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From Proteopedia

Revision as of 04:55, 7 December 2013

Introduction

Threonine dehydratase (TDH) also known as Threonine Ammonia-Lyase and Threonine Deaminase is a tetrameric enzyme of about 200kD that catalyzes the dehydration of threonine to α-ketobutyrate. TDH is the first enzyme in the isoleucine biosynthetic pathway. It belongs to the enzyme class EC-4 which cleaves C-C, C-O, C-N through hydrolysis or oxidation. TDH EC 4.3.1.19 specifically cleaves C-N bonds. The enzyme was first discovered in E.coli around 1965 but has since been purified from other bacterial sources such as Salmonella typhimurium. TDH exists in two forms in bacteria: biosynthetic threonine dehydratase (BTD) and catabolic threonine dehydratase (CTD. While BTD catalyzes the first reaction in the biosynthetic pathway of isoleucine in an aerobic environment, CTD functions in the degradation of threonine to propionate in anaerobic conditions. The feedback inhibition of TDH in E.coli served as a model for metabolic regulation as it was one of the earliest examples observed.

Structure

Threonine deaminase, purified from Salmonella typhimurium has a molecular weight of about 200kD which exists as tetramer in its native state and consists of four identical subunits. In the absence of the cofactor pyridoxal phosphate, TDH dissociates from its tetrameric form to about 2 individual 100 kD dimers. It is 514 residues in length with a secondary structure that is 37% helical and 17% beta sheet.

Protein Fold and organization of Domain

The four identical subunits of TDH are organized into two domains with different sizes. The secondary structure of TDH can be seen in the Figure 3 below. Image:Ss. The N-terminal domain is the catalytic domain as it possesses the cofactor of the enzyme, pyridoxal phosphate. This domain consists of residues 1–320 and contains two folding units, N1 and N2. As can be seen in Figure 3, both N1 and N2 subdomains comprise of a collections of four-stranded parallel β sheet with flanking helices. While the N2 subdomain consists of a single contiguous zone of sequence (residues 63–153), N1 is composed of an N2-like core (residues 160–313) with an additional pair of antiparallel strands (residues 34–51) extending the β sheet to six strands. The catalytic domain is homologous (40 identities out of about 280 aligned residues [25]) to the β subunit of tryptophan synthase from Salmonella typhimurium, and the relation of the subdomains is largely as described for that enzyme

Cofactor

For TDH to be catalytically active, pyridoxal phosphate (PLP), its prosthetic cofactor must be attached to it. This is initiated by the formation of a Schiff base linkage between the aldehyde group of PLP and the Lys-62 residue of TDH. The electrophilic nitrogen of the pyridine ring of PLP acts as an electron sink where excess electrons can be delocalized. This effect draws electrons from TDH thereby stabilizing the carbanion intermediate . Below is an illustration of the mechanism of attachment of the cofactor PLP a lysine residue.

Mechanism of Action of TDH

Application