A Dynamic Substrate is Required for MhuD-catalysed Degradation of Haem to Mycobilin

02 November 2020, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

The non-canoncial haem oxygenase MhuD from Mycobacterium tuberculosis binds a haem substrate that adopts a dynamic equilibrium between planar and out-of-plane ruffled conformations. MhuD degrades this substrate to an unusual mycobilin product via successive monooxygenation and dioxygenation reactions. This article establishes a causal relationship between haem substrate dynamics and MhuD-catalysed haem degradation resulting in a revised enzymatic mechanism. UV/Vis absorption (Abs) and electrospray ionisation mass spectrometry (ESI-MS) data demonstrated that a second-sphere substitution favouring population of the ruffled haem conformation changed the rate-limiting step of the reaction resulting in a measurable build-up of the monooxygenated meso-hydroxyhaem intermediate. In addition, UV/Vis Abs and ESI-MS data for a second-sphere variant that favoured the planar substrate conformation showed that this change altered the enzymatic mechanism resulting in an alpha-biliverdin product. Single-turnover kinetic analyses for three MhuD variants revealed that the rate of haem monooxygenation depends upon the population of the ruffled substrate conformation. These kinetic analyses also revealed that the rate of meso-hydroxyhaem dioxygenation by MhuD depends upon the population of the planar substrate conformation. Thus, the ruffled haem conformation supports rapid haem monooxygenation by MhuD, but further oxygenation to the mycobilin product is inhibited. In contrast, the planar substrate conformation exhibits altered haem monooxygenation regiospecificity followed by rapid oxygenation of meso-hydroxyhaem. Altogether, these data yielded a revised enzymatic mechanism for MhuD where access to both substrate conformations is needed for rapid incorporation of three oxygen atoms into haem yielding mycobilin.

Keywords

MhuD
Enzyme Mechanism
enzyme kinetics

Supplementary materials

Title
Description
Actions
Title
ESI
Description
Actions

Comments

Comments are not moderated before they are posted, but they can be removed by the site moderators if they are found to be in contravention of our Commenting Policy [opens in a new tab] - please read this policy before you post. Comments should be used for scholarly discussion of the content in question. You can find more information about how to use the commenting feature here [opens in a new tab] .
This site is protected by reCAPTCHA and the Google Privacy Policy [opens in a new tab] and Terms of Service [opens in a new tab] apply.