3,6-anhydro-alpha-L-galactonate cycloisomerase

 

The catabolic fate of the major monomeric sugar of red macroalgae, 3,6-anhydro-L-galactose (AHG), is poorly understood. AHG is not catabolised by ordinary fermentative microorganisms, and it hampers the utilisation of red macroalgae as renewable biomass for biofuel and chemical production. However, characterisation of two enzymes in Vibrio sp., a marine bacterium capable of catabolising AHG as a sole carbon source, revealed two key metabolic intermediates of AHG, 3,6-anhydrogalactonate (AHGA) and 2-keto-3-deoxy-galactonate. This entry represents the isomerase that converts 3,6-anhydro-alpha-L-galactonate to 2-dehydro-3-deoxy-D-galactonate.

 

Reference Protein

Sequence
H2IFX0 UniProt (5.5.1.25) IPR034382 (Sequence Homologues)
Biological species
Vibrio sp. EJY3 (Bacteria) Uniprot
Cofactors
Magnesium(2+) (1)
 

Enzyme Reaction (EC:5.5.1.25)

3,6-anhydro-L-galactonate
CHEBI:83435ChEBI
→
2-keto-3-deoxy-L-galactonate
CHEBI:75545ChEBI
Alternative enzyme names: 3,6-anhydro-alpha-L-galactonate cycloisomerase, 3,6-anhydro-alpha-L-galactonate lyase (ring-opening),

Enzyme Mechanism

Introduction

Although there is no crystal structure of the enzyme currently known, it is a member of the Enolase superfamily and so it is assumed that it acts in a similar manner to other superfamily members. A general acid/base abstracts a proton alpha to the carbonyl group and a second general acid/base reprotonates the substrate in the new position.

Catalytic Residues Roles

UniProt
His300 Acts as a general acid/base. Likely to be the base that abstracts the initial proton from the substrate. proton shuttle (general acid/base)
Asp273 Modifies the pKa of the general acid/base histidine. modifies pKa
Asp198, Glu224, Glu250 Forms part of the magnesium binding site. metal ligand
Lys169 Acts as a general acid/base, likely the second acid that donates the proton back to the intermediate in the final stages of the isomerisation. proton shuttle (general acid/base)

Chemical Components

References

  1. Yun EJ et al. (2015), Environ Microbiol, 17, 1677-1688. The novel catabolic pathway of 3,6-anhydro-L-galactose, the main component of red macroalgae, in a marine bacterium. DOI:10.1111/1462-2920.12607. PMID:25156229.

Step 1.

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Catalytic Residues Roles

Residue Roles
Lys169 proton shuttle (general acid/base)
His300 proton shuttle (general acid/base)
Asp198 metal ligand
Glu224 metal ligand
Glu250 metal ligand
Asp273 modifies pKa

Chemical Components

Step 1.

Contributors

Gemma L. Holliday