Pyridoxal 5'-phosphate-dependent enzymes may be grouped into five structural superfamilies of proteins, corresponding to as many fold types. The fold type I is by far the largest and most investigated group. An important feature of this fold, which is characterized by the presence of two domains, appears to be the existence of three clusters of evolutionarily conserved hydrophobic contacts. Although two of these clusters are located in the central cores of the domains and presumably stabilize their scaffold, allowing the correct alignment of the residues involved in cofactor and substrate binding, the role of the third cluster is much less evident. A site-directed mutagenesis approach was used to carry out a model study on the importance of the third cluster in the structure of a well characterized member of the fold type I group, serine hydroxymethyltransferase from Escherichia coli. The experimental results obtained indicated that the cluster plays a crucial role in the stabilization of the quaternary, native assembly of the enzyme, although it is not located at the subunit interface. The analysis of the crystal structure of serine hydroxymethyltransferase suggested that this stabilizing effect may be due to the strict structural relation between the cluster and two polypeptide loops, which, in fold type I enzymes, mediate the interactions between the subunits and are involved in cofactor binding, substrate binding and catalysis.