The chemical reactivity of fully maleylated cytochrome c

Effect on protein stability of reversing the charge on amino groups

The amino groups of beta-lactoglobulins A and B, cytochrome c and ribonuclease were progressively converted to acidic groups by reaction with succinic anhydride. The mixtures of modified proteins generated in this way were analyzed by urea-gradient electrophoresis, which separates the molecules on the basis of their net charge and demonstrates visually their urea-induced unfolding transitions. Molecules succinylated to varying extents were resolved by the electrophoresis, so purification of the many modified species was not required. It is demonstrated that accurate estimates of the stability of the folded state of an individual species may be estimated very easily from its urea-gradient electrophoretic pattern. Changes in ionization of the protein upon unfolding may also be detected. The general electrostatic effect of varying the net charge on these proteins was small. Converting the normally basic ribonuclease and cytochrome c to neutral and then to acidic proteins caused the net stabilities of their folded states to vary by no more than a few kJ/mol. However, specific interactions between a few ionized groups appear to be more important in some instances. Succinylation of the 19th, and final, lysine residue of cytochrome c produced unfolding even in the absence of urea, whereas reaction of the first 18 had very little effect. Reaction of the initial amino groups of beta-lactoglobulins A and B produced a small increase in stability in a few instances, a decrease in others; modification of more than about ten groups abruptly caused unfolding in the absence of urea.

The transport of cations in mitochondria

The present paper has reviewed several factors related to ion transport and examined the properties of cation transport in mitochondria. The analysis suggests that: (1) The concept that a metabolically dependent electrical potential across the mitochondrial membrane plays a role in determining ion fluxes and steady-state concentrations is not justified and the data indicate that such exchanges are generally electroneutral. (2) Generally, the influx and efflux of an ion proceed by the same mechanism with at least one exception. (3) There are indications that some of the steps in transport are common to several cations. (4) The idea that carrier or ionophoric molecules are involved in cation transport has been examined in some detail together with the possible involvement of some known mitochondrial components. In particular, a model has been introduced in which local charge imbalances produced by H+ fluxes serve as the driving force of transport. The molecules of the complex are arranged in series in a tripartite arrangement including a filter or gate, a nonselective channel and an H+-transferring portion linked to either electron transport or the ATPase. Parts of this model have been introduced by other investigators. Models in which different portions of channels have differing functions have been proposed previously for other transport systems.

Transglutaminase-catalysed incorporation of putrescine into denatured cytochrome. Preparation of a mono-substituted derivative reactive with cytochrome c oxidase

Guinea pig liver transglutaminase has been used to incorporate putrescine into horse heart cytochrome c. The native protein showed essentially no incorporation, while ethanol-denatured cytochrome c incorporated almost 1 mol putrescine per mol protein. No increase in this level of modification was obtained when maleylated cytochrome c and the tryptic peptides of cytochrome c were used as substrates. Analysis of the modified ethanol-denatured cytochrome c by tryptic cleavage and peptide isolation showed that glutamine-42 of the intact protein is the site of incorporation of radioactively labelled putrescine. Ethanol-denatured cytochrome c that was specifically modified at glutamine-42 by incorporated of putrescine could be readily renatured. The renatured modified protein showed reactivity with cytochrome c oxidase comparable to that of the original native protein.