Tryptophan is produced from chorismate by a pathway that does not use the sigmatropic rearrangement.

• In fact, the pyruvate side chain is clipped off at the beginning of the sequence.

A crystal structure [Proc. Nat. Acad. Sci. US, 2001, 98, 6021] has been obtained for the anthranilate synthase from S. marescens:

Anthranilate Synthase (1i7q), Glutamyl and Pyruvate Bound	Anthranilate Synthase (1i7s), Inhibited by Tryptophan

• Note again the feedback inhibition

• No mechanism has been demonstrated; a suggestion:

  

The last two steps are carried out by a single unusual enzyme:

• The enzyme is a tetramer, containing two each of two types of subunits: a₂b₂

• The a-subunits, as shown, catalyze the formation of indole by cleavage of the glycerol phosphate side chain

• The b-subunits catalyze the condensation with serine, which requires pyridoxal phosphate as a cofactor

• The indole is not dissociated (recall the T-protein several pages back), but instead is passed internally from one subunit to the other through a sort of tunnel!

Below are a couple of pictures from X-ray structures of an a b dimer, with the indole precursor complexed in the a-unit.

Tryptophan Synthase (1qop), Substrate in Chain A, Pyridoxal Phosphate in Chain B (right)	The Tunnel

Many enzymes use the trick of a tunnel to pass a substrate from one functional site to another.

• Usually the traveller is a small molecule like ammonia

• In this case, a relatively large molecule, indole, is passed along.

The condensation uses pyridoxal phosphate as an enzyme cofactor. This is derived from Vitamin B6

It is bound as a Schiff base to a lysine at the enzyme active site, and undergoes a transimination to bind the serine used in the condensation.

A Digression

We have several points in mind in examining this biosynthetic sequence.

• To get an idea of the variety of chemical processes Mother Nature can carry out, and the variety of schemes she has devised for doing the work

• To show, without examining the enzyme mechanisms in detail (yet) that we can write good organic chemists' curly arrow schemes for most of them. There's no magic here!

• To illustrate the use of cofactors by enzymes

We're going to move on to look at another pathway, the formation of squalene and its cyclization to sterols, and then use both pathways to address two important questions:

• Can we regulate biosynthetic pathways?

• How did evolution produce biosynthetic pathways?