If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:9:54
The structure of Serine is drawn incorrectly. The side chain for Serine is CH2OH, not C2H4OH. It should be one Carbon shorter.

Introduction to amino acids

AP.BIO:
SYI‑1 (EU)
,
SYI‑1.B (LO)
,
SYI‑1.B.2 (EK)
,
SYI‑1.C (LO)
,
SYI‑1.C.1 (EK)

Video transcript

DNA gets a lot of attention as the store of our genetic information and then and it deserves that if we didn't have DNA there would be no way of keeping the information that makes us us and other organisms what those organisms are and DNA has some neat properties it can replicate itself and we go into a lot of depth on that in other videos so DNA producing more DNA we call that we call that replication but just being able to replicate yourself on its own isn't enough to actually produce an organism and to produce an organism you some have to take that information in a DNA and then produce things like structural molecules enzymes transport molecules signaling molecules that actually that act that actually do the work of the organism and that process the first step and this is all a review that we've seen in other videos the first step is to go from DNA to RNA and in particular messenger RNA messenger RNA and this process right over here this is called transcription transcription we go into a lot of detail on this in other videos and then you want to go from that messenger RNA it goes to the ribosomes and and then tRNA goes and grabs amino acids and they form actual proteins so you go from messenger RNA and then in conjunction so this is all this is in conjunction with tRNA and amino acids so let me say plus plus T RNA and amino acids and I'll write amino acids in I'll write it in a brighter color since that's going to be the focus of this video so tRNA and amino acids you are able to construct you are able to construct proteins you are able to construct proteins which are made up of chains of amino acids and it's the proteins that do a lot of the work of the organisms or get proteins which are nothing but chains of amino acids or they're made up of sometimes multiple chains of amino acids so you can imagine I'm just going to that's an amino acid that's another amino acid this is an amino acid and this is an amino acid you could keep going so the these chains of amino acids based on how these different based on the the properties of these different amino acids and how and how the protein takes shape and how it might interact with its surrounding these proteins can serve all sorts of different functions anything from from part of your immune system antibodies they can serve as enzymes they can serve as signaling hormones like insulin they're involved in muscle contract at contraction actin and myosin we actually have a fascinating video on that transport of oxygen hemoglobin so proteins the way at least my brain thinks they do a lot of the work the DNA says well what contains the information but a lot of the work of the organism is actually done is actually done by the proteins and as I just said the building blocks of the proteins are the amino acids so let's focus on that a little bit so up here are some examples of amino acids and there are 20 common amino acids other or some you can have there are a few more that depending on what organism you look at and theoretically there could be many more but in most biological systems there are 20 common amino acids that the DNA is coding for and these are two of them so let's just first look at what is common so we see that both of these had actually all three of this this is just a general form you have an amino group you have an amino group and this is where this is why we call it an amino an amino acid so you have an amino group amino group right over here now you might say well it's called an amino acid so where is the acid and that comes that comes from this carboxyl group right over here so that's why we call it an acid this carboxyl group is this carboxyl group is acidic it likes to donate this proton and then in between we have a carbon and we call that the alpha carbon we call that the Alpha carbon alpha carbon and that alpha carbon is bonded it has a covalent bond to the amino group a covalent bond to the carboxyl group and a covalent bond to a hydrogen now from there that's where you get the variation in the different amino acids and actually there's even there's even some exceptions for for how the nitrogen is but for the most part the variation between the amino acids is what this fourth covalent bond from the Alpha carbon does so you see in you see and serine you have this what you could call it an alcohol you could have an alcohol side chain in valine right over here you have a a fairly pure hydrocarbon hydrocarbon sidechain and so in general we refer to these side chains as an R group and it's these are groups that play a big role in defining the shape of the proteins and how they interact with their environment and the types of things they can do and you can even see just from these examples how these different side chains might behave differently this one has an alcohol sidechain and we know that oxygen is electronegative it likes to hog electrons it's amazing how much of chemistry or even biology you can deduce from just pure electronegativity so oxygen likes to hog electrons you're gonna have a partial negative state a partially negative charge there hydrogen hydrogen likes to hydrogen it has low electronegativity relative to oxygen so it's going to have its electrons hog so you can have a partially positive charge just like that and so this this has a polarity to it and so it's going to be hydrophilic it's going to at least this part of the molecule is going to be able to be attracted and interact with water and that's in comparison to what we have over here this this hydrocarbon this hydrocarbon sidechain this has no polarity over here so this is going to be hydrophobic so this is going to be hydrophobic and so when we start talking about the structures of proteins and how the structures of proteins are influenced by its side chains you can imagine that parts of proteins that have hydrophobic side chains those are going to want to get onto the inside of the proteins if we're in an aqueous solution while the ones that are more hydrophilic we want to go on to the outside and you might have some side chains that are all big and bulky and so they might they might make it hard to tightly pack then you might have other side chains that are nice and small that make it very easy to pack so these things really do help define the shape and we're going to talk about that a lot more when we talk about when we talk about the structure but how do these things how do these things actually connect and we're going to go into much more detail in another video but if you have if you have sarin right over here and then you have valine valine right over here they connect through what we call peptide bonds and a peptide is the term for two or more amino acids connected together so this would be a dipeptide in the bond isn't this big I just actually let me just let me draw a little bit smaller so that's serine this is this is valine they can form a peptide bond and this would be the smallest peptide this would be a dipeptide right over here peptide peptide bond or sometimes called a peptide linkage and as this chain forms that can that polypeptide as you add more and more things to it as you add more as you add more and more as you add more and more amino acids this is going to be this can be a protein or can be part of a protein that does all of these things now one last thing I want to talk about this is the way the way these amino acids have been drawn is the way you'll often see them in a textbook but at physiological PHS the pH is inside of your body which is in that set you know that low sevens range so it's a pH pH of roughly seven point two to seven point four what you have is this this this carboxyl the carboxyl group right over here is likely to be deprotonated it's likely to have given away its hydrogen you're going to find that more likely than when you have it's going to be higher concentrations having been deprotonated than being protonated so at physiological conditions it's more likely that this that this oxygen has taken both of those electrons and now has a negative charge so it's given it's just given away the hydrogen proton but took that hydrogen's electron so it might be like this and then the amino group the amino group at physiological pH is it's likely to actually grab a proton so nitrogen nitrogen has an external own pair so it might use that lone pair to grab a proton in fact it's physiological pH is you'll find a higher concentration of it having grabbed a proton than not grabbing a proton so the nitrogen will have grabbed a proton use its lone pairs to grab a proton and so it is going to have so it is going to have a it is going to have a positive charge and and so sometimes you will see when you will you will see amino acids describe to this way this is actually more accurate for what you're likely to find at physiological conditions and these molecules have an interesting name a molecule that is neutral even though parts of it have charged like this this is called a zwitterion that's a fun fun word Witter Witter ion and zwitterion German means hybrid and ion obviously means that it's a it's it's going to have charge and so this has hybrid charge even though it has charges it these ends it is a the the charge is net out to be neutral
Biology is brought to you with support from the Amgen Foundation