Peptidoglycans – Complex Carbohydrates

by Kevin Ahern, PhD

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    00:01 there's no link to the endoplasmic reticulum.

    00:01 Another interesting group of modified saccharides are the peptidoglycans. Now peptidoglycans, as its name suggests are long polymers of carbohydrate residues and like we've been seeing, they're modified sugars and they're presents with a repeating disaccharide.

    00:20 These components are linked to short peptides, in the case of the bacterial cell wall as I will show in a second. The peptides that they're linked to only contain four amino acids.

    00:31 You can see the linkage down here beneath the green for the N-acetylmuramic acid. This is one of the modified forms of the repeating disaccharide. The other repeating disaccharide is N-acetylglucosamine, a molecule that we've seen before. Now the interesting thing about this linkage is that when we lay it out, the linkage makes a mesh or a network that looks like you see on the screen. You can see that there's long polymers running diagonally from the upper left to the lower right, those polymers and those diagonals contain the repeating glycan structure of N-acetylglucosamine and N-acetylmuramic acid as seen. Perpendicular to those are the tetrapeptides that are interacting with each other as you can see. Now those tetrapeptides are really interesting for two reasons; one is that the tetrapeptides contain within them at least one amino acid in the D configuration, this almost never happens in nature and this almost never happens for accidental purposes. And the reason that's important here is this is an example of where bacteria have used chemistry to their advantage.

    01:46 Proteases, that is enzymes that break down proteins, are enzymes that would break down peptides such as found here will not work on D amino acids; they are designed to work on L. So this is a defense mechanism bacteria have evolved over the years to protect their cell walls from being degraded.

    02:06 Well bacteria don't get the last word in this overall scheme though, and the reason that's the case is the antibiotic penicillin. Penicillin is killing to bacteria for a very important reason, it stops the bacterial cell wall from being assembled as you see here. Bacteria that can't make a cell wall can't divide, and bacteria that can't divide are doomed to die.

    About the Lecture

    The lecture Peptidoglycans – Complex Carbohydrates by Kevin Ahern, PhD is from the course Biochemistry: Basics.

    Included Quiz Questions

    1. Bacterial cell walls
    2. Nerve cells
    3. Plants
    4. Connective tissue
    5. Mitochondrial membranes

    Author of lecture Peptidoglycans – Complex Carbohydrates

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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