Acyl carrier protein , components

The success, albeit limited, of incorporation studies of polyketide assembly intermediates has resulted from feeding these in the form of their NAC thioesters which structurally mimic the thiol end of the phosphopantetheine moiety found in coenzyme A and the acyl carrier protein component of the PKS. This will be discussed further below, but it has also been shown that there are advantages to feeding starter units in the form of their NAC thioesters. 

The answer is e. (Murray, pp 230-267. Scriver, pp 2297-2326. Sack, pp 121-138. Wilson, pp 287-320.) The fatty acid synthase complex of mammals is composed of two identical subunits. Each of the subunits is a multienzyme complex of seven enzymes and the acyl carrier protein component. All the components are covalently linked together thus, all the components are on a single polypeptide chain, which functions in the presence of another identical polypeptide chain. Each cycle of fatty acid synthesis employs the acyl carrier protein and six enzymes acetyl transferase, malonyl transferase, p-ketoacyl synthase, p-ketoacyl reductase, dehydratase, and enoyl reductase. When the final fatty acid length is reached (usually C16), thioesterase hydrolyzes the fatty acid off of the synthase complex. 

Decker H, Summers RG, Hutchinson CR, Overproduction of the acyl carrier protein component of a type 11 polyketidc synthase stimulates production of teiracenomycin biosynthetic intermediates in Streptomyces glaucejceru, J Antibiot 1994 47 54-63. 

Biosynthesis of coen2yme A (CoA) ia mammalian cells incorporates pantothenic acid. Coen2yme A, an acyl group carrier, is a cofactor for various en2ymatic reactions and serves as either a hydrogen donor or an acceptor. Pantothenic acid is also a stmctural component of acyl carrier protein (AGP). AGP is an essential component of the fatty acid synthetase complex, and is therefore requited for fatty acid synthesis. Free pantothenic acid is isolated from hver, and is a pale yeUow, viscous, and hygroscopic oil. 

Phosphopantetheine tethering is a posttranslational modification that takes place on the active site serine of carrier proteins - acyl carrier proteins (ACPs) and peptidyl carrier proteins (PCPs), also termed thiolation (T) domains - during the biosynthesis of fatty acids (FAs) (use ACPs) (Scheme 23), polyketides (PKs) (use ACPs) (Scheme 24), and nonribosomal peptides (NRPs) (use T domain) (Scheme 25). It is only after the covalent attachment of the 20-A Ppant arm, required for facile transfer of the various building block constituents of the molecules to be formed, that the carrier proteins can interact with the other components of the different multi-modular assembly lines (fatty acid synthases (FASs), polyketide synthases (PKSs), and nonribosomal peptide synthetases (NRPSs)) on which the compounds of interest are assembled. The structural organizations of FASs, PKSs, and NRPSs are analogous and can be divided into three broad classes the types I, II, and III systems. Even though the role of the carrier proteins is the same in all systems, their mode of action differs from one system to another. In the type I systems the carrier proteins usually only interact in cis with domains to which they are physically attached, with the exception of the PPTases and external type II thioesterase (TEII) domains that act in trans. In the type II systems the carrier proteins selectively interact... 

Figure 11.4 Condensation, dehydration and reduction reactions in fatty add synthesis. These reactions constitute the major components of the pathway of fatty acid synthesis and are all catalysed by fatty acid synthase. The reduction reactions, indicated by addition of 2H in the diagram, involve the conversion of NADPH to NADP . (The re-conversion of NADP back to NADPH occurs in the pentose phosphate pathway.) The condensation reaction results in an increase in size of acyl-ACP by two carbon units in each step. The two carbons for each extension are each provided by malonyl-CoA. ACP - acyl carrier protein.

This enzyme, officially known as 3-hydroxypalmitoyl-[acyl-carrier protein] dehydratase [EC 4.2.1.61], is the fatty-acid synthase component that catalyzes the conversion of (3 i )-3-hydroxypalmitoyl-[acyl-carrier protein] to form 2-hexadecenoyl-[acyl-carrier protein] and water. This enzyme displays specificity toward 3-hydroxyacyl-[acyl-carrier protein] derivatives (with chain lengths from Ci2 to Cie), with highest activity on the palmitoyl derivative. See also Fatty Acid Synthetase... 

Figure 8-2. Pathway for synthesis of palmitate by the fatty acid synthase (FAS) complex. Schematic representation of a single cycle adding two carbons to the growing acyl chain. Formation of the initial acetyl thioester with a cysteine residue of the enzyme preceded the first step shown. Acyl carrier protein (ACP) is a component of the FAS complex that carries the malonate covalently attached to a sulfhydryl group on its phosphopantatheine coenzyme (-SH in the scheme).

The remaining series of reactions of fatty acid synthesis in eukary-l otes is catalyzed by the multifunctional, dimeric enzyme, fatty acid synthase. Each fatty acid synthase monomer is a multicatalytic polypeptide with seven different enzymic activities plus a domain that covalently binds a molecule of 4 -phosphopantetheine. [Note 4-Phosphopantetheine, a derivative of the vitamin pantothenic add (see p. 379), carries acetyl and acyl units on its terminal thiol (-SH)j group during fatty acid synthesis. It also is a component of 00-enzyme A.] In prokaryotes, fatty acid synthase is a multienzyme complex, and the 4 -phosphopantetheine domain is a separate protein, referred to as the acyl carrier protein (ACP). ACP is used below to refer to the phosphopantetheine-binding domain of the eukaryotic fatty acid synthase molecule. The reaction numbers in1 brackets below refer to Figure 16.9. [Note The enzyme activities listed are actually separate catalytic domains present in each mulf-1 catalytic fatty acid synthase monomer.]... 

Endogenous pantothenic acid occurs in food primarily in the bound form as a component of coenzyme A (CoA or CoASH), acyl-coenzyme A, and acyl carrier protein (ACP) (185,186). These are the principal vitamers in foods free pantothenic acid (Fig. 9) is much less common. Only the D( + ) or (R) enantiomer of pantothenic acid occurs naturally. 

One system in which pantetheine almost certainly performs such a carrier role is the fatty acid synthase from E. coli, in which 4 -phosphopantetheine is a component of the acyl carrier protein (see chapter 18). 

Panthenol is absorbed via passive diffusion after topical or oral application and then enzymatically oxidized to pantothenic acid. This is a component of coenzyme A and acyl carrier protein, and as such of great importance in fatty acid, carbohydrate, and amino acid metabolism. Deficiency leads to uncharacteristic symptoms such as headaches, apathy, gastrointestinal disturbances, palpitations, and paraesthesia typically in the feet, also known as burning feet syndrome. Wound healing is impaired. The recommended daily allowance is 5 to 7 mg.112... 

The animal fatty acid synthase (FAS EC 2.3.1.85) is one of the most complex multifunctional enzymes that have been characterized, as this single polypeptide contains all the catalytic components required for a series of 37 sequential transactions (Smith, 1994). The animal FAS consists of two identical polypeptides of approximately 2500 amino acid residues (MW, ca. 270 kDa), each containing seven catalytic subunits (1) ketoacylsynthase, (2) malonyl/acetyl transferase, (3) dehydrase, (4) enoyl reductase, (5) (3-kcto reductase, (6) acyl carrier protein (ACP), and (7) thioesterase. Although some components of the complex are able to carry out their respective catalytic steps in the monomeric form, only in the FAS dimer do the subunits attain conformations that facilitate coupling of the individual reactions of fatty acid synthesis to occur (Smith et al., 2003). 

The most important functions of pantothenic acid are its incorporation in coenzyme A and acyl carrier protein (AGP). Both CoA and AGP/4-phosphopantetheine function metabolically as carriers of acyl groups. Coenzyme A forms high-eneigy thioester bonds with carboxylic acids. The most important coenzyme is acetyl CoA. Acetic acid is produced during the metabolism of fatty acids, amino acids, or carbohydrates. The active acetate group of acetyl CoA can enter the Krebs cycle and is used in the synthesis of fatty acids or cholesterol. AGP is a component of the fatty acid synthase multienzyme complex. This complex catalyzes several reactions of fatty acid synthesis (condensation and reduction). The nature of the fatty acid synthase complex varies considerably among different species (91). 

Stable isotope labelling is also proving to have an important role in studies on the enzymology of polyketide biosynthesis. The acyl carrier protein (AGP) components of polyketide synthases (PKSs) are believed to play a central role in the control of the assembly and stabilisation of polyketide intermediates, especially of the highly oxygenated intermediates necessarily involved in biosynthesis of... 

The presentation in 1993 of the structure of the hydroxylase component of methane monooxygenase (MMOH) by Rosenzweig et al. (15) is the third published three-dimensional structure of a diiron-oxygen protein (Fig. 1). The previous two are from hemerythrin (Hr) (16,17) and protein R2 of E. coli ribonucleotide reductase (RNR-R2) (18, 19). Some other dinuclear iron proteins with known fi-oxo or p.-hydroxo bridges are purple acid phosphatases (PAP) [(e.g., uteroferrin (Uf)] (20, 21), ferritins (in early stages of nucleation) (22), rubrerythrin (Rr) (23-26), nigerythrin (26), and soluble stearoyl-acyl carrier protein A desaturase (A-AGP) (27, 28). 

ACP = acyl carrier protein ACPA D = ACPA desat-urase AlkB = octane 1-monooxygenase AOX = alternative oxidase DMQ hydroxylase = 5-demethoxyquinone hydroxylase EXAFS = extended X-ray absorption fine structure spectroscopy FMN = flavin mononucleotide FprA = flavoprotein A (flavo-diiron enzyme homologue) Hr = hemerythrin MCD = magnetic circular dichroism MME hydroxylase = Mg-protophorphyrin IX monomethyl ester hydroxylase MMO = methane monooxygenase MMOH = hydroxylase component of MMO NADH = reduced nicotinamide adenine dinucleotide PAPs = purple acid phosphatases PCET = proton-coupled electron transfer, PTOX = plastid terminal oxidase R2 = ribonucleotide reductase R2 subunit Rbr = rubrerythrin RFQ = rapid freeze-quench RNR = ribonucleotide reductase ROO = rubredoxin oxygen oxidoreductase XylM = xylene monooxygenase. 

A structural module comprising tetra(carboxylato)di-iron(II) centers with two additional A-donor groups has been identified at the active sites of the hydroxylase component (MMOH) of soluble methane monooxygenase, the R2 subunit of class I ribonucleotide reductase (RNR-R2), " " and stearoyl-acyl carrier protein (AGP) A desaturase (A9D). d8 structural comparisons of these enzymes reveal striking architectural similarities involving an approximately 18 A segment of a pseudo-222-symmetric four-helix bundle that encapsulates their di-iron active... 

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