The fructose-1,6-bisphosphate aldolases: same reaction, different enzymes.

Two forms of the enzyme fructoseI ,6-bisphosphate aldolase (EC 4. I .2.13) are known, designated class I and class 11. The enzymes o f class I function by imine formation between the substrate and a catalytically essential lysine residue in the active site, which acts to stabilize the intermediate carbanion (for review, see [ I , 21). The enzymes of class I1 utilize a divalent metal ion to act as the electron sink in what otherwise appears to be a similar catalytic mechanism [3] . As proteins, however, the two classes o f aldolase differ in several ways 141. The enzymes of eukaryotes generally fiill into class I and are tetramers (M, approx. 160000) of identical polypeptide chains. They occur as isoenzymes in diffcrent tissues (e.g. brain. liver, muscle), but their amino acid sequences are homologous, especially in the region o f the imine-forming lysine residue, consistent with a common evolutionary origin (2, 51. The complete amino acid sequencc o f rabbit muscle aldolase was established some years ago [ 5 7 ] . Nucleotide sequences of cDNAs for the rabbit muscle [ X I , human liver [Y, 101, rat liver 1 I I / and Tiyputzosoma hnic.ei \ 12) enzymes have since been determined, from which the primary structures of the proteins could be inferred. An X-ray crystallographic analysis of human muscle aldolase is in progress [ 131. At least some prokaryotes, notably Escherichiu coli [ 14, 15 [, Luc~tohucillirs cusei [ 161 and M y c w hwteriirm srnegrnutis [ 1 71, have also been shown t o contain class I enzymes, but comparatively little is known about these still somewhat rare proteins. Similarly. until recently, comparatively little was known about any of the class I1 enzymes. The aldolase of yeast is mctal-dependent, as are the aldolases of most prokaryotes studied [ 41. The class I I aldolases of Succhurornyes cwxvisiue [ 4, 1 X ] and E. c d i [ 1 Y I are perhaps the best characterized. Both are dimers ( M , approx. 80000) of identical polypeptide chains [18, 191. There are indications that thiol groups of cysteine residues may be involved in the catalytic activity of the yeast enzyme [ 201 and in the co-ordination o f the zinc ion [21]. However. class I1 aldolases have proved relatively hard to purify in quantity, only fragments of their primary structures have been determined, and no crystals have been obtained. I t was reported some years ago 1221 that a gene, p g k , encoding the enzyme phosphoglycerate kinase (EC 2.7.2.3) o f E. coli, was carried on plasmid pLC33-5 of the collection of Clarke & Carbon [23]. It was also noted that E. coli cells