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Definition
Acyl Carrier Protein (ACP) is a component of plastid-located plant fatty acid synthetase. It binds acyl groups covalently via the prosthetic group, 4-phosphopantetheine, during the biosynthesis of fatty acids.

Discovery
Hansen L in 1987 identified three isoforms of ACP in barley leaves. Protein sequence data have been obtained for ACP I and II, and in addition genomic clones encoding ACP I and III have been characterized 1,2. A scheme has been devised for the preparation of semisynthetic derivatives of acyl carrier protein (ACP). Acetylated synthetic ACP is coupled via its activated pentachlorophenol ester to native ACP, which had previously been acetylated and converted to the S-5′-dithiobis(2-nitrobenzoate)(DTNB) derivative. Removal of the DTNB moiety after the coupling yielded active ACP in good yield 3.

Structural Characteristics
Residue 1-74 of ACP, a 77-residue single-chain protein of E.coli with a 4'-hosphopantetheine prosthetic group, is a suitable, simple model to study the chemical synthesis of small proteins. The sequence of Acyl Carrier Protein (ACP) (65-74) (acid) is H-Val-Gln-Ala-Ala-Ile-Asp-Tyr-Ile-Asn-Gly-OH. The sequence of Acyl Carrier Protein (ACP) (65-74) (amide) is H-Val-Gln-Ala-Ala-Ile-Asp-Tyr-Ile-Asn-Gly-NH2.

The solution structure of B. subtilis ACP (9 kDa) has been determined using two-dimensional and three-dimensional heteronuclear NMR spectroscopy. The overall ACP structure consists of a four α-helical bundle in which 4-PP is attached to the conserved Ser36 that is located in α helix II. Structural data suggest that the two forms of ACP are essentially identical. The structural difference between B. subtilis ACP and both E. coli and act apo-ACP is not attributed to an inherent difference in the proteins, but is probably a result of a limitation in the methodology available for the analysis for E. coli and act apo-ACP. Comparison of the structure of free ACP with the bound form of ACP in the ACPACPS complex reveals a displacement of helix II in the vicinity of Ser36. The induced perturbation of ACP by ACPS positions Ser36 proximal to coenzyme A and aligns the dipole of helix II to initiate transfer of 4-PP to ACP.

Mode of Action
Acyl carrier protein (ACP) is a fundamental component of fatty acid biosynthesis in which the fatty acid chain is elongated by the fatty acid synthetase system while attached to the 4-phosphopantetheine prosthetic group (4-PP) of ACP. Activation of ACP is mediated by holo-acyl carrier protein synthase (ACPS) when ACPS transfers the 4-PP moiety from coenzyme A (CoA) to Ser36 of apo-ACP. Both ACP and ACPS have been identified as essential for E. coli viability and potential targets for development of antibiotic 3. Circular dichroism of recombinant Vibrio harveyi ACP and mutant derivatives of conserved residues Phe-50, Ile-54, Ala-59, and Tyr-71 revealed that, unlike Escherichia coli ACP, V. harveyi-derived ACPs are unfolded at neutral pH in the absence of divalent cations; all except F50A and I54A recovered native conformation upon addition of MgCl2. Mutant I54A was not processed to the holo form by ACP synthase. Some mutations significantly decreased catalytic efficiency of ACP fatty acylation byV. harveyi acyl-ACP synthetase relative to recombinant ACP,e.g. F50A (4%), I54L (20%), and I54V (31%), whereas others (V12G, Y71A, and A59G) had less effect. By contrast, all myristoylated ACPs examined were effective substrates for the luminescence-specific V. harveyi myristoyl-ACP thioesterase 4.

Functions

Synthesis of fatty acid sand phospholipids, acyl carrier protein (ACP) interacts with many different enzymes during the synthesis of fatty acids, phospholipids, and other specialized products in bacteria .

ACP structure and conformation, fatty acid attachment stabilizes mutant ACPs in a chain length-dependent manner, although stabilization was decreased for mutants F50A and A59G. Results indicate that (i) residues Ile-54 and Phe-50 are important in maintaining native ACP conformation, (ii) residue Ala-59 may be directly involved in stabilization of ACP structure by acyl chain binding, and (iii) acyl-ACP synthetase requires native ACP conformation and involves interaction with fatty acid binding pocket residues, whereas myristoyl-ACP thioesterase is insensitive to acyl donor structure 4.

Synthesis of membrane-derived oligosaccharides, the function of ACP in the synthesis of membrane-derived oligosaccharides is thus clearly different from that involved in lipid biosynthesis. The same molecular species of ACP that undergo enzymic acylation with long-chain fatty acid residues also function in the synthesis of membrane-derived oligosaccharides 5.

References

1.     Hansen L (1987). Three cDNA clones for barley leaf acyl carrier proteins I and III. Carlsberg Res Commun., 52:381-392.

2.      Hoj PB, Svendsen IB (1984). Barley chloroplasts contain two acyl carrier proteins coded for by different genes. Carlsberg Res Commun., 49:483-492

3.     Suo Z, Tseng CC, Walsh CT (2001). Purification, priming, and catalytic acylation of carrier protein domains in the polyketide synthase and nonribosomal peptidyl synthetase modules of the HMWP1 subunit of yersiniabactin synthetase. PNAS., 98(1):99-104.

4.     Flaman AS, Chen JM, Van Iderstine SC, Byers DM (2001). Site-directed mutagenesis of acyl carrier protein (ACP) reveals amino acid residues involved in acp structure and acyl-acp synthetase activity J Biol Chem., 276(38):35934-35939.

5.     Therisod H, Kennedy EP (1987). The function of acyl carrier protein in the synthesis of membrane-derived oligosaccharides does not require its phosphopantetheine prosthetic group. PNAS., 84(23):8235–8238. 

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