Molecular mechanisms of acid denaturation. The role of histidine residues in the partial unfolding of apomyoglobin.


Apomyoglobin adopts a partly folded intermediate conformation (I), sometimes referred to as a molten globule intermediate, near pH 4. To determine which histidine residues trigger this partial unfolding reaction, we made mutants in which nine of the twelve histidine residues in the protein are substituted individually. We then measured acid and urea-induced unfolding curves for these substituted proteins. Two acid unfolding transitions are observed: native (N) to intermediate (I), and I to unfolded (U). These data were fitted using a simple three-state model which has been shown to give an adequate description of acid and urea-induced unfolding of wild-type apomyoglobin. The aim is to quantify changes in the apparent standard Gibbs energy differences between N, I and U, as well as the unfolding mechanism, that result from these substitutions, and to test how well the model fits data for substituted proteins. In most cases, the model fits the data reasonably well, and significant changes in fitted unfolding parameters of various mutants are also clearly visible in the primary data. The following conclusions are drawn. (1) Histidines 24 and 119 synergistically stabilize native apomyoglobin (N) at pH 8, but together destabilize N as pH is decreased below seven. (2) Histidine 36 stabilizes N when it is protonated. (3) Histidine substitutions in the heme-binding pocket (residues 64, 93 and 97) have little effect on the stability of N, suggesting that the heme-binding pocket is open. (4) Histidine substitutions affect the N to I transition but have little effect on the I to U transition. (5) The simple model we use to describe the unfolding of apomyoglobin cannot account for all the data, particularly the effects of the H36Q mutation. The effect of protonated histidine 36 on stabilizing N is not included in the model. We suggest that breaking the hydrogen bond between histidines 24 and 119 by protonation when the pH is decreased from 6 to 4 is an important part of triggering the partial unfolding of N to I, and likewise that formation of the hydrogen bond between histidines 24 and 119 may be a rate-determining step in the kinetic process of forming N from I during refolding. Study holds ProTherm entries: 7493, 7494, 7495, 7496, 7497, 7498, 7499, 7500, 7501, 7502, 7503, 7504, 7505, 7506, 7507, 7508, 7509, 7510, 7511, 7512, 7513, 7514 Extra Details: the transition is from native to intermediate protein folding; protein strucutre; protein stability;,molten globule; apomyoglobin

Submission Details


Submitter: Connie Wang

Submission Date: April 24, 2018, 8:34 p.m.

Version: 1

Publication Details
Barrick D;Hughson FM;Baldwin RL,J. Mol. Biol. (1994) Molecular mechanisms of acid denaturation. The role of histidine residues in the partial unfolding of apomyoglobin. PMID:8158639
Additional Information

Structure view and single mutant data analysis

Study data

No weblogo for data of varying length.
Colors: D E R H K S T N Q A V I L M F Y W C G P

Data Distribution

Studies with similar sequences (approximate matches)

Correlation with other assays (exact sequence matches)

Relevant UniProtKB Entries

Percent Identity Matching Chains Protein Accession Entry Name
100.0 Myoglobin P02185 MYG_PHYMC
96.8 Myoglobin Q0KIY5 MYG_KOGBR
96.8 Myoglobin P02184 MYG_KOGSI
92.9 Myoglobin Q0KIY1 MYG_BALBO
92.9 Myoglobin Q0KIY2 MYG_BALED
92.9 Myoglobin P02177 MYG_ESCRO
92.2 Myoglobin P02178 MYG_MEGNO
91.4 Myoglobin Q0KIY3 MYG_PENEL
91.4 Myoglobin P02181 MYG_INIGE
92.1 Myoglobin P02174 MYG_GLOME
90.9 Myoglobin P02179 MYG_BALAC
91.4 Myoglobin P02173 MYG_ORCOR
90.8 Myoglobin Q0KIY7 MYG1_STEAT
90.8 Myoglobin P68276 MYG_DELDE
90.8 Myoglobin P68279 MYG_TURTR
90.8 Myoglobin P68277 MYG_PHODA
90.8 Myoglobin P68278 MYG_PHOPH
90.3 Myoglobin P02180 MYG_BALPH
90.1 Myoglobin P02183 MYG_MESCA
90.1 Myoglobin Q0KIY0 MYG_MESST
90.1 Myoglobin P02182 MYG_ZIPCA