intermediate for protein biotinylation and corepressor for tran-
scriptional regulation. The BirA: biotinyl-5′-AMP (holo) enzyme can
then adopt one of two different fates. When the cellular demand
for biotin is low holo BirA can dimerize and bind DNA where it func-
tions as the transcriptional repressor of the biotin biosynthesis
operon, thereby inhibiting the synthesis of more biotin. In con-
trast, in the presence of substrate requiring biotinylation the holo
BirA functions as a biotin ligase. Here BPL recognizes and binds to
a biotin carboxyl carrier protein (BCCP) present in the receiving
enzyme that contains the lysine residue targeted for biotinylation. 11
Protein biotinylation is an example of a post-translational modifi-
cation that is performed with exquisite specificity. For example, the
E. coli biotin ligase (BirA) modifies just one of the >4000 different
proteins in the bacterial cell. 12 Moreover, the biotin cofactor is co-
valently attached onto the side chain of one single, specific target
lysine residue present in the active site of biotin-dependent enzymes.
BPLs from a wide variety of species are able to modify BCCP from
unrelated organisms, 13–15 highlighting how highly conserved both
the catalytic mechanism and the protein:protein interactions
between enzyme and substrate have remained throughout evolu-
tion. The possible mechanisms through which BirA can switch
between its two functions are described later in this review.
All BPLs contain a conserved 2-domain catalytic core responsi-
ble for biotinyl-5′-AMP synthesis and protein biotinylation. 16 The
greatest divergence between the BPLs is in their N-terminal regions
(see Fig. 2A). Class I BPLs are composed only of the conserved cata-
lytic module that is required for protein biotinylation. Hence, these
are mono functional enzymes. X-ray crystal structures of Class I BPLs
have been reported for Mycobacterium tuberculosis 19 and Pyrococcus
horikoshii. 21 In contrast, the Class II BPLs are truly bi-functional having
both biotin ligase and transcriptional repressor activities due to an
N-terminal DNA binding domain. BirA from E. coli is the most ex-
tensively studied representative of a Class II BPL, having been the
subject of structural, genetic and biophysical studies (reviewed 22,23 ).