Cryo-EM maps how a TB enzyme flips on

Scientists are trying to understand how key bacterial enzymes switch between inactive and active forms. In a study led by corresponding author Ghader Bashiri, researchers focused on Isocitrate lyase 2 (ICL2) from Mycobacterium tuberculosis, an enzyme that undergoes dramatic conformational rearrangements when it interacts with a small molecule called acetyl-CoA. Rather than taking a single snapshot, the team used a time-resolved approach to capture the enzyme moving through states along its activation pathway. The work set out to record those transient shapes and to link them to how the enzyme’s active sites behave. By watching the structural journey of ICL2 as it responded to acetyl-CoA, the researchers aimed to reveal the sequence of changes that turn the enzyme on, and to test ideas about whether the effector drives a new structure or shifts an existing balance of forms. The study centers on direct observations of structural change, staying close to what can be seen and measured in the laboratory experiment reported by Ghader Bashiri and colleagues.

The team applied time-resolved cryo-EM to capture multiple conformational states of Isocitrate lyase 2 (ICL2) as it responded to binding by acetyl-CoA. Time-resolved cryo-EM allowed them to collect structural images at different moments, assembling a trajectory of shapes the enzyme adopts during activation. Analysis of those states showed how acetyl-CoA engages with allosteric sites away from the catalytic centres and triggers rearrangements that do not affect both active sites identically. Instead, the structures reveal asymmetric, half-of-site activity at the catalytic centres, meaning that the two catalytic regions behave differently as the enzyme transitions. These observed states support a conformational selection model of allostery: acetyl-CoA binding shifts a pre-existing equilibrium of shapes toward an active form rather than creating a wholly new structure. The combined time-resolved structural data and interpretation provide a step-by-step view of allosteric activation in ICL2 from Mycobacterium tuberculosis.

Seeing the activation trajectory of ICL2 in this way has several implications for basic science. First, the findings strengthen the idea that some enzymes are regulated by selection among pre-existing conformations rather than by a single induced fit; in this case acetyl-CoA nudges ICL2 toward active conformations that already exist in equilibrium. Second, the discovery of asymmetric, half-of-site behaviour highlights that different parts of the same protein can respond unequally to the same effector, a detail that matters for interpreting biochemical measurements and for modelling enzyme function. Finally, providing a time-resolved structural map gives researchers a clearer picture of the mechanical steps that link allosteric binding to catalytic output in Mycobacterium tuberculosis ICL2. These insights are poised to shape follow-up studies that probe how such conformational dynamics influence bacterial metabolism and regulation.