The third step of the shikimate pathway is catalyzed by DHQD
|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / QuickGO|
|Type I 3-dehydroquinase|
The structure of type i 3-dehydroquinate dehydratase from salmonella typhi
|SCOPe||2dhq / SUPFAM|
|Dehydroquinase class II|
|SCOPe||2dhq / SUPFAM|
- 3-dehydroquinate 3-dehydroshikimate + H2O
This enzyme belongs to the family of lyases, specifically the hydro-lyases, which cleave carbon-oxygen bonds. This enzyme participates in phenylalanine, tyrosine and tryptophan biosynthesis.
Role in the shikimate pathway
3-Dehydroquinate Dehydratase is an enzyme that catalyzes the third step of the shikimate pathway. The shikimate pathway is a biosynthetic pathway that allows plants, fungi, and bacteria to produce aromatic amino acids. Mammals do not have this pathway, meaning that they must obtain these essential amino acids through their diet. Aromatic Amino acids include Phenylalanine, Tyrosine, and Tryptophan.
This enzyme dehydrates 3-Dehydroquinate, converting it to 3-Dehydroshikimate, as indicated in the adjacent diagram. This is the third step in the Shikimate pathway. It belongs to the family of lyases, specifically the hydro-lyases, which cleave carbon-oxygen bonds. The systematic name of this enzyme class is 3-dehydroquinate hydro-lyase (3-dehydroshikimate-forming). This enzyme is one of the few examples of convergent evolution. The two separate versions of this enzyme have different amino acid sequences.
3-Dehydroquinate dehydratase is also commonly referred to as Dehydroquinate dehydratase and DHQD. Other names include 3-dehydroquinate hydrolase, DHQase, 3-dehydroquinase, 5-dehydroquinase, dehydroquinase, 5-dehydroquinate dehydratase, 5-dehydroquinate hydro-lyase, and 3-dehydroquinate hydro-lyase.
Purposes of the products
The aromatic amino acids produced by the shikimate acid pathway are used by higher plants as protein building blocks and as precursors for several secondary metabolites. Examples of such secondary metabolites are plant pigments and compounds to defend against herbivores, insects, and UV light. The specific aromatic secondary metabolites produced, as well as when and in what quantities they are produced in, varies across different types of plants. Mammals consume essential amino acids in their diets, converting them to precursors for important substances such as neurotransmitters.
As mentioned previously, two classes of 3-Dehydroquinate Dehydratase exist, known as types I and II. These two versions have different amino acid sequences and different secondary structures. Type I is present in fungi, plants, and some bacteria, for the biosynthesis of chorismate. It catalyzes the cis-dehydration of 3-Dehydroquinate via a covalent imine intermediate. Type I is heat liable and has Km values in the low micromolar range. Type II is present in the quinate pathway of fungi and the shikimate pathway of most bacteria. It catalyzes a trans-dehydration using an enolate intermediate. It is heat stable and has Km values one or two orders of magnitude higher than the Type I Km values.
The best studied type I enzyme is from Escherichia coli (gene aroD) and related bacteria. It is a homodimeric protein. In fungi, dehydroquinase is part of a multifunctional enzyme which catalyses five consecutive steps in the shikimate pathway. A histidine is involved in the catalytic mechanism.
3-Dehydroquinate Dehydratase is also an enzyme present in the process of the degradation of quinate. Both 3-Dehydroquinate and 3-Dehydroshikimate are intermediates in the reaction mechanism. The following image shows this process in Quinate Degradation.
The Shikimate pathway has become a focus of research into the development of herbicides and antimicrobial agents because it is an essential pathway in many plants, bacteria, and parasites but does not exist in mammals.
Most of the 3-dehydroquinate-dehydratase in bacteria and higher plants is type I DHQD.
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