Lanthanide Catalyzed Cyclization of Uridine3\u27-p-Nitrophenyl Phosphate

Steady state kinetics and 15N isotope effects have been used to study the cyclization reaction of uridine 3′-p-nitrophenyl phosphate. The cyclization reaction is catalyzed by transition metal ions and lanthanides, as are substitution reactions of many phosphate esters. Kinetic analysis reveals that the erbium-catalyzed cyclization reaction involves the concerted deprotonation of the 2′-OH group and departure of the leaving group. The transition state is very late, with a very large degree of bond cleavage to the leaving group, which could be due to a large degree of polarization of the PO bonds by erbium

Vitamin K epoxide reductase and its paralogous enzyme have different structures and functions

Abstract Vitamin K epoxide reductase (VKOR) is an essential enzyme for vitamin K-dependent carboxylation, while the physiological function of its paralogous enzyme VKOR-like (VKORL) is yet unknown. Although these two enzymes share approximately 50% protein sequence homology, the membrane topology of VKOR is still in debate. Here, we explored the differences in the membrane topology and disulfide-linked oligomerization of these two enzymes. Results from mutating the critical amino acid residues in the disputed transmembrane (TM) regions revealed that the second TM domain in the proposed 4-TM model of VKOR does not function as an authentic TM helix; supporting VKOR is a 3-TM protein, which is different from VKORL. Additionally, altering the loop sequence between the two conserved cysteine residues of VKORL affects its activity, supporting the notion that the conserved loop cysteines of VKORL are involved in its active site regeneration. However, a similar mutation in VKOR does not affect its enzymatic activity. Finally, our results show that although both VKOR and VKORL form disulfide-linked oligomers, the cysteine residues involved in the oligomerization appear to be different. Overall, the structural and functional differences between VKOR and VKORL shown here indicate that VKORL might have a different physiological function other than recycling vitamin K