The Complexity of Glycan Structures, Functions, and Origins

Sugars have more reactive groups than either amino acids or nucleotides. This allows them to form branched oligo- and polysaccharides. Therefore, the number of possible structures is far greater than comparably sized peptides or nucleic acids. The result is that carbohydrates have far greater information-carrying potential than either proteins or nucleic acids. Carbohydrates are present in, and essential for, all three of life’s kingdom’s; archaea, bacteria, and eukaryotes. This article illustrates the structural, functional, and evolutionary complexity of animal carbohydrates, i.e. glycans. These glycans include glycosaminoglycans, glycolipids, and glycoproteins. Attachment of glycans to proteins increases their functional range and allows their functions to be fine-tuned in response to the environment. Different copies of a single protein have different glycans attached. Each copy is a glycoform and the combination of a protein’s glycoforms is called its glycotype. Protein glycotypes are cell-, tissue- and developmentally-specific. The combination glycoproteins, glycosaminoglycans, and glycolipids form glycopatterns that are essential for organization of cells into tissues during development. The synthesis of a single glycan structure requires numerous enzymes, and the production of the necessary functional protein glycotypes and cellular glycopatterns requires integrated regulated expression of these enzymes by (glyco)gene regulatory networks (gGRNs) and micro RNA (miRNA) regulatory networks. Explanations for the origin of glycans must include mechanisms for the origin of glycosyl transferases, hydrolases, metabolic pathways for the activated sugar precursors, and all the components and information necessary to form the aforementioned regulatory networks. The challenges that this poses for a standard neo-Darwinian explanation are discussed.