= Solution
\b[Divergence preserves a structural framework while function changes; convergence produces similar solutions within otherwise different frameworks.] Evidence should compare the entire <protein fold>, the sequence order and connectivity of structural elements, and the geometry of functional residues. Similar activity alone cannot distinguish either history, because many unrelated structures can perform the same reaction.
The <globin fold> illustrates divergence. <myoglobin> and the alpha and beta chains of <haemoglobin> share a predominantly alpha-helical architecture and a homologous pocket for <haem>. The proximal <histidine> coordinates the iron, and the surrounding pocket modulates ligand binding. <Myoglobin> is principally a monomeric oxygen-binding <protein>, whereas <haemoglobin> subunits interact within a tetramer to give cooperative <oxygen> transport. Their conserved core architecture and alignable sequence positions support descent from an ancestral globin, with duplication and subsequent specialization. Structural similarity remains recognizable even when many surface residues have changed. The functional difference arises partly from new intersubunit interactions rather than construction of an entirely new oxygen-binding chemistry.
<Trypsin>, <chymotrypsin> and <elastase> provide a second divergent series. They retain the chymotrypsin-like fold and corresponding <catalytic triad> residues, but changes in the specificity pocket favour different side chains next to the scissile peptide bond. A negatively charged pocket in <trypsin> favours basic residues; a hydrophobic pocket in <chymotrypsin> favours bulky aromatic residues; obstruction of the pocket in <elastase> favours smaller side chains. Conserved topology, residue correspondence and related sequences support common ancestry. The particular numbering of homologous catalytic residues is useful shorthand, but matching residue numbers alone is not evolutionary evidence.
For convergence, compare chymotrypsin-like proteases with <subtilisin>. Both are <serine proteases> using a <serine> nucleophile, <histidine> as a proton-transfer partner and an aspartate that organizes the <histidine>. Both stabilize a tetrahedral intermediate and form an acyl-enzyme. Nevertheless, their global folds differ, and the catalytic residues occur in different orders along the polypeptide: His–Asp–Ser in the <chymotrypsin> family and Asp–His–Ser in <subtilisin>. The similar local catalytic arrangement embedded in distinct topologies is evidence for independent assembly of effective serine-protease chemistry. Early three-dimensional structures made comparison of these spatial arrangements possible.<Structure of Subtilisin BPN′ at 2.5 Å Resolution by C. S. Wright and others>[Wright, Alden and Kraut, 1969, structure of subtilisin BPN′ and active-site comparison]{ref}
The contrast is between a conserved scaffold with modified chemistry or specificity and independently organized scaffolds converging on a favourable active-site geometry. Chemical constraints help explain convergence: catalysis benefits from appropriately positioned acids, bases, nucleophiles and stabilizing charges. They do not require the rest of the <protein> to become similar.
Structural inference has limits. A common small motif can occur by chance or reflect physical constraints; failure to detect sequence similarity does not prove absence of ancient homology. Strong conclusions combine global structural alignment, sequence-profile comparisons, taxonomic distribution and <molecular phylogeny>. Domain duplication, rearrangement and loss can complicate a simple family tree. Thus \b[structural resemblance is most persuasive when its extent and topology are evaluated alongside independent evolutionary evidence], rather than classifying every shared helix or catalytic residue as proof of common descent.
Back to article page
