Metabolism Flashcards
Define the BMR, and detail how this can be accurately measured.
The BMR (basal metabolic rate) is the base amount of energy we need to intake in order to survive (maintain physiological homeostasis). Expired gas and heat production can be measured in a gas chamber, or via breath-by-breath spirometry to obtain accurate measurements of BMR.
How much energy is released when a gram of carbohydrate/fat/protein/ATP is metabolised? How does this compare to a bomb calorimeter?
By human metabolism, 1g of; carbs=4kcal, protein=4kcal, fat=9kcal, ATP=7.3kcal. In a bomb calorimeter, these values are higher (carb=4.2, protein=5.65, fat=9.4) because the body is not 100% efficient in digesting them (particularly proteins, which are difficult to digest by virtue of their nitrogen content).
Define monosaccharides, disaccharides, and polysaccharides.
Monosaccharides are carbs such as glucose, fructose, and galactose, which single carbohydrates units. Disaccharides are double-carbohydrate units, such as sucrose and lactose. Polysaccharides contain many units, and are starches.
Outline the processes of glycogenolysis and glucogenesis.
In the liver, glucose can be converted to glycogen for storage - hexokinase converts glucose to glucose-6P, which is then converted to glucose-1P, and then glycogen via glycogen synthase. This is known as glycogenolysis.
This glycogen can be mobilised by being converted back to glucose-1P via glycogen phosphorylase, then back to glucose-6P, and subsequently fructose. This is known as glucogenesis.
Describe what happens to pyruvate in anaerobic metabolism, and outline the Cori cycle.
Under anaerobic conditions, pyruvate which is derived from glucose is transformed to lactate by lactate dehydrogenase. The resulting bi-product, NAD+, is recycled into glycolysis.
When excess lactate is formed, it is moved to the liver and undergoes reverse glycolysis via the Cori cycle. This accounts for roughly 20-25% of lactate formed, whilst the other 75-80% is used in the muscle for energy. Here, lactate is converted back to pyruvate, and ATP is used to convert this back to glucose.
Describe the process of aerobic metabolism of carbohydrates.
Aerobic metabolism occurs in 3 stages:
1) Glycolysis - glucose is phosphorylated by ATP to form glucose-6P, which is then rearranged and phosphorylated again to form fructose-1,6-biphosphate. This is then split into 2x three-carbon mlecules (G3P). Oxidation, and subsequent phosphorylation then results in 2x BPG + 2x NADH. BPGs are then dephosphorylated (by ADP) to produce 2x ATP + 2x 3PG, which are then condensated to form 2x PEP + water. The removal of phosphates (by ADP) then produces 2x ATP + 2x pyruvate. There is a net gain of 2x ATP, 2x NADH, and the original glucose is split into 2x pyruvate.
2) Pyruvate from glycolysis enters the Krebs’ cycle, where it is converted to Acetyle co-enzyme A via removal of CO2 and addition of co-enzyme A. This joins with oxaoacetic acid, and CoA dissociates, to form citric acid, which quickly changes to isocitric acid. With the removal of CO2 and protons, and the re-joining of CoA after a 5-carbon intermediate, this is converted to succinyl-CoA. CoA dissociates again, and this is converted to succinic acid, producing 1xATP, then fumaric acid malic acid, and finally, returning to the pickup molecule, oxaloacetic acid, to restart the cycle. For each original glucose molecule, this results in a net gain of 2ATP, 6NADH, and 2FADH2.
Total net gain so far = 4ATP, 10NADH, and 2FADH2.
3) Electron transport chain - FADH2 and NADH are used as electron and proton carriers. When they reach the electron transport chain, they become FAD+ and NAD+, respectively. Resulting electrons travel through a chain of transmembrane proteins, and their free energy is used to pump the resulting protons across the membrane, producing a strong electrochemical gradient. Finally, these protons diffuse down their electrochemical gradient via ATP synthase, and their potential energy is converted to chemical energy by the production of ATP from ADP+Pi. Oxygen is the final electron acceptor after the ETC. Each NADH yields 3 protons (and thus, 3ATP), and each FADH2 yields 2 protons (and thus, 2ATP). This results in a net gain of 34 ATP from oxidative phosphorlyation, and 38ATP in total.
Describe the key differences between type I, IIa, and IIx muscle fibres.
Muscle fibres are characterised by distinct twitch spreed, rate of glycolysis, energy type used, and mode of metabolism:
1) Type I fibres are slow-twitch, low glycolysis, fat-burning, aerobic fibres.
2) Type IIa fibres are moderately fast-twitch, high glycolysis, glycogen and phosphocreatine using, long anaerobic fibres.
3) Type IIx fibres are fast twitch, high glycolysis, glycogen and phosphocreatine using, short anaerobic fibres.
More aerobically trained athletes have highest %slow twitch muscle fibres. More explosively trained athletes have faster twitch fibres, and are more competent at coping under anaerobic stress. Research has shown that it may not be possible to convert between types I and II fibres but that IIa and IIx fibres may be able to be changed between.
Describe how energy is produced from intramuscular phosphocreatine.
During short bouts of high intensity exercise phosphocreatine is used for energy. The phosphorylation of ADP by phosphocreatine produces ATP and creatine (with the catalyst creatine kinase). Once ATP is hydrolysed, the phophate joins back to the creatine to reform phosphocreatine again.
PCr has a higher phosphate transfer potential than ATP, so can transfer phosphate more readily. This PCr store depletes very fast, is is the main source of energy for the first 2-3 seconds of a sprint (depleting completely from 3-10 seconds of maximal effort). After 30 seconds of recover, roughly 50% of PCr stores will be restored. It takes 3-4 minutes for the store to be 90-95% restored, and full recovery takes around 20 minutes.
Discuss the advantages of creatine supplementation.
Supplementation of creatine can increase the Cr concentration in muscle from 110-120mmol/Kg (dry) to around 130-160mmol/Kg. This can result in increased ability to recover fast for training. However, there is a personal element to this - there are people who respond and those who don’t to supplementation (usually, based on their dietary creatine intake - therefore, vegetarians and vegans respond best).
Explain the key trade-off of metabolising fats for energy.
Oxidation of fats yields a huge amount of ATP (energy) per gram, and can be stored all over the body. However, fat metabolism requires a large oxygen investment (23xO2 as opposed to 6xO2 for carbohydrates).
Describe the process of fat metabolism, and detail the energy yield.
Most fats which we metabolise for energy are triglycerides (3x long-chain fatty acids bound to glycerol). Lipolysis is the breakdown of these triglycerides into their constituents, and is controlled by a range of hormones, including cortisol, human growth hormone, T4, etc, which all act upon hormone sensitive lipase. Triglycerides in the liver are combined with phospholipids, cholesterol, and proteins to form VLDLs, which enter adipose tissue via lipoprotein lipase, where they are then stores as triglycerides. In adipose tissue, or in the liver, these are then mobilised for energy - lipolysis takes place, and fatty acids are transported through the blood to skeletal muscle bound to albumin. Glycerol gets transformed to 3-phosphoglyceraldehyde, and then into pyruvates which enter the Krebs’ cycle to give 19ATP for each molecule of glycerol.
Meanwhile, fatty acids enter the muscle sarcoplasm via fatty acid transporters, and are converted to fatty acyl CoA. This is converted to acyl carnitine and transported into the mitochondria via the carnitine transporter. They are converted back to fatty acyl CoA to undergo beta oxidation - This is a cyclical pathway which gradually breaks down the fatty acid with each cycle removing 2 carbons in the form of acetyl CoA until the fatty acid is completely depleted. Each repetition of the cycle therefore produces acetyl CoA, NADH, and FADH2.
For example, palmitic acid (C16) undergoes 8 repetitions of beta oxidation, producing 8x acetyl CoA (12ATP each), 7x FADH2 (2ATP each), and 7x NADH (3ATP each) (minus 2 ATP which is required to “activate” the process) = 129ATP for every palmitic acid.
A triglyceride containing 3 palmitic acids will therefore break down for 406ATP (19 from glycerol, 129 from each palmitic acid).
Describe how lifestyle can be manipulated to maximise fat metabolism.
1) Diet - high fat, low carb diets can result in the oxidation of fats as a primary fuel source. After 3 days, insulin will begin to suppress hepatic glucose output. After 1 week, there will be an increased fat oxidation at rest. After 2 weeks, the enzymes involved in fat oxidation will increase in activity and levels, and there will be decreased muscle glycogen utilisation. After 3 weeks, muscle glycogen stores will deplete. After a month there will be a decrease in insulin receptor mRNA, and a decrease in hexokinase activity.
2) Exercise - between 60-70% HR pr VO2MAX is the maximum fat burning zone. Here, metabolism is still sub-anaerobic, but intensity is sufficiently high that the body must partially rely on fatty acid metabolism for energy.
What are the three ways by which amino acids can pass through the free pool of the body?
There is no body reservoir for protein - only free amino acids across all tissues of the body. Free amino acids pass through this pool in 3 ways:
1) From the digestion of proteins consumed in the diet.
2) From the degradation of tissue protein.
3) Non-essential amino acids produced in the tissues.
Describe the processes of transamination and oxidative deamination.
1) Transamination is the process by which amino acids combine with keto acids to produce new amino acids.
2) Oxidative deamination is the opposite process, where amino acids break down into keto acids and ammonia.
Both these processes occur in the liver, and are necessary for altering amino acids into other substrates for energy provision. Deamination of specific amino acids produces intermediate substrates which are found in the Krebs’ cycle.
Explain how protein metabolism can be measured.
Protein metabolism can be measured by examining nitrogen balance. Nitrogen balance is the ratio of nitrogen intake (dietary protein, where protein is assumed to be 16% nitrogen) to nitrogen excretion (urine, faeces, expired gas, and sweat). Nitrogen balance is where intake is equal to output, positive nitrogen balance is where intake is greater than output, and negative nitrogen balance is where nitrogen intake is less than nitrogen output.
