15: Most Unusual Chemical Reactions
Immediately after the discovery that complexes between antigenic peptides and MHC complexes could provide the signal for T-cell target cell reactions, it became of interest to investigate the chemical kinetics of peptide-MHC reactions. This of course was a tall order since the target complex was at the interface of two cells. The first attempts considered the question of the kinetics of the reaction,
Peptide + MHC MHC — Peptide
with a forward rate constant of Kf and a reverse rate constant of Kr. Attempts to measure the kinetics were made in two ways. In one method, isolated MHC molecules were inserted in supported lipid bilayers and the binding and dissociation rates of peptide measured. See (333, 356). In a second method, MHC molecules were solubilized in a mild detergent prior to the kinetic measurements. In some cases the kinetics were interpreted in terms of reaction intermediates. In other cases, peptide binding was limited by dissociation of pre-bound peptide.
The derived rate constants were unusual to say the least. Extremely slow ‘on’ rate constants were sometimes derived (~1 liter/mole-sec), along with sometimes extremely long dissociation half times – days, weeks, months. The lab carried out many studies (see 343, 346, 349, 351, 354,358, 359, 360, 361, 362, 364, 369, 370, 374,375,380, 381, 382, 388, 390, 391, 392, 401, 402, 405, 406, 407, 409, 413, 418, 419, 422, 423, 425, 432, 433, 434, 436, 439, 442, 444, 457, 458, 463, 467).
To summarize, the following kinetics were encountered:
1. Simple P+M PM; 381, (391, 392).
2. Kinetic intermediate: peptide binds to M, then dissociates or goes on to form stable complex (333, 390).
3. Binding requires predissociation of second peptide (356).
4. Two peptides can bind to one MHC molecule (358).
5. One peptide can enhance the binding of a second peptide (374, 442).
6. Antigenic peptides can form isomeric complexes with a single type of MHC molecule (406) (and are recognized by distinct T-cells (425)).
7. A peptide may dissociate from an MHC molecule, leaving the MHC molecule in an unstable state, whereby the peptide must quickly rebind or the MHC denatures irreversibly (444).
8. Many other experiments were carried out dealing with single chain reactivity and MHC stability (343, 346).
Thus, the kinetics and interactions amongst these molecules turned out to be much more complex than anticipated. Unfortunately my time to work in this field of science ran out and I really did not have the opportunity to analyze, systematize and exploit further the consequences of these results. Most importantly we did not have the chance to critically compare differences in reaction environments and mechanisms. And the involvement of additional proteins playing a role in peptide selection and processing (e.g., invariant chain, HLADM) has only added to these complexities. Of my former students, graduate and postdoctoral, Scheherazade Sadegh-Nasseri, Tania Watts and others have continued and extended this work on these more complex interactions involving antigen processing and presentation. All the students working in this area were remarkably productive.