BMB 401 Lecture 26 Transcript 1 Biochemistry 401 Lecture 26 Today we re going to talk about fatty acids We re going to begin with an overview of fatty acids and then we re going talk about triacylglycerols fats We re going to talk about the catabolism of triacylglycerols from dietary sources and from endogenous sources and then we re going to talk about fatty acid oxidation We re going to start with the catabolism of saturated fatty acids because this is more simple and they we re going to go on to the catabolism of unsaturated fatty acids So let s get started This is an overview of fatty acids Fatty acids are hydrocarbon chains that end in carboxylic acids Now at physiologic pH this carboxylic acid will be deprotonated to a carboxylate and therefore it will have a negative charge At the top of the side we see the 16 carbon saturated fatty acid palmitate This is an ionized form of palmitic acid On the right we see a carboxylate and on the left we see the long hydrocarbon chain The fatty acid on the bottom oleate the ionized form of oleic acid has a pronounced kink in the chain and this is because there is a cis double bond in oleate it is an 18 carbon monounsaturated fatty acid It s monounsaturated because there s one double bond there and cis double bonds that are put into fatty acids will kink the hydrocarbon chain This is very important not only for the structure of fatty acids but also for their function and we re going to see this little bit later on Fatty acids can have different characteristics They can have short medium or long hydrocarbon chains and these hydrocarbon chains can also have differing degrees of saturation Now these characteristics impact the function of these fatty acids For instance in panel A we see three stearates Now stearates are saturated fatty acids and the hydrocarbon chains are linear without kinks There are no kinks in stearate and so these hydrocarbon chains can pack against one another really easily to optimize van der Waals forces On the other hand in panel B we see two stearates but there s an oleate there also Now the oleate as we know and as we can see above is unsaturated It has one double bond It has a kinked confirmation and this has the effect that it reduces the amount of van der Waals forces that can form between the oleate and the two stearates and therefore there is a difference between the three stearates and the oleate with two stearates The three stearates are going to be less fluid in their interactions because the van der Waals forces are stronger among them BMB 401 Lecture 26 Transcript 2 However the fatty acids that include oleate panel B will be more fluid in nature They can slide past one another more easily because the van der Waals forces are less strong among these three molecules A similar effect would also be seen if we had shorter hydrocarbon chains Because these hydrocarbon chains are shorter say a C12 rather than C18 even if they were completely saturated there would be less van der Waals forces among those C12 fatty acids than there are among the C18 stearates and so for this reason those C12 fatty acids would also be more fluid in nature than the stearates And so structural characteristics of fatty acids impact the degree of fluidity that is seen and this has an important effect on membrane lipids and also on triacylglycerols Now there are two means of identifying the locations of the double bonds so that we can know what sort of fatty acid we re talking about In the figure above we see a saturated fatty acid palmitate which is a 16 carbon fatty acid Because it has no double bonds it is written as a C16 0 The zero stands for no double bonds On the other hand oleate that we see below is written as a C18 1 followed by notation that indicates the position of the double bond and whether it s cis or trans In the case of oleate we re talking about a cis 9 double bond When we re numbering from the carboxylate end the carboxylate carbon is carbon number one and we re going to count down the chain and we end up at the double bond starting at carbon number nine and ending at carbon ten We identify the double bond location by the number of the carbon in the double bond that s closer to the carboxylate end in this case carbon nine and so oleate is written C18 1 cis 9 You can also number from the methyl and This is called the omega end Omega is the Greek letter for the end of the alphabet In this case the carbon in the double bond that s closer to the omega end identifies the location of the double bond In the panel on the right we see an omega 3 fatty acid The carboxylate is shown at the bottom of the diagram and the methyl group is shown at the top The methyl group again is the omega end and contains omega carbon We re going to count down from the omega carbon and we reach carbon number three which is the start of the double bond and so this is an omega 3 fatty acid This is a list of some common names for fatty acids The number of carbons that they contain the number of double bonds their common name their systematic name and the formula for each fatty acid Now the thing that s important in this BMB 401 Lecture 26 Transcript 3 table is to understand the notation that we just went through If you see a cis nine hexadecenoate I would tell you that this fatty acid is a C16 1 cis 9 and you would have to be able to figure out which of these fatty acids would be identified by that sort of notation Please become familiar with going from one portion of this table to the other so that you can match the fatty acid and its notation Do you have to memorize the common names of these fatty acids with the number of carbons that each one contains You certainly don t You just have to be able to decode the notation Now in animals these fatty acids are unbranched even numbers from 16 to 24 carbons Most common are the 16 and 18 carbon fatty acids Double bonds are cis double bonds not trans and polyunsaturated double bonds are separated by a methylene group Now odd and branched chain fatty acids are synthesized by bacteria and oddly enough ruminant animal fat such as fat from cows contains odd number fatty acids but these come from bacteria that the cow has ingested in the course of regurgitation One of the things that fatty acids are used for is the production of triacylglycerol This is a triacylglycerol shown here It s comprised of glycerol and three fatty acids These …
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