I am so grateful you did this video. I've had a hard time with this in college and while studying for the MCAT. I now know and understand sugar structures. Thank you so much!
That's true! I have a whole series of chemistry basics on deck, including solubility foundations. I also cover the solubility and acid-base chemistry in depth in my course as they are essential MCAT topics to know for test day!
If a sugar molecule converts from an alpha sugar to a beta sugar (mutarotation you were talking about), will just C1 change orientation, or will the whole molecule invert? Thank you!!!
Great question! It will be just C1 - the rest of the molecule will stay as is. So if you convert from a-D-glucose to b-D-glucose, the orientation around C2 - C6 will all stay the same! HOWEVER! If you convert from D-glucose to L-glucose, that is a full enantiomer isomerization, which means all chiral centers will be flipped.
Can you explain why sucrose is still a 1-4 glycosidic bond instead of 1-5 or 1-2 ? Does the numbering of the fructose carbons change once it loses its free -OH group or am I just numbering wrong ? Also is sucrose neither alpha or beta since it only has the one anomeric carbon? Your videos have been the most helpful thing ever thank you SO much !!!!!!
Such a good question - sucrose is our funky dude and so breaks all the rules, and that was an error in my description in the video! Technically, sucrose as table sugar (which is what we would typically see) is a funky alpha-beta 1-2 bond, where both anomeric carbons are involved in the bond. This is why sucrose is a non-reducing sugar - we don't have an anomeric oxygen to reduce. The reason this gets tricky in drawings is that when sucrose is formed, fructose essentially does a "180" from how we normally draw it, with carbon 2 facing right. That's why it's easy to view it as a 1-4 bond, since we don't usually visualize our sugars "flipping" around. This is on the border of out-of-scope for the MCAT; they tend to focus on maltose and glycogen bonds since that's most directly related to metabolism, and they'll usually test sucrose as a non-reducing sugar. However, this was a great point to clarify, thank you so much for the question! Here's also a good reference sheet for the sugars and dissaccharides: basicmedicalkey.com/carbohydrates-3/
This is actually a cool clinical application! There's an enzyme, beta-galactosidase (aka lactase), that specifically breaks the beta bonds in lactose. Cellulose is a huge polymer that requires cellulase, not something that has evolved in most mammals. But lactose is in mammalian milk, so the theory is that we have evolved beta-galactosidase production while early in life, and that the levels drop off as we get older. This is the major reason for lactose intolerance - not enough lactase to break down lactose!
