**3. Dihydrodipicolinate reductase**

#### **3.1 Function of DHDPR**

*Dihydrodipicolinate reductase* (DHDPR, EC 1.3.1.26) was first isolated from *E. coli* in 1965 (Farkas & Gilvarg, 1965). Since then, the enzyme has been characterised from several species including *B. cereus* (Kimura & Goto, 1977), *Bacillus megaterium* (Kimura & Goto, 1977)*, Bacillus subtilis* (Kimura, 1975)*, C. glutamicum* (Cremer et al., 1988), *Methylophilus methylotrophus* (Gunji et al., 2004), *M. tuberculosis* (Cirilli et al., 2003), *S. aureus* (Dommaraju et al., 2011; Girish et al., 2011), and *T. maritima* (Pearce et al., 2008). DHDPR catalyses the second step in the lysine biosynthesis pathway (Fig. 1), the pyridine nucleotide-dependent reduction of dihydrodipicolinate (DHDP) to form *L*-2,3,4,5,-tetrahydrodipicolinate (THDP) (Dogovski et al., 2009; Hutton et al., 2007).

Fig. 7. Schematic representation of the catalytic mechanism of DHDPR.

In *E. coli*, DHDPR is encoded by the *dap*B gene, which is also an essential bacterial gene (Dogovski et al., 2009; Hutton et al., 2007). The open reading frame encodes a 273 amino acid polypeptide with a monomeric molecular weight of 28,758 Da. The enzyme functions by utilising either phosphorylated or non-phosphorylated pyridine nucleotides, NAD(P)H, as hydrogen donors to carry out its reaction. The kinetic mechanism of *E. coli* DHDPR is ordered and sequential (Reddy et al., 1995), involving binding of NAD(P)H followed by DHDP. The reaction is initiated by hydride transfer from the 4-pro-*R* position of NAD(P)H to the C4-position of DHDP, with the resultant enamine then undergoing tautomerisation to form THDP. Upon completion of the reaction, the release of the product THDP is followed by NAD(P)+ release (Reddy et al., 1995) (Fig. 7).

inhibition with *K*i = 12-24 mM (Hutton et al., 2007). Analogues of the straight chain hydrate form of ASA have also been investigated, with aspartic acid showing mixed type inhibition with *K*i = 90-140 µM (Hutton et al., 2007). Product analogues have also been investigated, exhibiting moderate DHDPS inhibition. More success was achieved with inhibitors based on the acyclic enzyme-bound DHDPS intermediates, such as diethyl (*E,E*)-4-oxo-2,5 heptadienedioate (Turner et al., 2005) and a bis-oxime ester (Boughton et al., 2008), which irreversibly inhibit DHDPS. Interestingly, several of these compounds have displayed clear differentiation in inhibition of DHDPS enzymes from different species (Mitsakos et al.,

*Dihydrodipicolinate reductase* (DHDPR, EC 1.3.1.26) was first isolated from *E. coli* in 1965 (Farkas & Gilvarg, 1965). Since then, the enzyme has been characterised from several species including *B. cereus* (Kimura & Goto, 1977), *Bacillus megaterium* (Kimura & Goto, 1977)*, Bacillus subtilis* (Kimura, 1975)*, C. glutamicum* (Cremer et al., 1988), *Methylophilus methylotrophus* (Gunji et al., 2004), *M. tuberculosis* (Cirilli et al., 2003), *S. aureus* (Dommaraju et al., 2011; Girish et al., 2011), and *T. maritima* (Pearce et al., 2008). DHDPR catalyses the second step in the lysine biosynthesis pathway (Fig. 1), the pyridine nucleotide-dependent reduction of dihydrodipicolinate (DHDP) to form *L*-2,3,4,5,-tetrahydrodipicolinate (THDP)

2008), suggesting the potential for targeting compounds to specific pathogens.

Fig. 7. Schematic representation of the catalytic mechanism of DHDPR.

In *E. coli*, DHDPR is encoded by the *dap*B gene, which is also an essential bacterial gene (Dogovski et al., 2009; Hutton et al., 2007). The open reading frame encodes a 273 amino acid polypeptide with a monomeric molecular weight of 28,758 Da. The enzyme functions by utilising either phosphorylated or non-phosphorylated pyridine nucleotides, NAD(P)H, as hydrogen donors to carry out its reaction. The kinetic mechanism of *E. coli* DHDPR is ordered and sequential (Reddy et al., 1995), involving binding of NAD(P)H followed by DHDP. The reaction is initiated by hydride transfer from the 4-pro-*R* position of NAD(P)H to the C4-position of DHDP, with the resultant enamine then undergoing tautomerisation to form THDP. Upon completion of the reaction, the release of the product THDP is followed

**3. Dihydrodipicolinate reductase** 

(Dogovski et al., 2009; Hutton et al., 2007).

by NAD(P)+ release (Reddy et al., 1995) (Fig. 7).

**3.1 Function of DHDPR** 
