Which enzyme does not use ATP and can move lipids in either direction across a membrane?

Scramblase is the enzyme that facilitates the movement of lipids across a membrane in either direction without the requirement for ATP. This is significant because scramblases operate in response to changes in the concentration of lipids and work by allowing the passive movement of lipids down their concentration gradient.

Unlike other enzymes that may be involved in lipid movement, such as flippases and floppases, which typically require ATP to transport lipids in a specific direction (inward or outward, respectively), scramblases do not utilize energy from ATP hydrolysis. This distinctive feature is essential in biological processes, particularly during events like apoptosis, where the asymmetric distribution of phospholipids in the lipid bilayer is altered, needing rapid redistribution.

Transferase enzymes, on the other hand, are primarily involved in the transfer of functional groups rather than the direct movement of lipids across membranes. Therefore, scramblase is unique in its ability to facilitate lipid movement bidirectionally and without energy investment.