Isoelectric Point of Amino Acids with MCAT Shortcut
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- เผยแพร่เมื่อ 20 ก.ย. 2024
- leah4sci.com/a... presents: Isoelectric Point of Amino Acids detailed tutorial with time-saving MCAT Shortcut
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This is video 6 in the MCAT amino acids tutorial video series. Learn how to quickly find the isoelectric point of any given amino acid as well as a time-saving shortcut for amino acids with 3 pKa values. Also learn how to verify your results quickly.
Note: The statement made at 12:39 holds true for all of the amino acids mentioned EXCEPT for tyrosine. Because the pKa of tyrosine's side chain is greater than the pKa of both the carboxy and amine group, you will use the pKa of the carboxy and amine (NOT that of the side chain) to determine its isoelectric point.
Referenced in this video:
MCAT Math series: leah4sci.com/m...
Catch this entire video series along with my amino acid cheat sheet, tutorials and practice quiz on my website:
leah4sci.com/a...
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The concept of pKa and pH "fighting" for the proton clarifies this concept for me SO much. Now I don't need to memorize this, I UNDERSTAND it! Thank you Leah!
Glad I could help!
My goodness, you have such a gift for teaching complicated material in such a way that us "less" genius folk can really grasp it! Thank you so much for sharing your gift!
You're very welcome, and just keep working on it. It took me a while to understand some of these concepts.
You are simply amazing. I never really thought that I would grasp the concepts after reading my textbooks, but your videos have proved me wrong. Your explanation in simple terms is just great.
Awesome! So glad you understand it now!
Your MCAT shortcuts are saving my life! Testing in 3 days and this is making things so much clearer than they have been during my whole study process.
I'll keep my fingers crossed for you! Glad I could help
How'd you do on the MCAT?
your teaching is awesome. clear. even for Korean. thanks a lot. this helped me so much.
Glad you liked it!
Shortcut: if it is an acidic aa, pI is the average of the pka of two carboxylic ends (acidic has negative charge) and if an aa is basic, the pI is the average of the two amine groups. For the rest of the aa, it is just a regular average between two numbers.
Don't forget the tricky ones though?
Thank you so much Leah, I'm an undergrad pre-vet student and in my biochemistry class I was so lost when the professor was doing these problems. I have a test Monday and now I understand this so well, thank you!!!
You're welcome! I'm glad I could help :)
been struggling with understanding PI for months. This was immensely helpful. Thank you!
You are so welcome, I'm glad to hear that this helped you so much!
best explanation and answering scheme ever! i wish i have known you earlier and now im rushing for my 2nd MCAT.. all the best for everyone who is running with me!
So happy to help! Regarding the MCAT, why are you rushing? I encourage you to let your full length practice scores guide you instead of the calendar so that you only test once you KNOW you are ready. Check out this for more: leah4sci.com/mcatready
Bless your soul!! I am a second year biochem student and this video was super helpful!!! Thank you
Glad you enjoyed it!
Jma3t l biochimie 😂😂
You just saved my biochemistry teste!!!! Thank you so much I was having troubles with this
You are very much welcome :)
My lifelong dream has been to be a doctor and I know I could do it. But your videos are really making me consider being a professor. You don't know how much of a relief it is to watch your videos and finally understand something that was confusing during a lecture. That ah hah moment when something makes complete sense is something that I would also like to do for other people. You are not only smart but you convey the material in a way that clicks with me. You are gifted, please don't stop making these videos.
Thank you so much! Whatever you choose to do, if it's your passion, go after it and don't give up!
Another high-recommendation comment. Clarity in simplicity; well-curated examples in efficient instruction.
Wow, thanks so much for your kind words!
I've been struggling in class!!! I haven't taken chemistry in 5 years. I am now in biochemistry and all I can say is thank you!!!! You're my new professor. You are precise, in depth, and relate all the information. Once again thank you and keep it up!!
Sorry do you mind posting the link to the following video please? I can't find it.
thank you. The next video will be uploaded shortly
Thankyou a lot......You came as a blessing......Was struggling with this topic for years .......U deserve more subscribes taking into account the content on ur channel.......A lot of useless channels have more subscribers than urs where they just blurt out the formulas without such beautiful explanations......Do not know why people arent accepting such amazing content......U have really good teaching skills......Please feel proud of yourself .......Just Amazing
Awww, thanks, I really appreciate your kind words. I'm glad to know my videos are helping you to understand!
Excellent simplification of a complicated topic. You are really gifted. Thank you for your beautiful explanation.
You're welcome, so glad it was helpful!
Easier way of thinking about it is if the amino acid is acidic, use the lower two. If it’s basic use the upper two.
Use whichever method helps you best!
I loveee your videos!! This is making studying for the mcat soo much easier! Thank you Leah! :D
awwww! Thank you and you're welcome :)
Hi Leah! Thank you so much for this video! Quick Question! Why do we know to deprotonate the hydrogen off of the carboxyl functional group rather than off the tyrosine (6:30 in the video). Thanks!!
The entire molecule is known as the amino acid tyrosine (not just the side chain). At this point in the video, we are determining the net charge of tyrosine between a pH of 2.2 and 9.1.
Since the pKa of the carboxyl group is 2.2, if we are above that value in pH, the carboxyl group will be deprotonated. Any pH above 2.2 will yield a deprotonated carboxyl group.
If still struggling on this topic, read my tutorial at leah4sci.com/amino-acid-charge-in-zwitterions-and-isoelectric-point-mcat-tutorial/
Nice, very well explained. Thanks for sharing this video.
You're so very welcome, glad you like it!
I purchased Magoosh MCAT to study and your videos have helped tenfold what their material has... The psych/soc material is excellent through Magoosh, but other than that, I wouldn't recommend.
I'm sorry to hear that hasn't worked out for you, but happy to hear my videos have helped you. Check out my website for even more resources: leah4sci.com
Definitely understand this way more now then when I learned it before. Amazing, thank you!
You are so welcome, happy to hear that it's helping you!
Seriously, you just explained it a whole lot better than my biochem teacher! Thanks, hope it helps on the test.
Glad to have helped you understand the topic better. You're welcome :)
You made it so simple!
Made my concepts clear .
Thank you
Glad I could help!
Thank you, I finally understand it!
You're very welcome! glad to help
This very educational...you are really good in explaining.
I am very happy for this, since this is a much more important subject than most Dairy technologists understand. I have used this many times to obtain better process results in my machine designs.
Happy to help!
For the tyrosine example, why is it -1 when the PH is 9.6? The ph is less than the pka for the R group, meaning we have to protonate it, giving it a charge of +1. The sum of +1 and negative one (from the carbonyl group) will be zero, not -1.
Thanks for your question! When the R group of tyrosine is protonated, it has a charge of zero. It exists as an -OH group off of the benzene ring. You will not see it exist at -OH2+. Therefore, at a pH of 9.6, the sum of 0 and -1 (from the carboxylate) gives a net charge of -1 for tyrosine.
Leah4sciMCAT thank you for your reply. I wasn’t aware of this rule. So if a NH3+ needs to be protonated, you keep it as a +1 charge. If you have to deprotonate NH3+, it become 0. If an OH group needs to be protonated, it will have a charge of 0 and if you have to deprotonate and OH group, it will have a charge of -1, correct? Also usually when finding pI, you start with the most protonated form of the amino acid, would that be used to see if the overall charge of the amino acid is a 0 or +1, or are you specifically looking at the charge of the side chain? Thank you
This video makes calculation of pi value really very easy.
So glad to hear my video helped you understand!
@16:50 Why are you adding a Hydrogen to the NH group if you're deprotonating it? The pH is greater than the pKa so it wins one for the solution and deprotonates the group so shouldn't the NH group become a N with a charge of -2?
Careful, every H has it’s own pKa value. When given pKa values for AAs you’re looking at the +H. To remove a neutral H (to get N-) you’d need another pKa value, for N-2 you’d need yet another. Both are well beyond the scope of our pH scale here
damm.... it's such a awesome video. Learned more than 10 hour biochem class !!!!!! thank you soooooo much!!!!!!!!!
You are very much welcome! Glad the video helped :)
wao thankyou so much...it really helped me to understand PI concept.👍
You are so very welcome!
Very clear and well broken down. Super easy to understand. Thanks!
You're very welcome :)
Thank you so much
Awesome video
You're very welcome!
This was extremely helpful! Thank you very much!
you're very welcome
I got an A in my orgo thanks to your orgo videos. I am sure that I will get an A in my biochem through your videos as well!
Yes, same here! Got a 99% and 97% both in Orgo 1 and Orgo 2 respectively only because of Leah. Hope to get an A as well in biochem. Good luck!
@kimbel Bradley great job! glad the videos helped. :). Best of luck on your Biochem exams! :).
@Gabe Nir awesome! Best of luck on your Biochem too. Hope you shared the channel to your friends ;)
@@gabenir5117 I am amazed by both of you.
You just simplified this to its simplest form. Thank you
You're very welcome!
I am grateful to you for this explanation😩❤️
You are so welcome!
All of your videos are wonderful. Thank you
You're welcome. So glad you like them!
your videos are always awesome. thank you
Glad you like them!
Thank very much. Very easy and simple to understand
You are very welcome!
Great video thanks 🤩
you are so very welcome!
Leah will the MCAT provide the pKa's? Or do we have to memorize them?
+Funksolid I recommend memorizing general values. Carboxy ~2, Amine ~10, Side chain carboxy ~4, HIS 6, basic ~11+
thank you for this video. it seriously helped me out.
You're welcome! :)
Such a genius. Thanks!
Awww thanks, you're so welcome!
Wow, clear explanation 👏
Glad it was helpful!
best isoelectric point vid I've watched (out of 4 others). Question though, why don't you include serine and threonine with their hydroxyl groups as part of the "if protonated it's neutral" group at 11:45 in the video? Thanks
good question. I chose to focus only on the amino acids that have pKa values significant enough to consider for the MCAT. Serine for example has an OH but is not highly ionizable due to lacking stability for the resulting negative charge. As such we just assume it's always protonated and neutral in regards to MCAT/biochemistry
When you are looking at the alcohol on the tyrosine side chain @ 11:17, why is it that the proton is being added to a deprotonated alcohol? Why not an already protonated alcohol?
the pKa refers to the ability to add/remove a proton to a specific form of the molecule. Tyrosine with an OH is very unlikely to accept a proton given that it is a very poor base. however, Tyrosine with an O- is likely to accept the proton. This works the same in reverse for donating the proton. O- has none to give, OH is likely to give it up.
OH MY GOD, my teacher didnt explain this, the book didnt explain this. THANK YOU SO MUCH.
You are so welcome!
Crazy easy! She is an awesome tutor !
Thank you for your kind words!
oh my goodness I can't believe I could understand this much easier...
Happy to help!
Thank you so much for these videos, Leah!! Your channel is a real blessing!! One question- why is the pKa on the amine group in tyrosine 9.1 instead of 9.6? I thought it was always the same on every amino acid being 9.6, so I've been using that for my calculations in these types of questions. But here you have it as 9.1, so is it different for every amino acid? If so, how do we know what it will be? Thanks so much again:))
Awww, thanks for your kind words!
Technically, the pKa of the amino group is slightly different for every amino acid. It’s fine to use 9.6 as an estimate for any one of them, when calculating pI. There’s no easy way to predict how the pKa of the amino will differ, based on the particular amino acid you’re looking at. You can find a chart of various values on my Amino Acids Practice quiz found here: leah4sci.com/amino-acids-practice
Your videos are amazing. Thank you!❤
You are so welcome!
You are awesome!!!!! Thank you so much!!
Do you only make videos for biochemistry?????
Mostly Organic Chemistry leah4sci.com/TH-cam and MCAT on this channel
thx for this video really helpfull for iit jee
You're very welcome
Thank you so much. You made it super simple
You're welcome :)
Extremely helpful. THANK YOU!
You're welcome! Glad the video helped.
Great video. Thanks for the help!
You're very welcome!
can you please put a video or message me on how you would find the pI of a polypetide that has 2 (R), 7 (k) ,1 (N) ,2 (H), 8 (E), 9 (D), 1 (C) ....i find your trick very very helpful when it comes to peptides that are like short chains of amino acids but how would i apply this to a polypeptide this long...
I'm sorry, but I don't offer tutoring through TH-cam comments. Try re-watching the video to really get the trick down. For more help with questions like this and more, I recommend joining the MCAT Study Hall. For more details visit join.mcatstudyhall.com/ or contact me through my website leah4sci.com/contact/
Hi Leah, you are awsome, thank you so much for this, just a small question, @ 12.39 you said for the upper 2 pKa values, and referring to the carboxyl group and the Side chain, is not the upper values 9,1 and 10,1 so it should be the amine group and the side chain?
We're looking at the 2 higher numbers. With basic amino acids that will be the alpha amino group and the side chain
@6:32 , how did you get the OH to have a net charge of 0? I figured that the charge would be +1 since the pH of 5 is less than 10.1 so there would be a protonation and thus would gain a +1 charge.
When looking at ionizable groups, take into account what happens when they get protonated/deprotonated.
You are correct that the OH will be protonated, but what happens when it IS protonated?
You get OH (not OH2+)
When deprotonated you get O-
Really basic question but how do you know whether to add a plus or hydrogen when things become pronated?
I'm sorry but I don't offer tutoring through TH-cam comments. For help with questions like this and more, I highly recommend joining the MCAT Study Hall. For more details visit join.mcatstudyhall.com/ or contact me through my website leah4sci.com/contact/
Hey Leah I am a bit confused on why Lysine @18:15 would have a charge of zero at the PI of 10. You mentioned at @12:22 that we look at zero for lower which referred to Asp, Glu, Cys, and Tyr, and we look at upper +1 which referred to His, Arg, and Lys. Please explain this to me because I am very confused. Thanks
The trick refers to identifying the charge when there are 3 pKa values. This is simply what you average to find the pI. However, the charge should be zero at the pI value given its definition. Or did I misunderstand your question?
it's really helpful to understand this!!
Glad it helped
At baseline, is the Amino group supposed to be NH3? At 16:40 I got a little confused about going from NH to NH2 with the lone pair after the solution won the H+. I just wanted to make sure I didn't miss a step! Thank you!
Thanks for asking! I wouldn’t refer to anything as a ‘baseline’. In this case, the amino group can only be considered protonated (NH3+) or deprotonated (NH2). At 16:40, don’t take any meaning from the fact that I have the amino written as NH. I simply was waiting to write in the number of hydrogen atoms, after my assessment of whether the group would be protonated or deprotonated.
Loved the way u taught
Thanks so much!
Thank you so much.
You're welcome!
Super helpful!
Thanks!
this was really useful, thanks!
You're welcome!
Lysine has an additional COO- that you didn't draw in your example? WHat affect does this have on
I don't think you finished your question. affect on what?
Awesome! Thank you!
You are very welcome!
Great! Thank you so much.
You are welcome!
Hi, this might sound like a very stupid question, but I'm just trying to self learn this whole subject; we havn't reached it in school yet:
Where do you get the pKa values from?
Thanks in advance!!
This series will help: leah4sci.com/mcat/mcat-chemistry/acid-base-ph-and-pka-calculations-in-mcat-chemistry/
LIFESAVER!
Glad to help!!!
great video helped me a lot
Glad it helped!
Excellent !
Glad you like it!
why are you giving the amine a hydrogen @16:55 when it's being deprotonated?
From NH3 to NH2...that is the deprotonation, she just wrote 2 later
Deprotonation refers to loss of the third acidic hydrogen, but there are still 2 hydrogens remaining (3 -> 2)
you are awesome!!! Go on plz never stop.
Awww, thanks!
For tyrosine why are the two lower pKa's being used? In my homework for biochem it says to use the higher two pKa's. I am very confused now, could you please explain.
The isoelectric point is the pH of the solution at which the net charge of the molecule is zero. Therefore, in order to calculate pI, we average the two pKa values that are just before and just after the pH range that gives us a net charge of zero. In this case, for tyrosine, that would be 2.2 and 9.1
Leah @ 5:10 you assume that tyrosine initially lacking hydrogen at the carboxyl why?
I started with a random structure and adjusted for the appropriate pH
Also, @ 15:14 you drew Lysine lacking H on the hydroxyl side and amine side? am I missing something here!
15:14 covers Asp
Do we use the structure of amino acid at pH = 7 to determine the IP?
No, not in all cases. The isoelectric point is defined as the pH at which the net charge of the molecule is zero. Not every amino acid has a net charge of zero at a pH of 7. The examples of aspartate and lysine in this video prove that.
can i clarify the something?
where did you get the pka value, or how can we solve this pka value?
thank u
Thanks for your question! The pKa values are constant for and specific to any given amino acid side chain. Some professors might have you memorize these values, but you can always find them in a table or appendix in the back of your textbook. It is important to remember the pKa of the free carboxylic acid group as 2.2 and the pKa of the free amino group as 9.1.
So for MCAT purposes: is it a good idea to memorize all the pKa's for amino acids or do they tend to be given in the passage/figures?
Diana Ramirez no, they are not given. Memorize the pka values for all basic and acidic amino acids. A good mnemonic: ST RKY CHED. Learn all the pka values of the amino acids of the mnemonic!
Yes and no. YES you want to memorize the pKa values, but not the exact number. Instead memorize an approximate value for each as I teach here leah4sci.com/amino-acid-charge-in-zwitterions-and-isoelectric-point-mcat-tutorial/
She makes me so happy
I'm glad I can make you happy :)
Thank you.
You're welcome
Thanks ma'am
You're welcome! :)
Thank u so much
You're welcome 😊
is there anyway to calculate pKa values for R groups, or do we have to just memorize them all
You have to memorize, however not the exact values. See the estimates I recommend in the video
isn't the R group for Aspartic acid negatively charged? I mean at physiological pH. explain please
It is! Watch the charge video in this series
+Leah4sciMCAT yeah you said it is negative but in this video you said it is neutral. anyways I got the idea tho. you r great
What point in the video? it sounds like you understand the concept which is the goal but if you point me to the error I can take a look
THANK YOU SO MUCH
You're very welcome
isn't Cysteine and Tyrosine neutral AAs?
Their side chains are neutral in charge, if that's what you're asking. For help with questions like this and more, I recommend joining the MCAT Study Hall. For more details, visit join.mcatstudyhall.com/ or contact me through my website leah4sci.com/contact/
You can also view my free Amino Acids series, practice quiz and cheat sheet at Leah4sci.com/AminoAcids
THANK YOU!
You're welcome! :)
Please send me your MCAT math cheat sheet, I requested but nothing has arrived. Thank you
I'm sorry to hear that! Please email at leah4sci.com/contact so my assistant can help you get your cheat sheets.
Leah! what does it mean to know the amino acid structures at ph 7.4? does this mean they are in zwitter ion form?
Thanks for your question! The physiological pH of the human body is known to be ~7.4. It’s important to know the structures at this pH, simply because of its relevance to human body chemistry. Zwitter ions contain an equal number of positively and negatively charged groups, meaning they are overall electrically neutral. Because of the pKa values of their side chains, not all amino acids are neutral zwitter ions at physiologic pH.
isnt aspartic acid positively chaged?
Great question! The net charge of aspartic acid (or of any amino acid with ionizable groups) changes with the pH of the solution. This is because each of the ionizable groups on the molecule has a different pKa value and can exist in either its protonated or deprotonated form depending on the pH.
Thank you so much ❤❤ ..ur the best :)
You're very welcome!
If the pH of a sol'n is the the same as a pka of a group, is that group protonated or deprotonated?
I'm sorry, but I don't offer tutoring through TH-cam comments. For help with this and more, I recommend joining the MCAT study hall: join.mcatstudyhall.com/
Thank youu ❤️❤️❤️❤️❤️😭😭😭
You're so welcome!
thanks a lot
you're welcome!
wtf jus happened...
?
same..