# Lipopolysaccharide: Recent Advances in Its Biosynthesis and Controlling Cell Envelope Homeostasis
###### Abstract
The accumulation of acyl-ACPs feedback inhibits multiple enzymes involved in fatty acid biosynthesis, exerting another level of regulation [11; 12]. Fatty acids comprise the acyl chains of glycerophospholipids, which are the major constituents of PL and the lipid A part of LPS. The first step in PL biosynthesis utilizes FabZ's _R_-3-hydroxyristoyl-ACP dehydratase activity. Thus, the fundamental common substrate of LpxC and FabZ, _R_-3-hydroxymristoyl-ACP, constitutes an essential branch point in the biosynthesis of PL and the lipid A part of LPS. As acyl-ACPs are also used in the biosynthesis of PL and LPS, their availability could be a regulatory checkpoint to maintain a balance between PL and LPS levels. This ensures that the synthesis of LPS and PL is tightly co-regulated, as for the viability of bacteria, it has to be held at a nearly constant ratio of 0.15 to 1.0 [13]. Overall, LPS is highly heterogenous in chemical composition and can be present in different glycoforms within a bacterium, exhibiting further structural diversity across bacterial species [14]. Thus, variations exist in the lipid A part and the inner core due to changes in acyl chain length, changes in phosphorylation, and additional substitutions such as GlcN, rhamnose, GlcUA, and additional Kdo [15; 16; 17]. In _E. coli_ and _Salmonella_, the most common non-stoichiometric substitutions are the incorporation of phosphoethanolamine (P-EtN) and 4-amino-4-deoxy-L-arabinose (Ara4N), which are critical for conferring resistance to cationic antimicrobial peptides (CAMPs). Variations in the Kdo group of Kdo\({}_{2}\)-lipid A have also been reported, including EpitB-dependent modification of the second Kdo in _E. coli_, addition of galacturonic acid in _Rhizobium leguminosarum_, and, in some cases, hydrolysis of the outer Kdo [18; 19; 20]. Significantly, some lipid A modifications are important for survival under stress conditions and even for evading the TLR4-mediated immune response [21]. Recent discoveries: (i) Establishing cross-talk of cell envelope machineries: Underlining this intricate metabolic network of PGN, PL, and LPS is the generation of acetyl-ACP by malonyl-ACP decarboxylase MadA. MadA participates in the initial steps and allows FabH-independent fatty acid synthesis initiation, which defines the architecture of these elements [22]. Acetyl-ACP supports the initiation of fatty acid (FASII) and PL synthesis by entering FASII at the FabB/F-condensing enzyme step. Acetyl-ACP is also a substrate for GlnU, required for UDP-GlcNAc synthesis, which acts as a metabolic precursor for PGN synthesis and for LpxA-mediated initiation of lipid A production. (ii) Evidence of two major hubs that coordinate LPS and PL biosynthesis: Based on the toxic accumulation of LPS in the absence of LapB, it was shown that LapB participates in FtsH-mediated proteolysis of LpxC, and the lethal phenotype of either \(\Delta\)_ftsH_ or \(\Delta\)_lapB_ can be overcome in the presence of a gain-of-function mutation in _fabZ_[23]. Such a _fabZ_ suppressor mutation acts by increasing the flux of acyl-ACPs towards PL, which are otherwise diverted to LPS by the stabilization of LpxC [24]. This interaction between LapB and FabZ was further reinforced by immunoprecipitations and, more importantly, the co-purification of LapB with LPS biosynthetic and transport proteins [23]. Thus, a broader function of LapB was proposed in acting as a scaffold for LPS assembly and regulating LpxC degradation. Such a scaffold-like function has been supported by additional observedinteractions of LapB with LpxA and LpxD for lipid A biosynthesis and with FabZ in the fatty acid biosynthesis cycle [25]. Recently, it was shown that LapB interacts with LapD [26]. The analysis of the LapD interactome revealed that LapD-interacting partners involve fatty acid/PL biosynthetic enzymes such as AccD, MadA (FabY), and committed enzymes of FASII FabH, FabF, and FabB. Physical interactions have also been observed with PssA (phosphatidylethanolamine synthesis) and myristoyl transferase LpxM [26].