Competitive Inhibitors Unveil Structure/Function Relationships in Human D-Amino Acid Oxidase.

Gianluca Molla

doi:10.3389/fmolb.2017.00080

Abstract

D-amino acid oxidase (DAAO) catalyzes the oxidative deamination of several neutral D-amino acids and is the enzyme mainly responsible (together with serine racemase) for degrading D-serine (D-Ser) in the central nervous system of mammals. This D-amino acid, which binds the coagonist site of the N-methyl-D-aspartate receptor, is thus a key neuromodulator of glutamatergic neurotransmission. Altered D-Ser metabolism results in several pathological conditions (e.g., amylotrophic lateral sclerosis or schizophrenia, SZ) for which effective “broad spectrum” pharmaceutical drugs are not yet available. In particular, the correlation between reduced D-Ser concentration and SZ led to a renaissance of biochemical interest in human DAAO (hDAAO). In the last 10 years, public and corporate research laboratories undertook huge efforts to study the structural, enzymatic, and physiological properties of the human flavoenzyme and to identify novel effective inhibitors which, acting as pharmaceutical drugs, could decrease hDAAO activity, thus restoring the physiological concentration of D-Ser. Although, none of the identified hDAAO inhibitors has reached the market yet, from a biochemical point of view, these compounds turned out to be invaluable for gaining a detailed understanding of the structure/function relationships at the molecular level in the mammalian DAAO, in particular of the interaction between ligand and the enzyme. This detailed knowledge, together with several recent studies concerning the interaction of the human enzyme with other protein regulative partners, its subcellular localization, and in vivo degradation, contributed to gaining comprehensive knowledge of the structure, function, and physiopathological role of this important human enzyme.

Highlights

D-amino acid oxidase (E.C. 1.4.3.3, DAAO), originally designated as “the new yellow enzyme,” was the second flavoprotein to be discovered at the beginning of the last century and it is considered the prototype of flavin-containing oxidases (Curti et al, 1992)
Notwithstanding the fact that human DAAO activity was detected for the first time in the human brain in 1966 (Neims et al, 1966), the function of this flavoenzyme in the mammalian central nervous system (CNS) remained elusive until 1992 when the development of novel sensitive analytical techniques allowed the detection in rat brain of substantial amounts of free D-Ser (Hashimoto et al, 1992)
The huge efforts by public and corporate research laboratories to identify novel pharmaceutical drugs to fight schizophrenia symptoms identified hundreds of different human DAAO (hDAAO) ligands which in vitro are able to inhibit the human flavooxidase with a potency up to 3 orders of magnitudes higher than classical inhibitors. These compounds were effective in inhibiting hDAAO activity ex vivo and/or in vivo as demonstrated by measuring residual hDAAO activity in cell lines and tissues from murine models

Summary

Introduction

D-amino acid oxidase (E.C. 1.4.3.3, DAAO), originally designated as “the new yellow enzyme,” was the second flavoprotein to be discovered at the beginning of the last century and it is considered the prototype of flavin-containing oxidases (Curti et al, 1992). In the mammalian enzyme, where Ser335 is replaced by Gly313, an active-site water molecule (placed at a H-bond distance from the α-NH+3 ) might play the same role, which could represent the first member of a proton relay chain between the substrate and the bulk solvent (Figures 1, 5; Boselli et al, 2004).

Results

Conclusion