Comprehending the constitution of early biological metabolism is indispensable for the understanding of the origin and evolution of life on Earth. Here, we analyzed the structural proteome before the Last Universal Common Ancestor (LUCA) based in the reconstruction of the ancestral sequences and structure for proteins involved in glycolysis/gluconeogenesis. The results are compatible with the notion that the first portions of the proteins were the areas homologous to the present-day catalytic sites. Those "proto-proteins" had a simple function: binding to cofactors. Upon the accretion of new elements to the structure, the catalytic function could have emerged. Also, the first structural motifs might have been related to the emergence of the different proteins that work in modern organisms.