Tryptophan synthase substrate channeling

Tryptophan Synthase Illustrates Substrate Channeling in Enzymatic Catalysis... 

Tryptophan Synthase Probing Substrate Channeling in a Bifunctional Enzyme... 

Indole-3-glycerol phosphate is the penultimate substrate which is fed into the tryptophan synthase complex. This enzyme is composed of two subunits (as an a2 and 2-tetramer) which utilizes substrate channeling to complete the construction of this amino acid. The a-subunit catalyzes the formation of indole from indole-3-glycerol phosphate and the -subunit catalyzes the condensation of indole to a serine-derived aminoacrylate moiety bound to pyridoxal monophosphate (PLP). 

Tryptophan synthase catalyzes the last two steps of the biosynthesis of tryptophan. It is typically found as a tetramer. The a subunits catalyze the reversible formation of indole and G3P from InGP. The p subunits catalyze the irreversible condensation of indole and serine to form tryptophan in a PLP-dependent reaction. Each a active site is connected to a p active site in a process known as substrate channeling [18]. Eigure 14.7 represents the stereo view of one ap heterodimer of tryptophan [19]. The a subunit contains bound indole-3-propanol phosphate and p subunit contains L-serine, which forms aldimine with the coenzyme PEP The orange sphere represents the course of tunnel running from the a to the p subunits. This tunnel is a 25 A long hydrophobic channel contained within the enzyme allowing for the diffusion of indole. If the channel did not exist, the indole formed at a active site would quickly diffuse always and be lost to the cell as it is hydrophobic and can easily cross membranes. 

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