Stereochemistry of Molecules with Three or More Asymmetric Carbons - Chemistry LibreTexts
For example, D-glyceraldehyde and L-glyceraldehyde (HERE) are mirror images of each Thus, for glucose, there are 16 possible stereoisomers. Figures and show both glucose and fructose in linear and circular projections. A ketose signifies that the sugar contains a ketone functional group. An example of an enantiomer is the D and L isomers of glucose, as shown by the figure to the have eight stereoisomers, and aldohexoses have sixteen stereoisomers. For carbohydrate derivatives, amino sugar is of amine group in place of hydroxyl group; . Overall the sugar has many stereochemistry such as enantiomers (D and L), Diasteromers, ***Deoxyribose (component of DNA)-The principal difference between RNA and DNA is .. C6H12O6 (a hexose) has 16 stereoisomers.
It happens to be a quick way of referring to enantiomers.Carbohydrates - Epimers, common names - Chemical processes - MCAT - Khan Academy
The enantiomer of L-glucose is D-glucose. The enantiomer of L-tryptophan is D-tryptophan. Plus, L- and D- refer specifically to absolute configuration, while as we noted previously there is no simple relationship between the sign of optical rotation and configuration. It turns out that most naturally occurring sugars are D- and most naturally occurring amino acids are L. Note It bears repeating: For other molecules, you can largely forget about it. So Fischer developed his own nomenclature.
Why is this so important? That might not be the clearest analogy. Four Carbon Aldehyde Sugars Aldotetroses Once the absolute configurations of L- and D- glyceraldehyde were proposed, the absolute configurations of other chiral compounds could then be established by analogy and a lot of chemical grunt work.
There are two four-carbon aldoses, throse and erythrose.
They each have two chiral centers. Each exist as a pair of enantiomers L- and D- giving four stereoisomers in total. See how L-Erythrose and L-Threose build on the stereocenter established in L-glyceraldehyde highlightedand D-Erythrose and D-Threose build on the stereocenter established in D-glyceraldehyde highlighted.
Well, you can count. There are one-- So, one, two, three, four. Now, that's not numbering it. I'm just counting the chiral centers, because this carbon up here, the carbonyl carbon, is double-bonded to an oxygen, so it's not a chiral center.
So, all of these aldohexoses have four chiral centers, and that means they have two to the four, or 16 stereoisomers. Now, half of those are going to have to have this OH at the bottom on the right side, and the other half would be left.
So, half of 16 is eight, and that's how we get to this idea that there are eight D-aldohexoses. And that's just kind of a thought that you can use, and you can translate that into pentoses.
Pentoses are going to have three chiral centers. So, there's going to be ultimately eight, and there would be four D and four L. So, the last thing I want to do is cover just the common names for maybe the five most commonly seen monosaccharides, and I've similarly kind of pre-drawn their structures in, and I'll give you their names and kind of the mnemonics that I was taught to remember them by. So, the first one that we have right here, number one, is ribose, so ribose, and the way I remember this, this is a pentose.
It's got an aldehyde and five carbons. So, it's an aldopentose, and all of the substituents, all of these hydroxyl groups are on the right side. So, I remember that ribose is all right. Now the next one we have, hopefully you can see here, because we've drawn it a couple different times, is glucose, and I should mention that this is D-glucose again, and I should mention that this is D-ribose here.
But the way that I remember glucose is actually a little bit racy, so keep in mind that I do not support flipping people off with your middle finger, but if you look at this, man, it sure does resemble somebody flipping off people. So, you can say, I don't know, whatever insults you want to glucose.
I'll just kind of like write some kind of expletive marks here to glucose. And you can remember that glucose, we'll just pretend that we're really frustrated with it, and we're kind of cursing it out.
And again, I don't condone you using your middle finger, but thank goodness that organic chemistry can redeem even the most heinous of societal insults. And so, we can remember that D-glucose looks like, if we're holding, if kind of this is our pointer finger, and you can curl your finger up and kind of stick your middle finger out with the fingernail down towards the page, and I'm sure you can make the connection of how your fingers resemble glucose.
And so, that's kind of my mnemonic for that. This next one is mannose, and again, it's D-mannose, and if you position your fingers in the same way that you were with glucose, and now you just extend your pointer finger as well. So, now we've got kind of two fingers extended and then two kind of curled up.
We can see that it's like a man holding his gun. Dolichol phosphate is a lipid molecule found in the ER lumen and is made of about twenty isoprene units.
The terminal phosphate group of dolical phosphate is the site of attachment of the oligosaccharide. With the help oligosaccharide-protein transferase, the oligosaccharide is transferred from dolichol phosphate to the asparagine molecule.
Kevin Ahern's Biochemistry (BB /) at Oregon State University
Proteins from the lumen of the ER and the ER membrane are then transferred to the Golgi complex, where the carbohydrate part of the glycoprotein is altered. Since the Golgi has three areas, each with its own set of enzymes, modifications to the precursor oligosaccharide allows for a range of oligosaccaride structures to form.
After the Golgi complex, proteins proceed to either lysosomes, secretory granules, or the plasma membrane, depending on the signals embedded within the amino acid sequences and the three-dimensional structures. Examples Erythropoietin EPO is a glycoprotein hormone that stimulate the production of red blood cells. The presence of three Asn residues and one Ser residues allow oligosaccharides to link the protein at the three N-linked glycosylation and one O-linked glycosylation sites.
It is secreted by the kidney.
- D and L Sugars
- Considering the chain structure of glucose:
- 6.9: Stereochemistry of Molecules with Three or More Asymmetric Carbons
Zona Pellucida The zona pellucida is a glycoprotein membrane, where it appears at multilaminar primary oocytes around the plasma membrane. The zona pellucida structures must initiate the acrosome reaction, in order to binds with the spermatozoa. Therefore, scientists found four zona pellucidas that are responsible binding the spermatozoa and the acrosome reaction within the mouse. The most important zona glycoprotein is the ZP3, because ZP3 is responsible for sperm binding.
The sperm protein is adhering with the plasma membrane of the oocyte. In addition, the ZP3 is involved with the acrosomal reaction; this lead to the releasing the spermatozoon of the acrosomal vesicle.
The ZP2 is responsible of mediating the subsequent of the sperm binding. The ZP4 is the protein that human encodes the genes. For humans, it takes five days after fertilization that the zona hatching was performed by the blastocyst. On the other hand, the zona pellucida is being replaced by the layer of trophoblastic cells, when zona pellucida is decomposes and degenerate.
Overall, the zona pellucida is has a great importance on the egg death and began the fertilization. Sequence Oligossacrides Oligosaccharides can be sequenced by enzymatic analysis and mass spectroscopy.
It is hard to know the structure of sugars so remove sugar from glycoprotein. You will use enzyme and mass spectroscopy to find out the order of these sugars that are attached. Glycosylation Errors Carbohydrate attachment to proteins is important for processing, stability, and targeting these proteins. Improper glycosylation of proteins can lead to inheritable human diseases called congenital disorders of glycosylation.
An example involves I-cell disease. I-cell disease is a lysosomal storage disease. A carbohydrate marker is used for directing degradative enzymes. The lysosomes of people with I-cell disease have large inclusions of undigested glycosaminoglycans. These inclusions are present because the lysosomes of I-cell patients lack the enzyme to degrade them. However, these enzymes are present in high volumes elsewhere in the body, thus indicating incorrectly delivered enzymes in I-cell patients.
Carbohydrate-binding proteins[ edit ] O-linkage It has been shown that carbohydrate-protein complexes function in cell-cell recognition processes as well as adhesion of cells to neighboring cells and the extracellular matrix. The diverse carbohydrate structures displayed on cell surfaces are well suited to serve as interaction sites between cells and their environments. A glycoprotein is formed when a carbohydrate group attaches to a protein through a covalent bond.
These glycosidic bonds link carbohydrates to the amino and hydroxy side chains of asparagine and serine or threonine, respectively. An N-linkage is the bond between a carbohydrate and the nitrogen in the asparagine side chain, and an O-linkage is the bond between a carbohydrate and the oxygen of serine or threonine. An asparagine residue can accept an oligosaccharide only if the residue is part of an Asn-X-Ser or Asn-X-Thr sequence, in which X can be any amino acid, except proline.
Thus, potential glycosylation sites can be detected in a proteins primary structure.
Not all potential sites are glycosylated, however. Glocosylated sites depend on protein structure within the region and the cell type in which the protein is expressed. All N-linked oligosaccharides have in commmon a pentasaccharide core consisting of three mannose and two N-acetylglucosamine residues. Glycoproteins play several roles in terms of the medical world. Modified carbohydrates have the ability to interfere with the interactions between carbohydrates and proteins.
This leads to the inhibition of the cell—cell recognition and adhesion that is a major factor contributing to cancerous growth. Thus, these the ligands of the carbohydrate-binding proteins could potentially evolve into new forms of cancer treatment.
There has been research on the development of protein serum-based cancer diagnostics.