Recent work has largely completed our understanding of the hydrogen-exchange chemistry of unstructured proteins and nucleic acids. Some of the high-energy structural fluctuations that determine the hydrogen-exchange behavior of native macromolecules have been explained; others remain elusive. A growing number of applications are exploiting hydrogen-exchange behavior to study difficult molecular systems and elicit otherwise inaccessible information on protein structure, dynamics and energetics.

An automated high-throughput, high-resolution deuterium exchange HPLC-MS method (DXMS) was used to extend previous hydrogen exchange studies on the position and energetic role of regulatory structure changes in hemoglobin. The results match earlier highly accurate but much more limited tritium exchange results, extend the analysis to the entire sequence of both hemoglobin subunits, and identify some energetically important changes. Allosterically sensitive amide hydrogens located at near amino acid resolution help to confirm the reality of local unfolding reactions and their use to evaluate resolved structure changes in terms of allosteric free energy.H ydrogen exchange (HX) measurements can, in principle, locate protein-binding sites and structure changes and can quantify otherwise unavailable dynamic and energetic parameters (1-4). For relatively small proteins, HX can be measured at an amino acid resolved level by NMR methods. For larger, functionally more interesting proteins, other strategies are required. Earlier work (5, 6) developed a ''functional-labeling'' approach that can selectively label, by hydrogen-tritium (H-T) or hydrogen-deuterium (H-D) exchange, just those sites that change in any functional process. In favorable cases, the label can then be located at medium resolution by a proteolytic fragmentation method in which the fragments are quickly produced and then separated by HPLC under conditions where the loss of isotopic label is slow (6-9).To move toward higher resolution and more comprehensive coverage of target proteins, recent work in many laboratories has coupled the HPLC separation to a second dimension of fragment resolution by online MS (10, 11). These methods tend to be labor intensive and time consuming, with limitations in throughput and comprehensiveness and in the structural resolution of functionally important changes. This article merges previous HX functional labeling and fragment separation methods with an automated MS approach termed deuterium exchange MS (DXMS) (12-18).We are using Hb as a model system to study how protein molecules manage intramolecular signal transduction processes. Hb functions by transducing a part of the binding energy of its initially bound O 2 ligands into structure-change energy. The energy is carried through the protein to distant heme sites in the form of energetic structure changes, and there transduced back into binding energy. The initial reduced binding energy and the later enhanced binding produces the physiologically important sigmoid binding curve. In short, the currency of allosteric interactions is free energy. Trying to understand allostery without measuring free energy is like trying to understand an economic system without measuring money. A great deal of information on regulatory structure change in many proteins is now available, but mainly in a qualitative pictorial sense from ''before and after'' crystallographic or NMR views. How these changes participate in energy transduction and translocation has been little explored (19)(20)(21...

The potential of hydrogen-exchange studies for providing detailed information on protein structure and structural dynamics has not yet been realized, largely because of the continuing inability to correlate measured exchange behavior with the parts of a protein that generate that behavior. J. Rosa and F. M. Richards (1979, J. Mol. Biol. 133, 399-416) pioneered a promising approach to this problem in which tritium label at exchangeable proton sites can be located by fragmenting the protein, separating the fragments, and measuring the label carried by each fragment. However, severe losses of tritium label during the fragment separation steps have so far rendered the results ambiguous. This paper describes methods that minimize losses of tritium label during the fragment separation steps and correct for losses that do occur so that the label can be unambiguously located and even quantified. Steps that promote adequate fragment isolation are also described.

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The inability to localize and measure the free energy of protein structure and structure change severely limits protein structure-function investigations. The local unfolding model for protein hydrogen exchange quantitatively related the free energy of local structural stability with the hydrogen exchange rate of concerted sets of structurally related protons. In tests with a number of modified hemoglobin forms, the loss in structural free energy obtained locally from hydrogen exchange results matches the loss in allosteric free energy measured globally by oxygen-binding and subunit dissociation experiments.

Protein hydrogen exchange is generally believed to register some aspects of internal protein dynamics, but the kind of motion at work is not clear. Experiments are being done to identify the determinants of protein hydrogen exchange and to distinguish between local unfolding and accessibility-penetration mechanisms. Results with small molecules, polynucleotides, and proteins demonstrate that solvent accessibility is by no means sufficient for fast exchange. H-exchange slowing is quite generally connected with intramolecular H-bonding, and the exchange process depends pivotally on transient H-bond cleavage. At least in alpha-helical structures, the cooperative aspect of H-bond cleavage must be expressed in local unfolding reactions. Results obtained by use of a difference hydrogen exchange method appear to provide a direct measurement of transient, cooperative, local unfolding reactions in hemoglobin. The reality of these supposed coherent breathing units is being tested by using the difference H-exchange approach to tritium label the units one at a time and then attempting to locate the tritium by fragmenting the protein, separating the fragments, and testing them for label. Early results demonstrate the feasibility of this approach.

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