The acute phase protein alpha1-acid glycoprotein: a model for altered glycosylation during diseases

Glycosylation is one of the most important post-translational modifications of proteins, and has been widely acknowledged as one of the most important ways to modulate both protein function and lifespan. The acute phase proteins are a major group of serum proteins whose concentration is altered during various pathophysiological conditions. The aim of this paper is to review the structure and functions of the alpha1-acid glycoprotein (AGP). AGP belongs to the subfamily of immunocalins, a group of binding proteins that also have immunomodulatory functions. One of the most interesting features of AGP is that its glycosylation microheterogeneity can be modified during diseases. This aspect is particularly remarkable, since both the immunomodulatory and the binding properties of AGP strongly depend on its carbohydrate composition. For these reasons, AGP can be considered an outstanding model for the study of glycan pattern modification during diseases. This review is focused on the most recent studies on the occurrence of different glycoforms in plasma and tissues and how the appearance of different oligosaccharide patterns during systemic inflammation or diseases can influence AGP's biological functions. The first part of the review will describe the structure of AGP and the several biological functions identified so far for this protein. The second part will be devoted to the post-translational modifications of the oligosaccharides micro-heterogeneity of AGP caused by pathological states. A critical evaluation of the impact of different AGP glycoforms on both its transport and anti-inflammatory features, and how the modifications of the glycan pattern can be utilized in clinical biochemistry, is also discussed.