Structure-Function Relationships of the Acyl-Carrier Protein of Escherichia coli

Specific chemical modifications of Escherichia coli acyl-carrier protein have been carried out. The salient results are: 1 The deletion of the N-terminal hexapeptide destroys the information required for molecular stability. A very drastic conformational change occurs which produces a total loss of biological activity. The fundamental structural rôle of the N-terminal sequence is compared to that found for proteolytic enzymes such as chymotrypsin or elastase. The possible existence of a salt-bridge between the α-ammonium of serine-1 and the side-chain of aspartic acid-35 (the neighbour of serine-36 which binds the 4′-phosphopantetheine group) is discussed. 2 Tyrosine-71 is not easily modified by tetranitromethane. Nitration decreases the pK of the phenol function from 10.4 to 7.3 at 20 °C. Nitrated acyl-carrier protein and nitrated palmityl-acyl-carrier protein have a decreased thermal stability as compared to their native forms. Activities in the malonyl-CoA-CO2 exchange reaction and in fatty-acid biosynthesis are similar for acyl-carrier protein and nitrated acyl-carrier protein. Nitrated palmityl-acyl-carrier protein has an activity in phospholipid biosynthesis of only one third that of its native form. 3 The side-chain of methionine-44 scarcely reacts with iodoacetamide in the native acyl-carrier protein. Its alkylation is easy, however, after the unfolding of the molecule in 9 M urea. Alkylation of methionine-44 does not appear to significantly influence the conformation or the thermal stability of the protein. However, modification of methionine-44 diminishes the activity in total fatty-acid biosynthesis by a factor of 3 and changes drastically the relative amounts of β-hydroxyacids and saturated acids. Alkylated palmityl-acyl-carrier protein is also less active than its native form in phospholipid biosynthesis. 4 The transformation of 11 of the 22 carboxylates into amide functions induces practically no change in the structural properties of acyl-carrier protein. Nevertheless the biological activity is completely lost. 5 The structural consequences of acylation of the –SH group of 4′-phosphopantetheine by bulky hydrophobic groups, the influence of cumulative chemical modifications, and the thermodynamic aspects of the thermal denaturation of acyl-carrier protein and its derivatives are discussed.