Distinctive iron requirement of tryptophan 5-monooxygenase: TPH1 requires dissociable ferrous iron

Melatonin biosynthesis pathways in nature and its production in engineered microorganisms

Melatonin is a biogenic amine that can be found in plants, animals and microorganism. The metabolic pathway of melatonin is different in various organisms, and biosynthetic endogenous melatonin acts as a molecular signal and antioxidant protection against external stress. Microbial synthesis pathways of melatonin are similar to those of animals but different from those of plants. At present, the method of using microorganism fermentation to produce melatonin is gradually prevailing, and exploring the biosynthetic pathway of melatonin to modify microorganism is becoming the mainstream, which has more advantages than traditional chemical synthesis. Here, we review recent advances in the synthesis, optimization of melatonin pathway. l-tryptophan is one of the two crucial precursors for the synthesis of melatonin, which can be produced through a four-step reaction. Enzymes involved in melatonin synthesis have low specificity and catalytic efficiency. Site-directed mutation, directed evolution or promotion of cofactor synthesis can enhance enzyme activity and increase the metabolic flow to promote microbial melatonin production. On the whole, the status and bottleneck of melatonin biosynthesis can be improved to a higher level, providing an effective reference for future microbial modification.

Gut dysbiosis and serotonin: intestinal 5-HT as a ubiquitous membrane permeability regulator in host tissues, organs, and the brain

The microbiota and microbiome and disruption of the gut-brain axis were linked to various metabolic, immunological, physiological, neurodevelopmental, and neuropsychiatric diseases. After a brief review of the relevant literature, we present our hypothesis that intestinal serotonin, produced by intestinal enterochromaffin cells, picked up and stored by circulating platelets, participates and has an important role in the regulation of membrane permeability in the intestine, brain, and other organs. In addition, intestinal serotonin may act as a hormone-like continuous regulatory signal for the whole body, including the brain. This regulatory signal function is mediated by platelets and is primarily dependent on and reflects the intestine’s actual health condition. This hypothesis may partially explain why gut dysbiosis could be linked to various human pathological conditions as well as neurodevelopmental and neuropsychiatric disorders.

Production and Peripheral Roles of 5-HTP, a Precursor of Serotonin

Serotonin (5-hydroxytryptamine [5-HT]) has been implicated in a variety of physiological and pathological functions. Multiple steps of enzyme reactions enable biosynthesis of 5-HT. The first and rate-limiting step of the reaction is the synthesis of 5-hydroxy-L-tryptophan (5-HTP) from L-tryptophan. This step is dictated by an enzyme, tryptophan hydroxylase (TPH). TPH requires 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4) as a co-substrate of TPH. 5-HTP has been simply regarded as a precursor of 5-HT and it is believed that the biological significance of 5-HTP is essentially ascribed to the production of 5-HT. However, recent works shed light on the specific functions of 5-HTP in the periphery. In this review article, we focus on the specific roles of exogenous 5-HTP as well as the endogenous 5-HTP in the gut epithelial cells. Since systemic treatment with 5-HTP is applied to patients with lower 5-HT levels, the studies on the specific role of 5-HTP might create an opportunity to explore the effects of exogenously-applied 5-HTP in the gut in man.

The catecholamine system in health and disease -Relation to tyrosine 3-monooxygenase and other catecholamine-synthesizing enzymes

Catecholamines [dopamine, noradrenaline (norepinephrine), and adrenaline (epinephrine); CAs] are neurotransmitters in the central and peripheral nervous systems as well as hormones in the endocrine system. CAs in the brain play a central role in versatile functions as slow-acting neurotransmitters functioning in synaptic neurotransmission, modulating the effects of fast-acting neurotransmitters such as glutamate and γ-aminobutyric acid (GABA). In this review, I focus on recent advances in the biochemistry and molecular biology of the CA system in humans in health and disease, especially in neuropsychiatric diseases such as Parkinson's disease (PD), in relation to the biosynthesis of CAs regulated by a pteridine-dependent monooxygenase, tyrosine 3-monooxygenase (tyrosine hydroxylase, TH) and its pteridine cofactor, tetrahydrobiopterin (BH4).

Developmental role of tryptophan hydroxylase in the nervous system

The serotonin 5-hydroxytryptamine (5-HT) neurotransmitter system contributes to various physiological and pathological conditions. 5-HT is the first neurotransmitter for which a developmental role was suspected. Tryptophan hydroxylase (TPH) catalyzes the rate-limiting reaction in the biosynthesis of 5-HT. Both TPH1 and TPH2 have tryptophan hydroxylating activity. TPH2 is abundant in the brain, whereas TPH1 is mainly expressed in the pineal gland and the periphery. However, TPH1 was found to be expressed predominantly during the late developmental stage in the brain. Recent advances have shed light on the kinetic properties of each TPH isoform. TPH1 showed greater affinity for tryptophan and stronger enzymic activity than TPH2 under conditions reflecting those in the developing brain stem. Transient alterations in 5-HT homeostasis during development modify the fine wiring of brain connections and cause permanent changes to adult behavior. An increasing body of evidence suggests the involvement of developmental brain disturbances in psychiatric disorders. These findings have revived a long-standing interest in the developmental role of 5-HT-related molecules. This article summarizes our understanding of the kinetics and possible neuronal functions of each TPH during development and in the adult.