The stereospecificity and catalytic efficiency of the tryptophan synthase-catalysed exchange of the alpha-protons of amino acids

Unusual reactivity of β-hydroxy-serotonin, a forgotten serotonin derivative

Serotonin (5-HT) is a well-known biogenic amine which regulates mood, sleep, and is involved in muscle contraction and blood coagulation. Based on an analogy to norepinephrine, a β-hydroxylated derivative of dopamine which has diverse physiological functions, beta-hydroxy-serotonin (β-OH-5-HT) originally encouraged interest as a potential pharmacological agent. Four decades ago, its organic synthesis was attempted. However, due to difficulties with the synthesis and the compound's instability, rigorous identification and characterization of β-OH-5-HT proved evasive. Here, we successfully synthesized β-OH-5-HT from 5-HT using a Pseudomonas enzyme, tryptophan side chain oxidase type I (TSOI), and we determined the structure by 2D-NMR and characterized β-OH-5-HT in detail. The CD spectra showed no optical activity, suggesting a racemic mixture. To separate DL-β-OH-5-HT, we synthesized L-Ala-5-HT and derivatized it into erythro- and threo-L-Ala-β-OH-5-HT with TSOI. Interestingly, both isolated fractions returned to a diastereoisomeric mixture within two hours at pH 5.0. Later, we found that, under acidic conditions, β-OH-5-HT readily reacted with nucleophiles like alcohols or thiols, yielding a variety of DL-β-substituted-5-HT. The unusual properties of β-OH-5-HT might be attributed to the unique nature of a β-hydroxyl group adjacent to an indole ring and amino group. The mechanism for the rapid racemization of β-OH-5-HT is discussed.

An efficient protocol for the complete incorporation of methyl-protonated alanine in perdeuterated protein

A strategy for the introduction of ((1)H,(13)C-methyl)-alanine into perdeuterated proteins is described. Specific protonation of alanine methyl groups to a level of 95% can be achieved by overexpressing proteins in M9/D(2)O based bacterial growth medium supplemented with 800 mg/l of 2-[(2)H], 3-[(13)C] L: -alanine. However, though simple, this approach results in undesired, non-specific background labeling due to isotope scrambling via different amino acid metabolic pathways. Following a careful analysis of known metabolic pathways we found that co-addition of perdeuterated forms of alpha-ketoisovalerate-d(7), succinate-d(4) and L: -isoleucine-d(10) with labeled L: -alanine, reduces undesired background labeling to <1%. When combined with recently developed methyl TROSY experiments, this methyl-specific labeling protocol permits the acquisition of excellent quality correlation spectra of alanine methyl groups in high molecular weight proteins. Our cost effective strategy offers a significant enhancement in the level of incorporation of methyl-labeled alanine in overexpressed proteins over previously reported methods.