Using NMR to Study Enzyme Function and Dynamics

While a 3D structure shows where atoms are located, dynamics tell us how they move. In enzymology, this motion occurs across a staggering range of timescales. For instance, bond vibrations and side-chain rotations happen in picoseconds ($10^{-12}$ s), while the actual chemical transformation or large-scale domain movement might take milliseconds (ms) or even seconds [2].

If an enzyme’s active site cannot move, the substrate cannot enter, and the product cannot leave. This “conformational exchange” is often the rate-limiting step of catalysis [3]. As we explored in our guide on how NMR reveals molecular structure and dynamics, the technique’s ability to link these movements to specific atoms is why it remains indispensable in drug discovery and biochemistry.

The fastest motions in an enzyme, occurring on the picosecond to nanosecond (ps–ns) scale, provide a measure of local flexibility. Researchers primarily use $^{15}$N Relaxation Experiments to study these movements [4].

Three specific parameters are typically measured:

• $R_{1}$ (Longitudinal Relaxation): Reflects the return of magnetization to equilibrium; sensitive to fast motions.

• $R_{2}$ (Transverse Relaxation): Sensitive to slower tumbling and chemical exchange.

• HetNOE (Heteronuclear Overhauser Effect): Indicates how much a specific N-H bond vector moves relative to the rest of the protein.

For Intrinsically Disordered Proteins (IDPs)—enzymes or regions that lack a fixed 3D shape—these experiments are vital. Research published in Journal of Visualized Experiments demonstrates that IDPs often exhibit low $R_{2}$ rates and negative HetNOE values, signaling high internal entropy that can be harnessed for signaling and regulation [4].

Most enzymatic steps, such as substrate binding and allosteric regulation, occur on the micro-to-millisecond (µs–ms) scale. This is where NMR truly shines through three primary techniques:

1. CPMG Relaxation Dispersion

The Carr–Purcell–Meiboom–Gill (CPMG) experiment is the gold standard for detecting “invisible” excited states. These are conformations with high free energy that are populated at less than 5% of the total enzyme population [1]. By applying 180° pulses at varying frequencies, researchers can “refocus” the signals from these transient states, allowing them to calculate the exact rate of the conformational switch ($k_{ex}$).

2. CEST (Chemical Exchange Saturation Transfer)

CEST is preferred for slower exchange processes (10–500 s$^{-1}$). It uses a weak radiofrequency field to “saturate” the minor state’s signal, which then transfers to the major state. This creates a “minor dip” in the NMR spectrum, essentially making the invisible visible [1].

3. $R_{1\rho}$ Relaxation Dispersion

This technique is highly versatile, bridging the gap between CPMG and CEST. It is particularly useful for studying enzymes that undergo extremely fast transformations or those involved in high-molecular-weight complexes, such as the 723-residue enzymes studied by the Kay Lab.

A breakthrough study using High-Resolution $^{31}$P Field Cycling NMR recently changed our understanding of GMP reductase (GMPR) [3]. While traditional NMR focuses on the protein, $^{31}$P relaxometry looks at the phosphate groups of the substrate and cofactor (NADP+).

The study revealed that the enzyme’s dynamics are “reaction-specific.” In the deamination step, the cofactor is more mobile than the substrate; in the hydride transfer step, the roles flip [3]. This demonstrates that interactions previously thought to be static tethers—like phosphate grippers—are actually active participants in the catalytic network. This type of high-resolution data is often a prerequisite for and complement to the work described in using NMR for metabolite profiling.

When designing an NMR study for enzyme dynamics, follow this prescriptive hierarchy based on your target timescale:

1. For local flexibility (ps–ns): Start with $^{15}$N $R_{1}$, $R_{2}$, and HetNOE. This is standard for identifying which parts of your enzyme “breathe” or are disordered.
2. For active site fluctuations (µs–ms): Use CPMG Relaxation Dispersion. If the exchange is slower than 1,000 s$^{-1}$, opt for CEST.
3. For large enzymes or solid-state samples: If your enzyme is part of a membrane or fiber, you may need to pivot. Check out our deep dive on using NMR to track ultrafast dynamics in solids for more specialized protocols.
4. For ligand tracking: If your substrate has a phosphorus group, Field Cycling $^{31}$P NMR provides the best signal for tracking how the ligand moves within the active site [3].

• Enzymes are Dynamic: Catalysis is governed by conformational changes that static structures cannot show.
• Timescales Matter: ps–ns motions represent local flexibility; µs–ms motions typically correspond to the actual catalytic steps.
• Invisible States: NMR is unique in its ability to detect “excited states” that exist for only a few milliseconds but are critical for function.
• Technique Selection: Use $^{15}$N relaxation for general maps, CPMG for catalytic bottlenecks, and CEST for slow structural rearrangements.

Action Plan

• Step 1: Assign the NMR resonances of your protein backbone to map specific residues.
• Step 2: Run a steady-state HetNOE experiment to identify disordered or highly flexible loops.
• Step 3: Perform a CPMG dispersion survey at multiple magnetic fields (e.g., 600 MHz and 800 MHz) to calculate exchange rates and population sizes of excited states.
• Step 4: Integrate your dynamic data with structural models to identify allosteric sites or drug-binding pockets.

Understanding enzyme dynamics isn’t just an academic exercise—it is the key to designing inhibitors that lock an enzyme in an inactive state, providing a blueprint for the next generation of precision medicine.