  This is a comment on a recently published paper in Langmuir: Mandal, N. S.;
Sen, A. Relative Diffusivities of Bound and Unbound Protein Can Control
Chemotactic Directionality. Langmuir 2021, PMID: 34647749 [arXiv:2103.13469].
In this study, Mandal and Sen claim to propose a new kinetic model to analyze
the directional movement of enzyme molecules in response to a gradient of their
substrate, with the supposedly new prediction that net movement occurs up the
substrate gradient when the diffusivity of the substrate-bound enzyme is lower
than that of the unbound enzyme, and movement down the substrate gradient when
the diffusivity of the substrate-bound enzyme is higher than that of the
unbound enzyme. They develop this theoretical scheme to present an alternative
to our previously published theoretical framework [Agudo-Canalejo, J.; Illien,
P.; Golestanian, R. Phoresis and enhanced diffusion compete in enzyme
chemotaxis. Nano Lett. 2018, 18, 2711-2717; arXiv:2104.02394]. We demonstrate
that despite their claim, what they present is exactly the same theory as our
work, and therefore, we conclude that their claim of novelty is
unsubstantiated.
