbound
1i41L02 Unbound Unbound Unbound Unbound Unbound Unbound
1i43A02 Unbound Unbound Unbound Unbound Unbound Unbound
1i43B02 Unbound Unbound Unbound Unbound Unbound Unbound
1i43C02 Unbound Unbound Unbound Unbound Unbound Unbound
1i43D02 Unbound Unbound Unbound Unbound Unbound Unbound
1i43E02 Unbound Unbound Unbound Unbound Unbound Unbound
1i43F02 Unbound Unbound Unbound Unbound Unbound Unbound
1i43G02 Unbound Unbound Unbound Unbound Unbound Unbound
1i43H02 Unbound Unbound Unbound Unbound Unbound Unbound
1i43I02 Unbound Unbound Unbound Unbound Unbound Unbound
1i43J02 Unbound Unbound Unbound Unbound Unbound Unbound
1i43K02 Unbound Unbound Unbound Unbound Unbound Unbound
1i43L02 Unbound Unbound Unbound Unbound Unbound Unbound
1i48A02 Unbound Unbound Unbound Unbound Unbound Unbound
1i48B02 Unbound Unbound Unbound Unbound Unbound Unbound
1i48C02 Unbound Unbound Unbound Unbound Unbound Unbound
1i48D02 Unbound Unbound Unbound Unbound Unbound Unbound
1i48E02 Unbound Unbound Unbound Unbound Unbound Unbound
1i48F02 Unbound Unbound Unbound Unbound Unbound Unbound
1i48G02 Unbound Unbound Unbound Unbound Unbound Unbound
1i48H02 Unbound Unbound Unbound Unbound Unbound Unbound
1i48I02 Unbound Unbound Unbound Unbound Unbound Unbound
1i48J02 Unbound Unbound Unbound Unbound Unbound Unbound
1i48K02 Unbound Unbound Unbound Unbound Unbound Unbound
1i48L02 Unbound Unbound Unbound Unbound Unbound Unbound

Reference for Active-site residues
resource references E.C.
literature [4], [7], [10]

Active-site residues
PDB Catalytic residues Cofactor-binding residues Modified residues Main-chain involved in catalysis Comment
1cs1A01 TYR 46;ARG 48;TYR 101;ASP 173;LLP 198 LLP 198(Modified by PLP)
1cs1B01 TYR 46;ARG 48;TYR 101;ASP 173;LLP 198 LLP 198(Modified by PLP)
1cs1C01 TYR 46;ARG 48;TYR 101;ASP 173;LLP 198 LLP 198(Modified by PLP)
1cs1D01 TYR 46;ARG 48;TYR 101;ASP 173;LLP 198 LLP 198(Modified by PLP)
1qgnA01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1qgnB01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1qgnC01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1qgnD01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1qgnE01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1qgnF01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1qgnG01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1qgnH01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i41A01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i41B01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i41C01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i41D01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i41E01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i41F01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i41G01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i41H01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i41I01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i41J01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i41K01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i41L01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i43A01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i43B01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i43C01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i43D01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i43E01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i43F01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i43G01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i43H01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i43I01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i43J01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i43K01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i43L01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i48A01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i48B01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i48C01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i48D01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i48E01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i48F01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i48G01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i48H01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i48I01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i48J01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i48K01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1i48L01 TYR 108;ARG 110;TYR 163;ASP 236;LYS 261 LYS 261(PLP binding)
1cs1A02
1cs1B02
1cs1C02
1cs1D02
1qgnA02
1qgnB02
1qgnC02
1qgnD02
1qgnE02
1qgnF02
1qgnG02
1qgnH02
1i41A02
1i41B02
1i41C02
1i41D02
1i41E02
1i41F02
1i41G02
1i41H02
1i41I02
1i41J02
1i41K02
1i41L02
1i43A02
1i43B02
1i43C02
1i43D02
1i43E02
1i43F02
1i43G02
1i43H02
1i43I02
1i43J02
1i43K02
1i43L02
1i48A02
1i48B02
1i48C02
1i48D02
1i48E02
1i48F02
1i48G02
1i48H02
1i48I02
1i48J02
1i48K02
1i48L02

References for Catalytic Mechanism
References Sections No. of steps in catalysis
[4]
Fig.7, p.6834-6839 9
[7]
Fig.7, p.987-990, p.992-994 8
[10]
p.797

References
[1]
Resource
Comments
Medline ID
PubMed ID 5646044
Journal Biochim Biophys Acta
Year 1968
Volume 151
Pages 664-9
Authors Guggenheim S, Flavin M
Title Proton retention in the gamma-elimination reaction catalysed by cystathionine gamma-synthase.
Related PDB
Related UniProtKB
[2]
Resource
Comments
Medline ID
PubMed ID 4205315
Journal J Biol Chem
Year 1974
Volume 249
Pages 1139-55
Authors Datko AH, Giovanelli J, Mudd SH
Title Homocysteine biosynthesis in green plants. O-Phosphorylhomoserine as the physiological substrate for cystathionine gamma-synthase.
Related PDB
Related UniProtKB
[3]
Resource
Comments
Medline ID
PubMed ID 9323022
Journal FEBS Lett
Year 1997
Volume 414
Pages 492-6
Authors Wahl MC, Huber R, Prade L, Marinkovic S, Messerschmidt A, Clausen T
Title Cloning, purification, crystallization, and preliminary X-ray diffraction analysis of cystathionine gamma-synthase from E. coli.
Related PDB
Related UniProtKB
[4]
Resource
Comments X-RAY CRYSTALLOGRAPHY (1.5 ANGSTROMS)
Medline ID 99059720
PubMed ID 9843488
Journal EMBO J
Year 1998
Volume 17
Pages 6827-38
Authors Clausen T, Huber R, Prade L, Wahl MC, Messerschmidt A
Title Crystal structure of Escherichia coli cystathionine gamma-synthase at 1.5 A resolution.
Related PDB 1cs1
Related UniProtKB P00935
[5]
Resource
Comments
Medline ID
PubMed ID 9914259
Journal Curr Opin Struct Biol
Year 1998
Volume 8
Pages 759-69
Authors Jansonius JN
Title Structure, evolution and action of vitamin B6-dependent enzymes.
Related PDB
Related UniProtKB
[6]
Resource
Comments
Medline ID
PubMed ID 10595588
Journal Biol Chem
Year 1999
Volume 380
Pages 1237-42
Authors Clausen T, Wahl MC, Messerschmidt A, Huber R, Fuhrmann JC, Laber B, Streber W, Steegborn C
Title Cloning, purification and characterisation of cystathionine gamma-synthase from Nicotiana tabacum.
Related PDB
Related UniProtKB
[7]
Resource
Comments X-ray crystallography
Medline ID
PubMed ID 10438597
Journal J Mol Biol
Year 1999
Volume 290
Pages 983-96
Authors Steegborn C, Messerschmidt A, Laber B, Streber W, Huber R, Clausen T
Title The crystal structure of cystathionine gamma-synthase from Nicotiana tabacum reveals its substrate and reaction specificity.
Related PDB 1qgn
Related UniProtKB
[8]
Resource
Comments
Medline ID
PubMed ID 11098400
Journal C R Acad Sci III
Year 2000
Volume 323
Pages 841-51
Authors Gakiere B, Ravanel S, Droux M, Douce R, Job D
Title Mechanisms to account for maintenance of the soluble methionine pool in transgenic Arabidopsis plants expressing antisense cystathionine gamma-synthase cDNA.
Related PDB
Related UniProtKB
[9]
Resource
Comments
Medline ID
PubMed ID 10673430
Journal Structure Fold Des
Year 2000
Volume 8
Pages R1-6
Authors Schneider G, Kack H, Lindqvist Y
Title The manifold of vitamin B6 dependent enzymes.
Related PDB
Related UniProtKB
[10]
Resource
Comments
Medline ID
PubMed ID 11518531
Journal J Mol Biol
Year 2001
Volume 311
Pages 789-801
Authors Steegborn C, Laber B, Messerschmidt A, Huber R, Clausen T
Title Crystal structures of cystathionine gamma-synthase inhibitor complexes rationalize the increased affinity of a novel inhibitor.
Related PDB 1i41 1i43 1i48
Related UniProtKB
[11]
Resource
Comments
Medline ID
PubMed ID 12121993
Journal J Biol Chem
Year 2002
Volume 277
Pages 36380-6
Authors Ominato K, Akita H, Suzuki A, Kijima F, Yoshino T, Yoshino M, Chiba Y, Onouchi H, Naito S
Title Identification of a short highly conserved amino acid sequence as the functional region required for posttranscriptional autoregulation of the cystathionine gamma-synthase gene in Arabidopsis.
Related PDB
Related UniProtKB

Comments
This enzyme was transferred from E.C. 4.2.99.9 to E.C. 2.5.1.48.
This enzyme belongs to the cystathionine beta-lyase subclass of the type-I PLP-dependent enzyme superfamily (Aminotransferase superfamily) (see [5] & [9]).
This enzyme catalyzes the following reactions:
(A) Formation of external aldimine (with amine group of O-succinyl-L-homocysteine),
(B) Isomerization (change in the position of double-bond),
(C) Elimination of carboxyl ester group, forming double-bonded carbon atoms, leading to beta-gamma unsaturated intermediate,
(D) Addition of the second substrate, sulfide, to the double-bonded beta-carbon of the unsaturated intermediate,
(E) Isomerization (change in the position of double-bond),
(F) Formation of internal aldimine, leading to the elimination of the product from PLP.
According to the literature [4], [7] & [10], these reactions proceed as follows:
Here, Tyr108 (of 1qgn) from the adjacent chain modulates the function of Lys261 as base-acid during catalysis. Moreover, Asp236 stabilizes the positively charged pyridine nitrogen of PLP, increasing its electrophilic character (see [4]).
(A) Formation of external aldimine (with amine group of O-succinyl-L-homocysteine):
(A1) Lys261 (of 1qgn) is covalently bound to the PLP cofactor. (Here, Arg110 (from the adjacent chain) and the protonated aldimine of the PLP lower the pKa of the sidechain of Tyr163)
(A2) Tyr163 acts as a general base, which deprotonates the amine group of the first substrate, O-succinyl-L-homocysteine.
(A3) The deprotonated amine group makes a nucleophilic attack on the C4' atom of the PLP, which is bound to Lys261, leading to a tetrahedral intermediate.
(A4) A proton on the amine from the first substrate must be transferred to the leaving nitrogen atom, somehow.
(A5) A lone pair on the amine group makes a nucleophilic attack on the C4' atom of the PLP, leading to the formation of the double-bond and the release of the leaving group.
(B) Isomerization (change in the position of double-bond) [This reaction does not involve quinoid intermediate. see [4]]:
(B1) Lys261 acts as a general base, to deprotonate the alpha-carbon of the first substrate, now covalently bound to the PLP cofactor, forming a carbanionic inermediate.
(B2) Lys261 acts as a general acid, to protonate the C4' atom of the PLP, leading to the formation of PLP-substrate ketimine.
(C) Elimination of carboxyl ester group, forming double-bonded carbon atoms, leading to beta-gamma unsaturated intermediate:
(C1) Lys261 acts as a general base, to deprotonate the beta-carbon (deprotonation site) of the ketimine intermediate, leading to the formation of transition state, alpha-beta unsaturated ketimine.
(C2) Tyr163 acts as a general acid, to protonate the ester bond of the eliminated group, succinate, resulting in the formation of a beta-gamma unsaturated intermediate and the release of succinate.
(D) Addition of the second substrate, sulfide, to the double-bonded beta-carbon of the beta-gamma unsaturated intermediate:
(D1) The sulfur atom of the second substrate, cysteine, makes a nucleophilic attack on the gamma-carbon (sp2) of the intermediate.
(D2) Lys261 acts as a general acid, to protonate the beta-carbon of the intermdiate, resulting in the formation of the PLP product ketimine.
(E) Isomerization (change in the position of double-bond):
(E1) Lys261 acts as a general base, to deprotonate the C4' atom of the PLP, forming a carbanionic intermediate.
(E2) Lys261 acts as a general acid, to protonate the alpha-carbon of the carbanionic intermediate, returning to the external aldimine.
(F) Formation of internal aldimine, leading to the elimination of the product from PLP: Although this final reaction has not been elucidated in the literature, Lys261 acts as a nucleophile to form the internal aldimine.

Created Updated
2004-07-01 2009-02-26