A new peer-reviewed paper published by carbohydrate researcher Russell Carlson at the University of Georgia has been published in the journal BioCosmos, which explains the high informational content and complexity of glycans, and why this is best explained on the hypothesis of intelligent design rather than unguided evolutionary processes.¹

Indeed, the structural diversity exhibited by glycans is immense — due to variations in the composition of monosaccharides, linkage positions, anomeric configurations (i.e., the arrangement of atoms around the anomeric carbon), and branching patterns. Glycopatterns vary across different cell types and stages of development.

The Complexity of Glycan Structures

Glycans significantly enhance the functional and structural complexity of proteins. Many posttranslational modifications of proteins involve glycan attachment. There are roughly 3,000 glycans that can attach to proteins. Carlson calculates that, if a protein has only three glycosylation sites (many proteins have many more than this), each of which “can be glycosylated by any one of three different glycan structures…and the protein can have one, two, or all three sites occupied by a glycan,” the potential number of glycoforms would be 63. If we increase the number of glycan structures that may glycosylate each site from three to 3,000, the number of possible glycoforms for the protein increases to more than 27 billion. This potential complexity is, of course, far greater for proteins with more glycosylation sites than the three assumed by this calculation.

Writing and Reading Glycan Structures

Carlson also discusses how glycan structures are “written” and “read.” The process of attaching glycans to proteins is carried out by glycosyltransferases, in addition to other enzymes within the endoplasmic reticulum and Golgi apparatus. N-linked glycosylation takes place at asparagine (N) residues in the consensus sequence Asn-X-Ser/Thr (where X is any amino acid other than proline). The process begins in the endoplasmic reticulum, and involves the transfer of a preassembled oligosaccharide to the protein. Further processing of the glycan structure occurs in the Golgi apparatus by trimming and modification. O-linked glycosylation, by contrast, takes place on serine or threonine residues, the glycan being built one sugar at a time, generally beginning with N-acetylgalactosamine (GalNAc).

How do cells “read,” or interpret, glycan structures? Key to this process are glycan-binding proteins. Lectins recognize glycan motifs and mediate cell adhesion, signaling, and immune responses. Glycans can also be dynamically modified by glycosidases and glycosyltransferases, allowing protein function to be changed over time.

Recognition of Self

Carlson also discusses the role of glycans as markers of tissue identity. He notes,

In addition to their role in fine-tuning the activity of individual proteins, glycans on the surface of cells serve as markers of tissue identity; as determinates of self vs. non-self, i.e. “me or not me.” This is, of course, important in order for a kidney cell to be and function as a kidney cell, a nerve cell to function as a nerve cell and so forth. It is also important in order for the immune system to recognize potential pathogens, i.e. nonself or “not me”; and mount an appropriate defense response. This process is accomplished by cell surface glycans forming tissue-specific patterns for individual proteins (tissue-specific glycotypes), as well as a ‘global’ cellular cell-type pattern (a glycopattern) comprised of the combination of glycotypes of cell surface glycoproteins, glycolipids, and GAGs.

Glycan Structural/Functional Patterns Are Determined by Regulatory Networks

Carlson also discusses the role of regulatory networks in determining glycan structural / functional patterns. As he explains, “The specific patterns are determined by the expression of various glycogenes at the correct time and at the right location. That is, it is the spatiotemporal regulation of the GTs and GHs, etc., that ultimately dictate the glycopatterns.” Furthermore,

Transcriptional regulation of glycogene expression is accomplished by gene regulatory networks (GRNs). These networks are composed of transcription factors (TFs) that work together with non-protein coding regions of the DNA called cis-regulatory modules (CRMs), and these networks can interact with one another to form interactomes that dictate the dynamic and precise glycan structures necessary for development. Developmental gene regulatory networks (dGRNs) are those that determine an organism’s body plan, and since glycans are essential for tissue formation and function, it is likely that glycosyl gene regulatory networks (gGRNs) are part of the dGRN family. As depicted in Figure 10, gGRNs involve TF factors interacting with CRMs of various glycogenes.

Carlson also discusses how “Glycan regulation also occurs at the translational level via the action of micro RNA (miRNA) networks.” He notes,

MiRNAs largely repress translation of mRNA via promoting mRNA degradation or by binding and preventing translation; however, recent work in the case of two sialyltransferases also showed that miRNA can also upregulate their activity. One miRNA can affect the mRNA transcripts of several glycogenes, while another miRNA can affect transcripts of a second set of glycogenes and these two sets can overlap so that several, but not all, glycogene transcripts may be common to both. Additionally, miRNAs can vary the level at which different glycogene transcripts are affected, thereby not only determining the presence or absence of a GT, but also the level of its activity. MiRNAs could also impact regulation at the transcriptional level by affecting the translation of glycogene TF mRNAs.

He also briefly discusses the involvement of epigenetic factors in regulating these networks — for instance, by altering expression of genes coding for glycogene transcription factors.

Implications for Design

The complexity of glycan structures obviously greatly increases the informational potential of the cell, and this carries implications for the plausibility of an unguided search generating these complex and information-rich patterns. Carlson offers three take-home points from his review paper:

1. “[C]arbohydrate structural complexity is due to the chemistry of sugars which allows many branching possibilities and, therefore an oligosaccharide can form many more possible structures than an oligopeptide or oligonucleotide.” The implication of this is that “carbohydrates have much more potential information than either proteins or nucleic acids.”
2. “[T]he immense potential structural complexity of carbohydrates gives the cell a great deal of flexibility to both fine-tune and increase the functional range of a protein’s activity…and form the pattern of the cell’s glycocalyx required for development of an organism, the function and maintenance of a cell type, as well as enabling the cell to adapt to changing environments.”
3. Glycan evolution is quite complex — for example, “numerous GTs [glycosyltransferases] involved in protein N-glycosylation are thought to be unique eukaryotic innovations, which others have multiple components in which only the functional domain of one component has similarity to a domain of similar function in archaea or bacteria.”

In every other realm of experience, we habitually associate complex information-rich systems with intelligent causes. Stochastic processes are unable to generate such systems by trial-and-error, since the search space is so large that it exhausts the available probabilistic resources. Carlson suggests the possibility that the best explanation of these information-rich structures is a “purposeful process” involving “an ‘agency’ of mind or consciousness as suggested by intelligent design advocates.” He concludes, “That the evidence of dGRNs, which include those involved in producing functional glycan structures, supports a systems level purposeful, goal-oriented evolutionary mechanism and that, given experience shows that such processes invariably are a product of mind, intelligent design is a reasonable explanation of their origin.”

Notes

1. Carlson, Russell W., “The Complexity of Glycan Structures, Functions, and Origins,” BioCosmos: New perspectives on the origin and evolution of life, vol. 4, no. 1, Sciendo, 2024, pp. 57-78. https://doi.org/10.2478/biocosmos-2024-0005