Post-Translational Modifications of Cytochrome c in Cell Life and Disease

Abstract

Mitochondria are the powerhouses of the cell, whilst their malfunction is related to several human pathologies, including neurodegenerative diseases, cardiovascular diseases, and various types of cancer. In mitochondrial metabolism, cytochrome c is a small soluble heme protein that acts as an essential redox carrier in the respiratory electron transport chain. However, cytochrome c is likewise an essential protein in the cytoplasm acting as an activator of programmed cell death. Such a dual role of cytochrome c in cell life and death is indeed fine-regulated by a wide variety of protein post-translational modifications. In this work, we show how these modifications can alter cytochrome c structure and functionality, thus emerging as a control mechanism of cell metabolism but also as a key element in development and prevention of pathologies.

Keywords: cytochrome c; mitochondrial diseases; post-translational modifications.

Figure 1

Cell localization and functions of cytochrome c. Cc is located in the mitochondrial intermembrane space, vacuole and zymogen granules under homeostasis. However, during DNA damage and apoptosis stimuli, mitochondrial Cc travels into the nucleus, the rough endoplasmic reticulum (RER) and the cytoplasm, respectively. The most relevant functions performed by Cc in each location are explained in the boxes. Color key: blue, physiological functions; red, functions performed under stress; and black, unknown functions. Created with BioRender.com (https://biorender.com/).

Figure 2

Cytochrome c sequence evolutionary conservation and post-translational modifications. (a) Evolutionary analysis by Maximum Likelihood method. The evolutionary history was inferred by using the Maximum Likelihood method and General Reversible Mitochondrial + Freq. model [158]. The bootstrap consensus tree inferred from 500 replicates [159] represents the evolutionary history of the taxa analyzed [160]. Branches corresponding to partitions reproduced in less than 50% bootstrap replicates are collapsed. The percentage of replicate trees in which the associated taxa clustered together in the bootstrap test (500 replicates) are shown next to the branches [160]. Initial tree(s) for the heuristic search were obtained automatically by applying Neighbor-Join and BioNJ algorithms to a matrix of pairwise distances estimated using the JTT model, and then selecting the topology with superior log likelihood value. This analysis involved 31 amino acid sequences. There was a total of 122 positions in the final dataset. Evolutionary analyses were conducted in MEGA X [161]. Labels are colored as a function of the number of amino acid positions that can undergo post-translational modifications (PTMs). (b) Sequence alignment and amino acid evolutionary conservation of Cc sequences. Residues are colored according to ConSurf conservation score [162]. Amino acids that can be modified are indicated with (#). (c) Ribbon representation of human Cc structure (PDB ID: 2N9I [163]). Left, residues are colored according to conservation score. Side chain of amino acids that undergo PTMs are labelled. Right, the different foldon units are colored as follows: foldon I in blue, foldon II in green, foldon III in yellow, foldon IV in red, and foldon V in purple.

Figure 3

Cause–effect of post-translational modifications of cytochrome c in human pathologies. Carbonylation, glycation and nitration of specific residues of Cc promote the development of several neurodegenerative diseases and strokes. However, phosphorylated Cc at positions 47 and 97, or nitrosylated in its tyrosine residues has been revealed to be a neuroprotective agent against this type of pathology. Cancer cells show many modifications of heme protein, such as phosphorylations, acetylations, homocysteinilations, and nitrations. Nitrosylation and phosphorylation are mutually exclusive modifications, so the presence of nitrosylated Cc species could be act as an anticancer agent. Notably, homocysteinilation is also related to cardiovascular diseases. Finally, it has been described that peroxynitrite ion and nitrated proteins (including Cc) trigger inflammatory processes. The green wheels represent Cc modifications that favor the development of diseases (see the direction of rotation of the gear indicated by the green arrows), white wheels represent the pathologies—gray arrows represent the direction of rotation that produces the development of the disease—and the red wedges indicate PTMs that impede them.