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Amino acid structure

Video transcript

Hey so we're going to be talking about me no acids and specifically with regards to amino acid structure and before we dive on into this topic I think it's nice to kind of take a step back and take a big picture view of amino acids and kind of figure out you know where do they exactly fit in the grand scheme of biochemistry and specifically it's a human metabolism and I think the best way to do this is to take a real-world example of a human protein called hemoglobin now what is hemoglobin while hemoglobin is found within the red blood cells that flow all through our bloodstream so here's my little red blood cell and each red blood cell is chock-full of this hemoglobin protein just going to write it as HGB here and this hemoglobin protein is what is actually responsible for picking up oxygen when this little red blood cell flows through the vessels in the lungs picks up oxygen and then transports this oxygen to all the various tissues within our bodies and so you can kind of think of hemoglobin as a car of sorts and you know my favorite car is a Porsche 911 so I mean if you want to be more environmentally friendly you could pick uh you can think of a Prius or something like that so whatever your car is oxygen is like the passenger for that car and so hemoglobin goes by the lungs picks up oxygen delivers it to the tissues and then tissues are just groups of cells that are of this of a similar type and so each of the cells and these tissues then takes the oxygen and uses it to generate adenosine typhus fee or ATP which is the energy source for all the various metabolic processes that go on within ourselves to help keep us alive so now where do amino acids fit into all of this well I mean Oh acids are the building blocks of this hemoglobin protein and so without amino acids this entire vitally important process wouldn't be able to occur now sticking with the car analogy just a little bit longer just like we have different types of components that come together to form different types of cars whether it be a Porsche or Prius we'll have you you can have different types of amino acids and there are 20 of them to be exact that can come together to form countless countless different types of proteins and so now that you have an idea of where amino acids fit in this bigger picture of a metabolic process let's go ahead and take a closer look at what the actual structure of an amino acid is first we have the amino group and then we have the carboxylic acid group and already you can start to see where the named amino acid comes from you have amino from the amino group and then you have acid from this carboxylic acid group here and then linking the two groups is this carbon atom which we call the Alpha carbon and then bound to the Alpha carbon is a hydrogen atom as well as a unique sidechain or our group we just use our to denote any generic sidechain so each of the amino acids has the same generic structure and what makes each of the twenty amino acids different from each other is this our group or the sidechain so each of the side chains for the amino acids is going to look different one thing that's important to note is that this carbon atom the alpha carbon is also known as a chiral carbon and what does a chiral carbon mean again well a chiral carbon is a carbon atom that has four unique groups bound to it so if we take a look at this carbon we can see that one group that's bound is the amino group another group is the carboxylic acid group the hydrogen atom makes the third group and then the fourth group bound to it is the R group or the side chain and so the Alpha carbon and amino acids is considered a chiral carbon and remember that chirality really refers to optical activity in other words if you were to shoot plane polarized light at an amino acid then because this carbon is chiral it would rotate to that light and so that's what chirality is really referring to it's referring to optical activity now it's important to note that there is one exception among the amino acids for chirality and that is the amino acid glycine and that's because the sidechain or our group for glycine is just a hydrogen atom it is the simplest of all side chains just one hydrogen atom and so if you were to substitute a hydrogen atom in place of this R group you would see that you have a duplication of atoms coming off of this alpha carbon in the case for glycine and so glycine is the only amino acid that does not have a chiral carbon so that's just important to make note of so let's give ourselves a little bit more room here now another way that you can portray the structure of an amino acid is with something called a Fischer projection and Fischer projections help to highlight the relationship of the four groups around a chiral carbon so let me draw that for you here first you have your amino group and up top you have your carboxylic acid here's your hydrogen atom and at the bottom is the sidechain or R group and just to orient you a bit here in the center is the chiral carbon the Alpha carbon here and then you have the four groups coming off of the chiral carbon and the horizontal bonds here you can kind of picture those as coming out of the plane of the computer towards you and then these vertical bonds here are coming out of the plane of the computer away from you and this particular configuration is called an L amino acid and conversely you can have the mirror image of this which I'll draw for you here and this particular configuration is called ad amino acid and these two configurations are called enantiomers and enantiomers are mirror image molecules that are not superimposable so you can picture these are mirror images of each other but if you were to take this D amino acid and try to superimpose it on the L you wouldn't be able to do that it's you can kind of think of these two configurations like your left and right hand and although your left and right hand are mirror images of each other you can't superimpose them on one another and that's the relationship between an L and a D configuration for a Fischer projections and these two configurations look awfully similar and are really easy to mix up and so the way that I like to keep them straight is if I look at where the amino group is let's take the L amino acid if I look at where the amino group is I can see that it is to the left of the projection so L is for left amino group now if I look at the D amino acid I again look for the amino group and I see that it's to the right of this configuration and so D which actually means dextro or right and Latin is for right amino group and that's kind of how I like to keep them straight so why is it important to distinguish between L and D amino acids well the L form of an amino acid is the only form that you will find within the human body and so that's really important to remember that the L configuration is the kind that you find within humans all right now how about we review a little bit about everything that we learned first we sort of got a bit picture of where amino acids fit in a larger metabolic process such as in the example of hemoglobin and then we learned about the structure of an amino acid and the fact that the central alpha carbon is a chiral carbon with optical activity and the one exception to this rule is the amino acid glycine which just has the simplest sidechain of a hydrogen atom and therefore it is not a chiral molecule and then we also learned about the Fischer projections for amino acids and the fact that the L configuration of an amino acid is the only one that you find within the human body and there you have it