how's it going everyone this is high yield mcat and today we're going to be talking about the 20 amino acids encoded by the human genetic code so to jump right into it one common point of confusion is that amino acids are sometimes referred to as residues so on the mcat they're not going to explain every single term for you so if you see residue think amino acid now i also want to clarify that there are more than 20 amino acids but only 20 are going to be encoded directly by our human genome meaning that they are encoded by our dna in a gene translate or excuse me transcribed to mrna and translated into a polypeptide which will fold to make a protein so we call these protein ogenic amino acids and a little bit about what the etymology of this word is is that protein of course means our protein but genic just like gene means to produce or to generate so a gene produces some sort of functional product almost always a protein can sometimes be an rna as well but proteinogenic means protein producing and that should make sense because a protein after all is a polypeptide made up of a bunch of amino acids all connected by peptide bonds hence the name polypeptide so the good news for the mcat is that you only need to know one mrna codon that codes for an amino acid and there are a lot of these codons but the one that you need to know is the start codon which is aug and do you know what amino acid this codes for you may know it from your intro biology class already it's actually methionine so methionine so congratulations you already know one of the 20. so that's the good news but the bad news is that you are responsible for memorizing all 20 of these proteinogenic amino acids but the good news of the bad news is they are the number one most high yield thing on your mcat exam so studying these will get you a long way and it doesn't take as much time as you may think all right so let's get into what you actually need to know about each of these 20 amino acids so to start out let's take a look at methionine so our full name of this amino acid we already know as a methionine however to keep things brief we often use abbreviations for each amino acid so our first three-letter abbreviation that we will learn will be for our amino acid methionine and that is quite simply m-e-t not all of them will be this straightforward however all right moving on to our one letter abbreviation we have m again not all of them will be as intuitive as methionine and then we also need to know very very importantly our structure so we'll go over this a bit more in depth in a bit but we want to draw our backbone of our peptide with a little we could call them a mini n and c terminus our alpha carbon which we'll get to in a bit and what makes methionine unique is its r group so this little chain coming off with the sulfur so that is what makes methionine methionine and not any other amino acid next we want to know its classification which we will go over in subsequent videos and methionines classification is nonpolar we also want to know the pkrs and this is shorthand for the pka of the functional side group what makes an amino acid an amino acid and in this case it's not really applicable to methionine because it doesn't really have any hydrogens that can accept or donate very easily we also want to know any special characteristics of the amino acid and in this case the special characteristic we've already gone over it is encoded by aug the start codon and i want to make a little bit of a note about the pkrs before we move on is that we don't need to memorize the pis also known as the isoelectric point so we don't need to know the pis we'll get to those in a subsequent video as well finally one common point is how do we learn these things well i recommend flash cards and you can use virtual flash cards which include apps like anki which is very popular among medical students so that might be something you want to check out before you enter medical school and also quizlet which is a bit more well known you can also of course make your own flashcards between anki and quizlet i prefer anki because it is a bit more user-friendly for making pictures so you can draw out an amino acid and upload that picture into a flashcard on that note i want to say that drawing out the structures is very important to learning so drawing out these structures by hand and that's going to really help you ingrain the amino acid structure in your memory very well now finally let's get into some fundamental characteristics before we wrap up our introduction of amino acids so here i've drawn the basic structure of an amino acid and this little green line do you recognize what this might be this is a hydrogen and it's something that most people don't think about so remember that there's a hydrogen attached to this middle carbon in our backbone of an amino acid now the one of the first things i want to get across is that an amino acid is wider ionic so you might have noticed we have this negative charge and this positive charge here and that means our net charge is zero so zwitterionic molecules have a net charge of zero but they have a net charge of zero because they have a positive and a negative canceling each other out so that is the definition of zwitter ionic that positive and negative canceling each other out and that will be true at physiological ph which is ph of 7.4 also something you should know so physiological ph being 7.4 next we want to cover the source of these ions the n-terminus so this amino group on the right and the c-terminus this carboxylic acid group on the left these both are going to form either a peptide bond or the end of our polypeptide and we'll get to that when we go over polypeptides but this is the formation or this is where the terms n-terminus and c-terminus come from and what makes a polypeptide polar now one thing i want to know uh want you all to know is that we go from the n to the c-terminus when we are translating a protein from the five prime end of the mrna to the three prime end of the mrna oh i already mentioned this briefly with methionine but the r group is what makes each amino acid unique so this r group it doesn't stand for an atom you won't find it on the periodic table but it stands for some sort of functional group that is going to distinguish this amino acid from any other amino acid because all of the amino acids are going to share this part right here they're going to be common with that what makes them distinct is this r group so that's what we need to know as far as that goes now continuing on we have something called a c alpha carbon and this is going to be this carbon right here now this is a common point of kind of confusion misconception so one thing we want to know is that there's only going to be one c alpha carbon per amino acid and it is named alpha because it is one carbon away so one carbon away from our carbonyl carbon so a carbonyl carbon is a carbon double bonded to an o so this can be in a carboxylic acid group an aldehyde a ketone etc and then one final note that i want to say is that in eukaryotes all amino acids are of the l configuration stereochemistry so that is going to be the c alpha because it is the only chiral carbon in the amino acid backbone now some r groups will also have a chiral carbon but we are talking about the c alpha that is common among all amino acids so that's an introduction to amino acids and that's it for high-yield mcat today so leave a like subscribe and let me know in the comments what you would like to see next for your mcat exam