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‐CO 2 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.