Importance of tyrosine for structure and function of mitochondrial malate dehydrogenases

Mitochondrial malate dehydrogenase from pig and chicken both contain one tyrosine/subunit with highly red-shifted spectrum, most probably involved in a hydrogen bond with a carboxylate group. The spectral changes of this tyrosine can be used as an indicator for alkaline denaturation, acid transition and coenzyme binding. Acid transition is coupled with breaking of this bond by protonation as monitored by loss of absorbance at 290 nm. Activity is lost and fluorescence intensity is increased at slightly higher pH, thus indicating increased mobility of the indicator and most probably of the whole protein prior to protonation of the indicator-tyrosine.