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My reference book (Princeton Review for SAT Chemistry Subject Test) mentions that:

Proteins and carbohydrates are both polymers; however, only carbohydrates commonly form branched polymers. Glycogen and cellulose are both carbohydrate polymers made up of glucose monomers; glycogen is a highly branched polymer while cellulose is primarily straight-chained.

But I don't get why proteins cannot form branched polymers. Proteins have both an amino group and a carboxylic acid group, and I understand that the two groups join together with the elimination of a water molecule during polymerization.

Branched polymers are formed when a hydrogen atom (or any other substituent) is replaced by another monomer unit, and I don't see why this can't take place in the case of proteins. Could someone please explain?

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  • $\begingroup$ Hmm how you imagine branching there? BTW there's lots of posttranslational modification in there, so your premise is a bit flawed. $\endgroup$ – Mithoron Oct 25 '20 at 13:49
  • $\begingroup$ If we have an amino acid (a monomer group) with say 2 amino groups and a carboxylic acid, then there can be branches right... $\endgroup$ – Buckbitter Inning Oct 26 '20 at 7:25
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Note that branching is not unheard of, for instance glutathione is an abundant short branched peptide formed by condensation of the carboxylic group on a glutamic acid side chain to the main chain amino group of a cysteine aminoacid. This requires a dedicated enzyme both for synthesis (glutamate cysteine ligase) and degradation of the gamma peptide bond. Such gamma peptide bonds could serve as the basis for branching but afaik occur only in special cases.

An important example of branching also occurs during ubiquitylation, in which ubiquitin is attached to a protein's C-terminus via lysine side chains to target the protein for degradation. A variety of polyubiquitin chain arrangements (including multiple branching) have been identified.

I suppose one can only speculate from its absence (or non-ubiquity) that branching either (1) does not generally confer a significant evolutionary advantage or that (2) there are evolutionary barriers that have impeded development of advantageous biochemical machinery for generation and regulation of branching.

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Proteins are built from monomers with only two linking groups

The monomers that make up proteins join in very specific ways. The general structure is:

amino acid core

where R can be a variety of possible groups. To form a protein the monomers join when the amino group reacts with the carboxylic acid group. this gives a product with a specific orientation and which still contains one free amino group and one free carboxylic acid. Another monomer can react at either end but, inevitably, the core chain of the backbone is linear (at least in the sense that no branches are possible).

Glucose, on the other hand, has several groups that can react in condensation reactions:

glucose

so, depending on which groups react, there is the possibility for branches to form in the core chain of the polymers that result.

It is worth noting that a limited from of other links can form with proteins, though these would not normally be called "branches" as they don't affect the core chain making up the protein. These can occur when the R group in the amino acid has some sulfur groups which can form S-S cross links by reacting with other amino acids elsewhere in the protein chain.

But the basic idea explaining why proteins don't form branches in their core backbone is that, the monomers have only two sites where polymer links can form. Branched chain polymers need each monomers to have more than two sites for forming polymer links.

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    $\begingroup$ Theoretically, there could be amidic bridges via basic and acidic aminoacids like lysine and glutamic acid. But it would be rather by means of organic chemistry than biochemistry, as AFAIK enzymes are not focused this way. Ribosomal synthetic factories use a linear way, but I cannot exclude such option at post synthesis step. $\endgroup$ – Poutnik Oct 26 '20 at 6:57
  • $\begingroup$ What about proteins that have more than one amino group, like L-tryptophan or arginine? Wont this provide more than two sites for forming polymer links? I am a high school student, so pardon me if im missing something obvious. $\endgroup$ – Buckbitter Inning Oct 26 '20 at 7:20
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    $\begingroup$ @WolframtheTungsten Yes, there are some amino acids where the R-group has a carboxylic acid or an amino group. But these are nowhere near the backbone. In principle, if the reactions were random, branching might be possible. But, in living systems, proteins are not assembled randomly but by precision biological machinery which controls which monomer is added next and this machinery is designed around the backbone of an amino acid and will completely ignore the side groups. $\endgroup$ – matt_black Oct 26 '20 at 10:07

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