# Enzyme‐Substrate Complex Formation and Electron Transfer in Nitrogenase‐Like Dark‐Operative Protochlorophyllide Oxidoreductase (DPOR)

## TL;DR

This paper studies how a key enzyme in photosynthesis, DPOR, forms a complex with its substrate and transfers electrons during a critical reaction.

## Contribution

The study reveals the formation of the enzyme–substrate complex and identifies cooperative behavior in DPOR's mechanism.

## Key findings

- The enzyme–substrate complex forms before electron transfer and MgATP hydrolysis.
- Rate constants for complex formation and electron transfer show a rate-limiting interplay.
- Cooperativity in enzyme–substrate complex formation is observed, potentially influencing enzymatic turnover.

## Abstract

Nitrogenase‐like dark‐operative protochlorophyllide oxidoreductase (DPOR) is a two‐component metalloenzyme involved in (bacterio)chlorophyll biosynthesis. DPOR enables photosynthesis in photosynthetic bacteria by catalyzing the MgATP hydrolysis‐dependent, stereoselective two‐electron reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide). This requires the repeated transient association of DPOR's two component proteins (BchL and BchNB), and involves a series of individual and unresolved sequence of events (including MgATP‐hydrolysis, electron transfer, protein association/dissociation, substrate binding, etc.). DPOR shares structural and mechanistic similarities with nitrogenase, although the spectroscopic properties of Pchlide and Chlide permit the reaction to be followed in situ with visible spectroscopy. Here, we investigate DPOR's mechanism through vis‐spectroscopy in the absence of an electron donor in the system, where we were able to observe the formation of the enzyme–substrate (ES) complex prior to substrate reduction (electron transfer and MgATP hydrolysis). The determination of rate constants for ES formation as well as overall electron transfer reveals the complex rate‐limiting interplay between these two processes. Further, we observe evidence of cooperativity for ES complex formation in DPOR, which may be the origin of cooperativity during enzymatic turnover.

Dark‐operative protochlorophyllide oxidoreductase (DPOR) participates in photosynthesis by catalyzing the ATP‐dependent reduction of protochlorophyllide to chlorophyllide. Structurally and mechanistically resembling nitrogenase, DPOR′s mechanism was investigated using visible spectroscopy. We used the vis‐spectra of substrate, product and Enyzme – Substrate complex (prior to the electron transfer) to study the kinetics of the mechanism and we observed cooperativity in the process.