Enzyme - [Acyl-carrier-protein] S-malonyltransferase

Alternative Name(s)
  • Malonyl-CoA/dephospho-CoA acyltransferase.
  • Malonyl transacylase.
  • Acyl carrier protein malonyltransferase.
  • Malonyl coenzyme A-acyl carrier protein transacylase.
  • MCAT.
  • Malonyl transferase.
  • Malonyl-CoA-acyl carrier protein transacylase.

Catalytic Activity

holo-[ACP] + malonyl-CoA = CoA + malonyl-[ACP]


There are no Cofactors for this Enzyme

Reaction Mechanism

    In Escherichia coli Fatty acid synthesis, FAS II includes a specific malonyl-CoA:ACP transacylase (MCAT), which catalyses specifically the elongation step. The initiation of each elongation step in the fatty acid synthesis cycle requires the transfer of a malonyl moiety from the respective CoA thioester to the -SH group of the phosphopantetheine arm of the acyl carrier protein (ACP), the central component of any FAS. MCAT, along with Acyl-carrier-protein S-acetyltransferase (EC, is essential for the initiation of fatty-acid biosynthesis in bacteria. This enzyme also provides the malonyl groups for polyketide biosynthesis.

    The catalytic residues consist of a conventional Ser-His dyad, which is unusually hydrogen to the main chain carbonyl of a glutamine residue. The reactions consists of an acylation step and the subsequent transfer of the acyl moiety proceed via tetrahedral intermediates, analogously to serine proteases. His201 deprotonates Ser92 to form a nucleophile. This subsequently attacks the carbonyl group-subsequent break down of this tetrahedral intermediate eliminates CoA forming a thioester with the enzyme. His201 then deprotonates a water molecule, which nucleophilically attacks the carbonyl group a second time break down results in elimination of the enzyme.
    Catalytic Residues
    AA Uniprot Uniprot Resid PDB PDB Resid
    Leu P0AAI9 93 1mla 93
    Ser P0AAI9 92 1mla 92
    His P0AAI9 201 1mla 201
    Arg P0AAI9 117 1mla 117
    Gln P0AAI9 11 1mla 11
    Gln P0AAI9 250 1mla 250
    Step Components

    intermediate formation, enzyme-substrate complex formation, bimolecular nucleophilic addition, proton transfer, native state of enzyme regenerated, enzyme-substrate complex cleavage, overall reactant used, unimolecular elimination by the conjugate base, bimolecular nucleophilic substitution, intermediate collapse, overall product formed

    Step 1.

    Ser92 is activated towards nucleophilic attack at the thio-carbonyl of malonyl-CoA by the general base His201. A catalytic dyad is present between Ser92A and His201A, with the protonated form of His201A being stabilised through a hydrogen bond with the main chain carbonyl of Gln250A [PMID:7768883].

    Step 2.

    The tetrahedral enzyme-substrate intermediate collapses, eliminating the coenzyme A component.

    Step 3.

    The terminal thiolate of the previously eliminated CoA acts as a general base towards His201.

    Step 4.

    The enzyme-malonyl carbonyl linkage acts as an electrophile towards the terminal thiol of the acyl carrier protein which is deprotonated by the general acid His201.

    Step 5.

    The tetrahedral intermediate collapses eliminating the malonyl acyl-carrier protein adduct and free serine side chain, which is reprotonated by the general acid His201A.


    The products of the reaction.

Reaction Parameters

  • Kinetic Parameters
    Organism KM Value [mM] Substrate Comment
    Plasmodium falciparum 0.0092 malonyl-CoA acyl carrier protein mutant T65A, pH 7.5, 25°C
    Escherichia coli 0.0095 malonyl-CoA wild-type acyl carrier protein, pH 7.5, 25°C
  • Temperature

    There are no reaction parameters information for this Enzyme.

  • pH
    Organism pH Range Comment
    Escherichia coli 5 - 10 about half-maximal activity at pH 5.0, about 80% of maximal activity at pH 10.0
    Persea americana 6.2 - 9.8 about half-maximal activity at pH 6.2, about 65% of maximal activity at pH 9.8
    Glycine max 7.5 - 8.5 maximal activity

Associated Proteins

Protein name Organism
Polyketide biosynthesis protein BaeE Bacillus velezensis (strain DSM 23117 / BGSC 10A6 / FZB42)
Malonyl-CoA-acyl carrier protein transacylase, mitochondrial Human
Malonyl CoA-acyl carrier protein transacylase, mitochondrial Baker's yeast
Probable malonyl-CoA-acyl carrier protein transacylase, mitochondrial Fruit fly
Polyketide biosynthesis malonyl CoA-acyl carrier protein transacylase BaeC Bacillus velezensis (strain DSM 23117 / BGSC 10A6 / FZB42)