Question #1
(12%) What is the biochemical makeup of a lipoprotein? How do chylomicrons, VLDL, and LDL differ from one another in terms of lipid content and functionality?
Lipoprotein is made up of lipids and proteins. It has a lipid core composed of cholesterol esters and triacylglycerols. It also has a phospholipid bilayer along with apolipoprotein: `B-48, C-III, and C-II.
Chylomicrons donate triacylglycerols to peripheral tissue. Chylomicrons are mainly composed of triglycerides, ~80%.
VLDLs leave the liver and donate triacylglycerol to peripheral tissues where they become IDL to eventually form LDL. Mainly triglycerides, -49%, but also contain phospholipids, cholesterol and protein which make up the other half.
LDL provides cholesterol esters and cholesterol to peripheral tissue. LDL is mainly composed of cholesterol, -49%.
(16%) The figure depicts plasma levels of fuel sources (fatty acids, ketone bodies, and glucose) during fasting. Name which lines correspond to which fuel source. Explain your reasoning for labeling the figure the way that you did. Be brief in your response. It is not necessary to discuss hormones or enzymes in your response. However, include in your answer, why it is vital to longevity to be able to switch fuel sources during starvation.
Line A is glucose because it utilized right away, and it depleted during starvation. Then line B is ketones because, are produced as an energy source during fasting because they are able to cross the BBB and be utilized as an energy by the brain. So line C, is fatty acids because initially we break down fatty acids to be utilized as energy but cannot supply the brain as an energy source so ketogenesis is needed and that is why it is the highest line because during longer states of starvation it provides two energy sources: the body and the brain instead of just one, the body in fatty acids. It is important the ability to switch fuel sources in order to maintain homeostasis and energy within our bodies.
(45%) During long-term fasting, free fatty acid concentrations increase in your blood. From what tissue do these metabolites originate? What biochemical pathway increases free fatty acid concentrations in the blood? Describe this biochemical pathway (enzymes and regulatory mechanisms) and explain how starvation upregulates it. Be sure to discuss the role of perilipin. Conversely, how is this pathway slowed down during the post-prandial state?
During fasted states, starvation, epinephrine, and glucagon levels increase which stimulates lipolysis. These fatty acids from lipolysis originate from adipose tissue. Lipolysis increases free fatty acid concentrations in the blood. Initially ATP is made into cAMP through adrenal cyclase. This increase in cAMP is going to create and activate protein kinase a period adipose cells PKA is going to bind to inactive lipase which will activate hormone sensitive light peace dash P. The activated HL-P or add a post triglyceride lipase is going to phosphorylate, remove one fatty acid from triacylglycerol that is present in adipose tissue to diacylglycerol and one free fatty. Then HSL-P continues to degrade DAG by removing one fatty acid to monoacylglycerol and one free fatty acid. MAG will then be degraded to free fatty acids and glycerol by monoacylglycerol lipase. All these free fatty acids will be circulating in the blood to then enter peripheral tissue to be made into energy via beta oxidation starvation up regulates this reaction because of low concentration of insulin. Insulin acts to slow lipolysis. Epinephrine and glucagon will stimulate perilipin and once stimulated it will unprotect lipid droplets, allowing for lipolysis to occur. Therefore, during the post-prandial state lipolysis is slowed down because after eating insulin levels increase which increases phosphodiesterase. Phosphodiesterase prevent cAMP to be activated therefore PKA is not activated as well which doesn’t activate HSL causing lipolysis to slow down.
(14%) What is the estimated protein loss of patient with a protein intake of 50 grams and urinary nitrogen of 10 grams? Show your work and explain what you are calculating at each step. Round your answer to the nearest 100th decimal place.
Equation = (Protein intake (g) / 6.25) – (UUN + 4)
Equation = (50 g / 6.25) – (10 g + 4) = -6
First part calculates nitrogen intake
Equation = -6 x 6.25 = -37.5 g of protein
(15%) Starting from after a micelle has been absorbed by the enterocyte, describe the biochemical process by which long-chain fatty acids and cholesterol become incorporated into a lipoprotein. Name enzymes involved and described the biochemical steps.
Long chain fatty acids and monoglycerides leave the micelle and enter the enterocyte for further processing. In the enterocyte (in the SI), monoacylglyceride (monoglyceride) and fatty acids are recombined to make triglycerides. Long chain fatty acids, CoA and ATP are synthesized by fatty acyl-coenzyme A synthetase and produce fatty acyl-CoA with byproducts of AMP + PPi. Then two acylation reactions occur to form triacylglycerol. The first one uses monoacylglycerol and fatty acyl reacted with the enzyme acyl-CoA monoacylglycerol acyltransferase to make diacylglycerol and CoA. Then again fatty acyl-CoA is combined with diacylglycerol and synthesized by the enzyme acyl-CoA diacylglycerol acyltransferase to produce triacyclglycerol and CoA. By making triacylglycerol and cholesterol ester they are then stored at the core to make the chylomicron, a lipoprotein. Then fatty acyl-CoA combines with cholesterol and is esterified by using acyl-CoA-cholesterol acyltransferase to make cholesterol ester and CoA. These triglycerides are then packaged into a chylomicron and transported into the lymphatic system.
(36%) Skeletal muscle and other tissues are able to utilize fatty acids as a source of energy. Discuss the steps of beta-oxidation. For the “transport” step, provide the enzymes involved. For the “chop” step, it is not necessary to name the enzymes involved, but do describe the reactions and intermediate metabolites.
There are three steps in beta-oxidation: transport, chop, and entry.
Transport: long chain fatty acyl CoA is transported from the cytosol to the inside of the mitochondria. This is done by converting long chain fatty acids to fatty acyl-CoA by using fatty acyl CoA synthetase. Then fatty acyl CoA and carnitine are catalyzed by carnitine palmitoyl transferase I to free CoA and acyl-carnitine. Acyl-carnitine is then shuttled across the intermembrane space to intermembrane where carnitine-acylcarnitine translocase moves acyl-carnitine into the matrix. Finally, the enzyme carnitine palmitoyl-transferase II replaces carnitine with CoA to get fatty acyl-CoA in the mitochondrial matrix.
Chop: initially an oxidation reaction occurs in which a double bond is generated between alpha and beta carbons of the fatty acyl CoA using FAD (an electron acceptor), which created 2 ATP by oxidative phosphorylation. Then a hydration reaction occurs that breaks the double bond and adds a hydroxyl group to the beta carbon. After thus another oxidation reaction creates a double bond between the oxygen and carbon atoms using NAD+ (electron acceptor) and creates 3 ATP by oxidative phosphorylation. Lastly a thiolytic cleavage occurs where there is a CoA-dependent release of acetyl CoA and it then enters the TCA cycle.
Entry: Acetyl CoA, NADH, FADH are produced by extracting 2 acetyl CoA to enter ATP-yielding pathways
(45%) You are walking home from work, and you come across an older adult male who is passed out face down on the sidewalk. There is no obvious injury so you turn the person on their back to make sure they can breathe unobstructed. You detect a fruity odor on their breath. You believe that you are smelling acetone. Discuss the biochemical pathway (and enzymes involved) that synthesizes acetone, as well as the other ketone bodies. Next, discuss the biochemical pathway and enzymes involved by which ketone bodies can be converted to energy.
Acetyl-CoA can be used to make the ketone body acetoacetate. Acetoacetate can the create the other ketone bodies: 3-hydroxybutyrate and acetone. Acetoacetate can be synthesized to acetone with a byproduct of carbon dioxide by using the enzyme acetoacetate decarboxylase. Furthermore, acetoacetate and 3-hydroxybutyrate can be reversibly converted to one another with the use of the enzyme 3-hydroxybutryrate dehydrogenase and the electron transporter, NADH or NAD+.
To be converted to energy the ketone body, 3 hydroxybutyrate is converted to acetoacetate by using 3-hydroxybutyrate dehydrogenase. Then acetoacetate and succinyl-CoA are synthesized by thiophorase to create acetoacetyl-CoA with a byproduct of succinate. Acetoacetyl-CoA and CoA are synthesized by thiolase to make two acetyl-CoA groups which then enter the TCA cycle to create ATP, energy.
(19%) What is positive nitrogen balance? What are TWO situations when a person would be in positive nitrogen balance? What is negative nitrogen balance? What are TWO situations when a person would be in negative nitrogen balance?
Positive nitrogen balance is when protein synthesis is greater than protein degradation. Two situations in which someone would have this balance is growth and pregnancy.
Negative nitrogen balance is when protein degradation is greater than protein synthesis. Two situations in which someone would have this balance is with physical inactivity and infection/burns.
(21%) In the postprandial period, your body will preferentially deposit triglycerides from the gravy that you just consumed into adipose tissue for storage, rather than muscle for ATP synthesis. Describe biochemically how the body regulates this process.
Chylomicrons and VLDL, composed mainly of triglycerides, bind to lipoprotein lipase and activate it. When activated this enzyme converts lipoproteins to triacylglycerol within adipose tissue and skeletal muscle. Which then increases lipoprotein lipase secretion and induction in adipose tissue. This has the opposite effect in skeletal muscle, causing it to be stored rather than utilized for ATP synthesis.
(24%) How much ATP can be generated from the oxidation of arachidic acid (20 carbons)? Explain how you calculated your answer.
2 Carbons = 12 ATP / 1 Carbon = 6 ATP
20 C x 6 ATP = 120 ATP
9 FADH2 x 2 ATP = 18 ATP
9 NADH x 3 ATP = 27 ATP
120 + 18 + 27 – 2 = 163 ATP
(10%) Explain the role that ketogenesis plays in increasing longevity during long-term fasting?
During long-term fasting states ketogenesis is very important to survival. Ketones are water soluble, so they do not need transported by carriers like fatty acids or albumin. They also can be utilized by extra hepatic tissues for energy including the brain and muscle. Ketones are also able to cross the BBB.
(32%) Explain biochemically how during protein degradation in muscle, nitrogen is transferred to the liver and will eventually enter the urea cycle. What two biochemicals serve as nitrogen shuttles from the muscle to the liver? In the liver each of these shuttles is converted to glutamate. For each nitrogen shuttle, describe the chemical reactions that generate glutamate. Name enzymes involved.
Alanine and Glutamine are the biochemical nitrogen shuttles.
(27%) Excess glucose from your mashed potatoes will be converted to fatty acids to ultimately be stored in adipose tissue. Explain in biochemical detail the hormonal and allosteric regulation of lipogenesis in the postprandial state, and then also in the early fasted state.
Insulin (hormonal regulation) and citrate (allosteric regulation) increase acetyl-CoA carboxylase activity. When this activity is increase it converts acetyl-CoA through the use of the enzyme acetyl CoA carboxylase into malonyl CoA.
(24%) Discuss how hormonal and allosteric factors during the fasted state, favors the shunting of acetyl-CoA into ketogenesis within the liver.
During the fasted state, epinephrine is increased which increases lipolysis leading to increased hepatic fatty acids which then increases acetyl-CoA within the liver mitochondria. Also glucagon levels are increased as well. From both acetyl-CoA and glucagon being increased this stimulates gluconeogenesis. Acteyl-CoA allosterically activates pyruvate carboxylase while glucagon increases PEPCK expression, important parts of the GNG. When GNG is increased oxaloacetate is reduced which leaves less substrate for citrate synthase reaction to occur, requiring less acetyl-CoA to be used in the TCA cycle. Therefore, since acetyl-CoA is no longer needed in the TCA cycle it is shunted toward other metabolic pathways like ketogenesis.
(35%) The urea cycle is important in amino acid degradation. What enzyme is highly regulated in the urea cycle? What chemical reaction does it catalyze? Aside from the presence of more ammonia, how does elevated amino acid degradation/catabolism increase the activity of this enzyme?
In the urea cycle the enzyme carbamoyl phosphate synthetase 1 (CPS-I) is highly regulated. This enzyme catalyzes ammonia, carbon dioxide and 2 ATP molecules into carbamoyl phosphate and 2 ADP + 2 Pi. Elevated amino acid degradation increases the activity of CPS-I because of the presence of the amino acid, arginine. Arginine is an allosteric activator of N-acetylglutamate (NAG) synthase which synthesizes NAG. NAG is an allosteric activator of CPS-I, therefore more arginine allows for more NAG which increases CPS-I activity.
(24%) The third phase of lipogenesis involves the Fatty Acid Synthase (FAS) complex. Describe the reactions that the FAS complex catalyzes. You don’t need to name specific enzymes of the complex, but you should name intermediate molecules made.
First cytosolic acetyl-CoA is joined to an acyl carrier protein which form acetyl-ACP. Then malonyl-CoA is joined to ACP which forms malonyl-ACP. Acetyl-ACP, formed in the first step, and malonyl-ACP, formed in the second step, are joined together to create a keto-acyl ACP. The malonyl-CoA also acts as a 2-carbon donor. Then a reduction reaction occurs in which the keto-acyl ACP is reduced at the b-carbon using NADPH to create hydroxy-acyl ACP. Hydroxyl-acyl ACP then goes through a dehydration reaction, removing H2O, which makes enoyl ACP. The next step includes another reduction reaction where enoyl ACP is reduced to fatty acyl ACP using NADPH. In the first cycle of this complex, the first fatty acyl created is butyryl-ACP. This is repeated 7 times total, until palmitoyl ACP is created. Palmitoyl ACP dissociated from ACP creating 16 C palmitic acid (palmitate) which is the end product the FAS catalyzed reaction.