Analogues of NADP+ as inhibitors and coenzymes for NADP+ malic enzyme from maize leaves

Abstract

Structural analogues of the NADP(+) were studied as potential coenzymes and inhibitors for NADP(+) dependent malic enzyme from Zea mays L. leaves. Results showed that 1, N(6)-etheno-nicotinamide adenine dinucleotide phosphate (∈ NADP(+)), 3-acetylpyridine-adenine dinucleotide phosphate (APADP(+)), nicotinamide-hypoxanthine dinucleotide phosphate (NHDP(+)) and β-nicotinamide adenine dinucleotide 2': 3'-cyclic monophosphate (2'3'NADPc(+)) act as alternate coenzymes for the enzyme and that there is little variation in the values of the Michaelis constants and only a threefold variation in Vmax for the five nucleotides. On the other hand, thionicotinamide-adenine dinucleotide phosphate (SNADP(+)), 3-aminopyridine-adenine dinucleotide phosphate (AADP(+)), adenosine 2'-monophosphate (2'AMP) and adenosine 2': 3'-cyclic monophosphate (2'3'AMPc) were competitive inhibitors with respect to NADP(+), while β-nicotinamide adenine dinucleotide 3'-phosphate (3'NADP(+)), NAD(+), adenosine 3'-monophosphate (3'AMP), adenosine 2': 5'-cyclic monophosphate (2'5'AMPc), 5'AMP, 5'ADP, 5'ATP and adenosine act as non-competitive inhibitors. These results, together with results of semiempirical self-consistent field-molecular orbitals calculations, suggest that the 2'-phosphate group is crucial for the nucleotide binding to the enzyme, whereas the charge density on the C4 atom of the pyridine ring is the major factor that governs the coenzyme activity.