ALA Synthase is the committed step of the heme synthesis pathway, and is usually rate-limiting for the overall pathway.
Heme synthesis begins in mitochondria with condensation of glycine & succinyl-CoA, with decarboxylation, to form delta-aminolevulinic acid (ALA). Pyridoxal phosphate (PLP) serves as coenzyme for delta-Aminolevulinate Synthase (ALA Synthase). Condensation with succinyl-CoA takes place while the amino group of glycine is in Schiff base linkage to the aldehyde of PLP. Coenzyme A and the carboxyl of glycine are lost following the condensation reaction.