How enzyme activity can accelerate molecular movement inside cells

The fluid surrounding a cell is not as passive as it may seem. It actively aids in the process of transporting molecules into the cell. This is the finding of a study conducted by researchers from the Indian Institute of Technology Gandhinagar, the University of Pennsylvania, IIT Jodhpur, and the Indian Institute of Science Education and Research Kolkata.

The study, published in Small, demonstrates that enzymes in the fluid outside the cell can independently increase the cell's uptake of a particular cargo, without entering the cell or modifying the cargo itself.

To illustrate this concept, imagine a pond with a tennis ball floating in it. Without any disturbance, the ball would drift aimlessly, barely moving. However, if the pond were filled with tiny, active swimmers who were stirring the water, the ball would begin to move more freely, reaching the pond's edge more frequently and sooner than it would have on its own.

This concept is mirrored in the cellular experiments conducted by the research team. The movement of these "swimmers," or enzymes, causes the extracellular fluid to become more agitated, which in turn helps push nearby molecules toward the cell, resulting in the cell absorbing more of them.

The study focused on transferrin, a protein that the body uses to transport iron into cells. The researchers chose this particular molecule because its journey into the cell has been extensively studied, allowing them to compare the ordinary uptake against the changes induced by the enzymes. By using fluorescently labeled transferrin as a tracking mechanism, the researchers were able to observe that cells with working enzymes absorbed roughly 17% more transferrin compared to cells without the enzymes.

This increased uptake was also confirmed by observing the transferrin moving 50% faster when the enzymes were active, as measured using total internal reflection fluorescence (TIRF) microscopy and fluorescence correlation spectroscopy.