# FUT8 Catalysis Involves GDP-Fucose–Induced Loop Activation Promoting a Reaction at the SN1‑SN2 Frontier

## TL;DR

This study reveals how the enzyme FUT8 uses structural changes to catalyze a key sugar transfer reaction in glycoprotein function.

## Contribution

The study provides a detailed mechanistic and structural model of FUT8 catalysis involving GDP-fucose-induced loop activation and a unique reaction pathway.

## Key findings

- GDP-fucose binding induces conformational changes in two loops to stabilize the active site.
- The reaction proceeds through a late transition state with an energy barrier of ~18 kcal/mol.
- A transient ion pair forms during catalysis, but no stable intermediate is observed.

## Abstract

α1,6-Fucosyltransferase
8 (FUT8) catalyzes the core α1,6-fucosylation
of N-glycans, a modification essential for the biological
function of many mammalian glycoproteins. Despite its importance,
the structural and mechanistic aspects of FUT8 catalysis remain incompletely
understood. Here, we combine molecular dynamics, QM/MM, and metadynamics
simulations to delineate the full catalytic cycle of FUT8. We reveal
that GDP-fucose binding induces a concerted conformational rearrangement
of two flexible loops, which cooperatively stabilize a closed, catalytically
competent active site. Formation of the Michaelis complex primes the
enzyme for fucose transfer via a slightly late and highly asynchronous
SN2 inverting mechanism. In fact, the reaction proceeds
through a late transition state in a single kinetic step (energy barrier
∼18 kcal/mol, consistent with experimental k
cat values) but in three different stages, i.e.: (i) cleavage
of the glycosidic bond between fucose and GDP, (ii) formation of the
glycosidic bond and (iii) H-transfer from the acceptor to the catalytic
Glu373, underscoring the asynchronous nature of this process. Moreover,
topological calculations of the electron localization function (ELF)
along the reaction coordinate reveal the transient formation of an
intimate ion pair, with a lifetime of 350–800 fs, as a transient
intermediate; notably, despite the cationic character of the transition
state, no stable intermediate is formed. These findings highlight
how structural rearrangements enable a chemically distinct catalytic
process and provide a structural framework for rational inhibitor
design.