Threonine ammonia lyase

Interestingly, although a number of such reversible ammonia lyases are known, only a few seem to need pyridoxal as a cofactor, for example, threonine ammonia lyase (EC 4.3.1.19) (Scheme 12.1), where the ammonia is presumably lost via a hydrolytic mechanism to generate the corresponding carbonyl compound or (as is not shown in the Scheme 12.1) the corresponding pyridoxamine but not the alkene. 

With threonine (Tlir, T) in hand, the path to the biosynthesis of isoleucine (He, I) is clear. The first part of the biosynthesis is shown in Scheme 12.15 where, in the presence of the pyridoxal (cofactor) protein (enzyme) threonine ammonia lyase (EC 4.3.1.19), threonine (Thr, T) is converted to 2-oxobutanoate. Then, with thiamine diphosphate (cf. Chapter 11, Scheme 11.7) as a cofactor for the enzyme aceto-lactate synthase (EC 2.2.1.6), an acetyl group is added at C2 of the 2-oxobutanoate... 

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