Structural changes of proteins in liver cirrhosis and consequential changes in their function

The liver is the site of synthesis of the majority of circulating proteins. Besides initial polypeptide synthesis, sophisticated machinery is involved in the further processing of proteins by removing parts of them and/or adding functional groups and small molecules tailoring the final molecule to suit its physiological purpose. Posttranslational modifications (PTMs) design a network of molecules with the common protein ancestor but with slightly or considerably varying activity/localization/purpose. PTMs can change under pathological conditions, giving rise to aberrant or overmodified proteins. Undesired changes in the structure of proteins most often accompany undesired changes in their function, such as reduced activity or the appearance of new effects. Proper protein processing is essential for the reactions in living beings and crucial for the overall quality control. Modifications that occur on proteins synthesized in the liver whose PTMs are cirrhosis-related are oxidation, nitration, glycosylation, acetylation, and ubiquitination. Some of them predominantly affect proteins that remain in liver cells, whereas others predominantly occur on proteins that leave the liver or originate from other tissues and perform their function in the circulation. Altered PTMs of certain proteins are potential candidates as biomarkers of liver-related diseases, including cirrhosis. This review will focus on PTMs on proteins whose structural changes in cirrhosis exert or are suspected to exert the most serious functional consequences.