Metabolism of fat and amino acids.pptx

METABOLIS M OF FAT  Fatty Acid Metabolism LOCATION of FA Synthesis = cell cytoplasm - Inner mitochondrial membrane is impermeable to Acetyl-CoA; Tricarboxylate transporter translocates it out of mitochondria as citrate....

METABOLIS M OF FAT  Fatty Acid Metabolism LOCATION of FA Synthesis = cell cytoplasm - Inner mitochondrial membrane is impermeable to Acetyl-CoA; Tricarboxylate transporter translocates it out of mitochondria as citrate. 2 http://anitapopescu.me/projects/Notes/10-FA-Synthesis.html  Fatty Acid Synthesis intermediate of beta oxidation, the beta intermediat Starting with two acetyl-CoA, onetheis D-configuration. converted to malonyl-Next, water is removed from c CoA by carboxylation and 3 of the hydroxyl intermediate to produce a tran catalyzed by the enzyme bonded molecule. acetyl-CoA Last, the double bond is hydrog carboxylase (ACC), yield athe only intermediate. saturated regulatory The process cycles enzyme of fatty acid synthesis. addition Next, bothof another molecules malonyl-ACP have to the growing cha theirultimately CoA portions replaced with an intermediate by 16 carbons is produ a carrier protein known as ACP (palmitoyl-CoA). At this point, the cytoplasmic synth (acyl-carrier protein) to form ceases. and malonyl-ACP. acetyl-ACP Joining of a fatty acyl-ACP (in this case, acetyl-ACP) with malonyl-ACP splits out the For fatty acid synthesis, I must reverse the p carboxyl that was added and Of breaking creates fatty acids,atthough the intermediate the you’ll wonder ‘bout upper right in the figure at left. Each First, the cycle is ketone of reduced addition starts to a with carbons one tw hydroxyl Yet products using NADPH. of reactions Next, number carbons ev water is removed from carbons 2 and The 3 answer of the hydroxyl is that CO2 plays peek-a-boo like g intermediate to produce a trans By linking to an Ac-CoA then popping off ag doubled bonded molecule. Last, the double bond is Reactions are hydrogenated tolike yield oxidations a ‘cept they’re backwa saturated Reduction, dehydration, intermediate. Thethen two hydrogens a process cycles with the 3 Fatty Acid Synthesis addition of Theanother product of malonyl- the process is a 16 carbon ch Metabolism of Fat Synthesis of fat starting with glycerol-3- phosphate requires action of acyl transferase enzymes, such as glycerol-3- phosphate acyl transferase, which catalyze addition of fatty acids to the glycerol Synthesisbackbone. of fat requires glycerol-3- phosphate (or DHAP) and three fatty acids. In the first reaction, glycerol-3- phosphate is esterified at position 1 with a fatty acid, followed by a duplicate reaction at position 2 to make phosphatidic acid. This molecule, which is an intermediate in the synthesis of both fats and phosphoglycerides, gets dephosphorylated 4 to form diacylglycerol before the third Metabolism of Fat Breakdown of fat in adipocytes requires catalytic action of three enzymes, hormone sensitive triacylglycerol lipase (called LIPE) to remove the first fatty acid from the fat, diglyceride lipase to remove the second one and monoglyceride lipase to remove the third. Of these, only LIPE is regulated and it appears to be the rate limiting reaction. Interestingly, there appear to be few controls of the metabolism of fatty acids. The primary control of their oxidation is availability. One way to control that is by control of the breakdown of fat. This process, which can be stimulated by the epinephrine kinase cascade, is controlled 5  Fatty Acid beta-Oxidation In biochemistry and metabolism, beta-oxidation is the catabolic process by which fatty acid molecules are broken down in the cytosol in prokaryotes and in the mitochondria in eukaryotes to generate acetyl-CoA, which enters the citric acid cycle, and NADH and FADH2, which are co- enzymes used in the electron transport chain. It is named as such because the beta carbon of the fatty acid undergoes oxidation to a carbonyl group. Beta-oxidation is primarily facilitated by the mitochondrial trifunctional protein, an enzyme complex associated with the inner mitochondrial membrane, although The overallvery long chain reaction fatty for one acids cycle of beta are oxidized oxidation is: in peroxisomes. Cn-acyl-CoA + FAD + NAD++ H2O + CoA → Cn-2-acyl-CoA + FADH2 + NADH + H++ acetyl-CoA 6  Fatty Acid alpha-Oxidation Alpha – oxidation is defined as the oxidation of fatty acid (methyl group at beta carbon) with the removal of one carbon unit adjacent to the α-carbon from the carboxylic end. The carbon unit is removed in the form of CO2. Alpha oxidation occurs in those fatty acids that have a methyl group (-CH 3) at the beta-carbon, which blocks beta oxidation. There is no production of ATP. 7  Membrane Transport Free fatty acids cannot penetrate any biological membrane due to their negative charge. Free fatty acids must cross the cell membrane through specific transport proteins, such as the SLC27 family fatty acid transport protein. Once in the cytosol, the following processes bring fatty acids into the mitochondrial matrix so that beta-oxidation can take place. 1.Long-chain-fatty-acid—CoA ligase catalyzes the reaction between a fatty acid with ATP to give a fatty acyl adenylate, plus inorganic pyrophosphate, which then reacts with free coenzyme A to give a fatty acyl-CoA ester and AMP. 2.If the fatty acyl-CoA has a long chain, then the carnitine shuttle must be utilized: 1. Acyl-CoA is transferred to the hydroxyl group of carnitine by carnitine palmitoyltransferase I, located on the cytosolic faces of the outer and inner mitochondrial membranes. 2. Acyl-carnitine is shuttled inside by a carnitine-acylcarnitine translocase, as a carnitine is shuttled outside. 3. Acyl-carnitine is converted back to acyl-CoA by carnitine palmitoyltransferase II, located on the interior face of the inner mitochondrial membrane. The liberated carnitine is shuttled back to 8 the cytosol, as an acyl-carnitine is shuttled into the matrix.  Carnitine Shuttle Regulation of Carnitine Shuttle This carnitine shuttle is a rate limiting step in the oxidation of fatty acids in the mitochondria and thus fatty acid oxidation can be regulated at this step. Malonyl CoA, an intermediate of fatty acid synthesis present in the cytosol is an inhibitor of carnitine acyltransferase I. This indicates that when fatty acid synthesis is in progress, oxidation of fatty acid cannot occur at the same time as the carnitine shuttle is impaired by inhibition of carnitine acyltransferase I. 9 METABOLI SM OF AMINO ACIDS  Nitrogen Fixation The process of nitrogen fixation is important for life on earth, because atmospheric nitrogen is ultimately the source of amines in proteins and DNA. The enzyme playing an important role in this process is called nitrogenase and it is found in certain types of anaerobic bacteria called diazotrophs. Symbiotic relationships between some plants (legumes, for example) and the nitrogen- fixing bacteria provide the plants with access to reduced nitrogen. The overall reduction reaction catalyzed by nitrogenase is N2 + 6H+ + 6e− → 2NH3 In these reactions, the hydrolysis of 16 ATP is required. The ammonia can be assimilated into glutamate and other molecules. Enzymes performing nitrogenase catalysis are very susceptible to oxygen and must be kept free of it. It is for this reason that most nitrogen- fixing bacteria are anaerobic. Movement of amines through biological systems occurs largely by the process of transmination, discussed in amino acid metabolism. 11 Amino Acid Metabolism 12  Amino Acid Metabolism An important general consideration in amino acid metabolism is that of transamination. In this process, an exchange of amine and oxygen between an amino acid and an alpha-ketoacid occurs (see below) Alpha-ketoacid + amino acid amino acid + alpha- ketoacid An example reaction follows Pyruvate + Amino acid Alanine + Ketoacid This reaction is catalyzed by an enzyme known as a transaminase. 13  Amino Acid Metabolism Transamination Transamination is an exchange of functional groups between any amino acid (except lysine, proline, and threonine) and an α-keto acid. The amino group is usually transferred to the keto carbon atom of pyruvate, oxaloacetate, or α-ketoglutarate, converting the α-keto acid to alanine, aspartate, or glutamate, respectively. Transamination reactions are catalyzed by specific transaminases (also called aminotransferases), which require pyridoxal phosphate as a coenzyme. Pyridoxal phosphate PLP acts as a coenzyme in all transamination reactions, and in certain decarboxylation, dea mination, and racemization reactions of amino acids. 14  Amino Acid Metabolism Oxidative Deamination In the breakdown of amino acids for energy, the final acceptor of the α-amino group is α-ketoglutarate, forming glutamate. Glutamate can then undergo oxidative deamination, in which it loses its amino group as an ammonium (NH4+) ion and is oxidized back to α- ketoglutarate (ready to accept another amino group): 15 Amino Acid Metabolism 16  Amino Acid Metabolism Amino acids, such as glutamate, can also gain nitrogen directly from ammonium ion, as shown below (by nitrifying bacteria) Alpha-ketoglutarate + NH4+ Glutamate Many amino acids can be synthesized from citric acid cycle intermediates. For example, synthesis of the non-essential amino acids occurs as follows: aspartic acid can be made by transamination of oxaloacetate. Glutamate comes from transamination of alpha- ketoglutarate. Pyruvate, as noted, is a precursor of alanine (via transamination). Amino acids that can be made from glutamate include glutamine (by addition of an additional ammonium ion), proline, and arginine, Asparagine is made from aspartate by addition of ammonium ion also. Serine is formed from 3-phosphoglycerate and is itself the precursor of both glycine and cysteine. Cysteine and serine are also made from methionine. Tyrosine is made by hydroxylation of phenylalanine. 17 Amino Acid Metabolism 18 Glucose- Alanine cycle 19  Urea Cycle Yet another cyclic pathway important in cells is the urea cycle. With reactions spanning the cytoplasm and the mitochondria, the urea cycle occurs mostly in the liver and kidney. The cycle plays an important role in nitrogen balance in cells and is found in organisms that produce urea as a way to excrete excess amines. The cycle scavenges free ammonia (as ammonium ion), which is toxic if it accumulates. The capture reaction also requires ATP, and bicarbonate, and the product is carbamoyl phosphate. This molecule is combined with the non-protein amino acid known as ornithine to make another non-protein amino acid known as citrulline. Addition of aspartate to citrulline creates argninosuccinate, which splits off a fumarate, creating arginine (a source Figure 1 Urea Cycle of arginine). If arginine is not needed, 20 Urea is the major disposal form of amino groups derived from amino acids, and accounts for about 90% of the nitrogen- containing components of urine. One nitrogen of the urea molecule is supplied by freeammonia, and the other nitrogen by aspartate. [Glutamate is the immediate precursor of both ammonia (through oxidative deamination by glutamate dehydrogenase) and aspartate nitrogen (through transamination of oxaloacetate by AST).] The carbon and oxygen of urea are derived from CO2. Urea is produced by 21 the liver, and then is  Amino Acid Metabolism Amino acids are also divided according to the pathways involved in their degradation. Amino acids can be classified as being “glucogenic” or “ketogenic” based on the type of intermediates that are formed during their breakdown or catabolism. The catabolism of glucogenic amino acids produces either pyruvate or one of the intermediates in the Krebs Cycle. The catabolism of ketogenic amino acids produces acetyl CoA or acetoacetyl CoA. Figure 1. Glucogenic amino acids are listed in GREEN boxes and ketogenic amino 22 acids are listed in YELLOW boxes.