Test: Fat and protein metabolism - MCAT MCQ

Triglycerides, which are the main components of dietary fats and oils, provide the highest amount of energy per gram among the listed options. Triglycerides contain more than twice the energy of proteins and carbohydrates. While proteins and carbohydrates provide approximately 4 kilocalories (kcal) of energy per gram, triglycerides provide around 9 kcal of energy per gram. Therefore, triglycerides are considered a concentrated source of energy in the diet.

The digestion and mobilization of dietary fats involve several steps. First, bile salts are released into the small intestine, where they emulsify dietary fats, breaking them down into smaller droplets. This emulsification process increases the surface area of the fat droplets, allowing lipases to act upon them more efficiently. Lipases then degrade the triglycerides into free fatty acids and monoglycerides. These smaller components are absorbed by the intestinal cells and converted back into triglycerides. The newly formed triglycerides are then incorporated into chylomicrons, which are large lipoprotein particles. Finally, the chylomicrons are released into the lymphatic system and eventually enter the bloodstream, where they transport the dietary fats to various tissues for utilization or storage.

The density of lipoproteins is a key factor that determines their function. Lipoproteins are complex particles composed of lipids (such as cholesterol and triglycerides) and proteins. The relative amounts of lipids and proteins in a lipoprotein influence its density. Lipoproteins with higher lipid content have lower density, while those with higher protein content have higher density. The density of lipoproteins affects their transport and distribution in the bloodstream and their interaction with specific receptors on target cells. Different lipoproteins, such as chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL), have distinct densities and play different roles in lipid metabolism and transport. For example, LDL is often referred to as "bad cholesterol" because it transports cholesterol to peripheral tissues and can contribute to the development of atherosclerosis, while HDL is known as "good cholesterol" because it helps remove cholesterol from tissues and transport it back to the liver for excretion.

The majority of fatty acids enter the outer membrane of the mitochondria through transferase-facilitated entry of carnitine bound fatty acids. Before entering the mitochondria, long-chain fatty acids need to be converted into their acyl-CoA form. This conversion takes place in the cytoplasm, where the fatty acid is first activated by attaching Coenzyme A (CoA) to form fatty acyl-CoA. The acyl group is then transferred to carnitine by the enzyme carnitine palmitoyltransferase I (CPT-I) located on the outer mitochondrial membrane. The resulting carnitine-bound fatty acyl group is then transported across the outer mitochondrial membrane by a carnitine-acylcarnitine translocase. Once inside the mitochondria, the carnitine group is transferred back to CoA by carnitine palmitoyltransferase II (CPT-II) located on the inner mitochondrial membrane. The fatty acyl-CoA can then undergo beta-oxidation, a series of enzymatic reactions that occur within the mitochondrial matrix, to produce acetyl-CoA, which enters the citric acid cycle for further energy production.

• Start simple-- a 20 carbon FA will yield 10 molecules of acetyl-CoA (20 carbons available, two carbons per acetyl-CoA). Recall that for each acetyl-CoA, the Krebs cycle produces 10 molecules of ATP.
• Remember that the process of oxidizing fatty acids to form molecules of acetyl-CoA also generates energy. Each pass of β-oxidation generates one molecule of NADH and one molecule of FADH₂
• To produce 10 acetyl-CoA, this fatty acid will require 9 passes of oxidation.
• 2.5 ATP can be generated per NADH, and 1.5 ATP per FADH₂
• ​2.5x9= 22.5; 1.5x9= 13.5; 13.5+22.5+100 = 136
• Approximately 136 molecules of ATP can be produced from the oxidation of a 20 carbon fatty acid, including those produced in the Krebs cycle.

During amino acid catabolism, amino acids are broken down through various metabolic pathways. The carbon skeletons of amino acids can be used for energy production or converted into glucose or ketone bodies. Glucose is an important energy source for the body, while ketone bodies can serve as an alternative fuel during periods of prolonged fasting or low carbohydrate intake.

Tetrahydrofolate, on the other hand, is a coenzyme involved in one-carbon metabolism and plays a role in the synthesis of nucleotides and amino acids, but it is not a major product of amino acid catabolism. Glycogen, a storage form of glucose, is not directly produced from amino acid catabolism.

Therefore, the correct answer is C. Ketone bodies and glucose.

The human body cannot synthesize these essential amino acids on its own, so they must be obtained from dietary sources. When we consume protein-rich foods, the proteins are broken down into individual amino acids during the process of digestion. These amino acids are then absorbed into the bloodstream and used by the body for various physiological processes, including the synthesis of new proteins, neurotransmitters, and other important molecules.

Therefore, the correct answer is D. Dietary protein only.

Chylomicrons are large lipoprotein particles that transport dietary fats, specifically triglycerides, from the intestine to other tissues, such as muscle and adipose tissue, for utilization or storage.

Gluconeogenesis is the process by which amino acids, along with other substrates like lactate and glycerol, are converted into glucose in the liver and kidneys. This process helps maintain blood glucose levels during fasting or low carbohydrate intake.

Hormone-sensitive lipase is the enzyme responsible for the hydrolysis of triglycerides into free fatty acids and glycerol during lipolysis. It is regulated by hormonal signals, such as epinephrine and glucagon.