Macronutrients and metabolism -
In Chapter 5 , you learned that enzymes are proteins and that their job is to catalyze chemical reactions. Recall that the word catalyzes means to speed up a chemical reaction and reduce the energy required to complete the chemical reaction, without the catalyst being used up in the reaction.
Without enzymes, chemical reactions would not happen at a fast enough rate and would use up too much energy for life to exist. A metabolic pathway is a series of enzymatic reactions that transform the starting material known as a substrate into intermediates, which are the substrates for the next enzymatic reactions in the pathway, until, finally, an end product is synthesized by the last enzymatic reaction in the pathway.
Some metabolic pathways are complex and involve many enzymatic reactions, and others involve only a few chemical reactions. To ensure cellular efficiency, the metabolic pathways involved in catabolism and anabolism are regulated in concert by energy status, hormones, and substrate and end-product levels.
The concerted regulation of metabolic pathways prevents cells from inefficiently building a molecule when it is already available. Just as it would be inefficient to build a wall at the same time as it is being broken down, it is not metabolically efficient for a cell to synthesize fatty acids and break them down at the same time.
Catabolism of food molecules begins when food enters the mouth, as the enzyme salivary amylase initiates the breakdown of carbohydrates. The entire process of digestion converts the large polymers in food to monomers that can be absorbed.
Carbohydrates are broken down to monosaccharides, lipids are broken down into fatty acids, and proteins are broken down to amino acids. These monomers are absorbed into the bloodstream either directly, as is the case with monosaccharides and amino acids, or repackaged in intestinal cells for transport by an indirect route through lymphatic vessels, as is the case with fatty acids and other fat-soluble molecules.
Once absorbed, blood transports the nutrients to cells. Cells requiring energy or building blocks take up the nutrients from the blood and process them in either catabolic or anabolic pathway. The organ systems of the body require fuel and building blocks to perform the many functions of the body, such as digesting, absorbing, breathing, pumping blood, transporting nutrients in and wastes out, maintaining body temperature, and making new cells.
monosaccharides, lipids are broken down into fatty acids, and proteins are broken down to amino acids. Energy metabolism refers more specifically to the metabolic pathways that release or store energy.
Some of these are catabolic pathways, like glycolysis the splitting of glucose , β-oxidation fatty-acid breakdown , and amino acid catabolism. Others are anabolic pathways, and include those involved in storing excess energy such as glycogenisis , and synthesizing triglycerides lipogenesis.
All cells are in tune to their energy balance. When energy levels are high cells build molecules, and when energy levels are low catabolic pathways are initiated to make energy. Glucose is the preferred energy source by most tissues, but fatty acids and amino acids can also be catabolized to the cellular energy molecule, ATP.
The catabolism of nutrients to energy can be separated into three stages, each containing individual metabolic pathways. The three stages of nutrient breakdown allow for cells to reassess their energy requirements, as end products of each pathway can either be further processed to energy or diverted to anabolic pathways.
Additionally, intermediates of metabolic pathways can sometimes be diverted to anabolic pathways once cellular energy requirements have been met.
The three stages of nutrient breakdown are the following:. The breakdown of glucose begins with glycolysis, which is a ten-step metabolic pathway yielding two ATP per glucose molecule; glycolysis takes place in the cytosol and does not require oxygen. In addition to ATP, the end products of glycolysis include two three-carbon molecules, called pyruvate.
Pyruvate has several metabolic fates. One, if there is insufficient oxygen, it is converted to lactate then shunted to the liver. Two, if there is sufficient oxygen and the cell needs energy, it is shunted to the mitochondria and enters the citric acid cycle or Cori cycle or Krebs cycle , or three, it may be converted to other molecules anabolism.
Pyruvate that is transported into the mitochondria gets one of its carbons chopped off, yielding acetyl-CoA. Acetyl-CoA, a two-carbon molecule common to glucose, lipid, and protein metabolism enters the second stage of energy metabolism, the citric acid cycle.
This is an irreversible process. The breakdown of fatty acids begins with the catabolic pathway, known as β-oxidation, which takes place in the mitochondria. In this catabolic pathway, four enzymatic steps sequentially remove two-carbon molecules from long chains of fatty acids, yielding acetyl-CoA molecules.
In the case of amino acids, once the nitrogen is removed deamination from the amino acid the remaining carbon skeleton can be enzymatically converted into acetyl-CoA or some other intermediate of the citric acid cycle.
In the citric acid, cycle acetyl-CoA is joined to a four-carbon molecule. In this multistep pathway, two carbons are lost as two molecules of carbon dioxide are formed. The energy obtained from the breaking of chemical bonds in the citric acid cycle is transformed into two more ATP molecules or equivalents thereof and high energy electrons that are carried by the molecules, nicotinamide adenine dinucleotide NADH and flavin adenine dinucleotide FADH 2.
NADH and FADH 2 carry the electrons hydrogen to the inner membrane of the mitochondria where the third stage of energy synthesis takes place, in what is called the electron transport chain.
In this metabolic pathway, a sequential transfer of electrons between multiple proteins occurs and ATP is synthesized. Water is also formed. The entire process of nutrient catabolism is chemically similar to burning, as carbon molecules are burnt producing carbon dioxide, water, and heat.
However, the many chemical reactions in nutrient catabolism slow the breakdown of carbon molecules so that much of the energy can be captured and not transformed into heat and light.
Complete nutrient catabolism is between 30 and 40 percent efficient, and some of the energy is therefore released as heat. Heat is a vital product of nutrient catabolism and is involved in maintaining body temperature. If cells were too efficient at transforming nutrient energy into ATP, humans would not last to the next meal, as they would die of hypothermia.
We measure energy in calories which are the amount of energy released to raise one gram of water one degree Celsius. Food calories are measured in kcal or Calories or calories. Some amino acids have the nitrogen removed then enter the citric acid cycle for energy production.
The nitrogen is incorporated into urea and then removed in the urine. The carbon skeleton is converted to pyruvate or enters the citric acid cycle directly.
These amino acids are called gluconeogenic because they can be used to make glucose. Amino acids that are deaminated and become acetyl-CoA are called ketogenic amino acids and can never become glucose. Fatty acids can never be made into glucose but are a high source of energy. These are broken down into two carbon units by a process called beta-oxidation enter the citric acid cycle as acetyl-CoA.
Thus, after a high-carbohydrate meal, our glycogen stores will reach capacity. After glycogen stores are filled, glucose will have to be metabolized in different ways for it to be stored in a different form.
The synthesis of glycogen from glucose is a process known as glycogenesis. Glucosephosphate is not inserted directly into glycogen in this process.
There are a couple of steps before it is incorporated. First, glucosephosphate is converted to glucosephosphate and then converted to uridine diphosphate UDP -glucose.
UDP-glucose is inserted into glycogen by either the enzyme, glycogen synthase alpha-1,4 bonds , or the branching enzyme alpha-1,6 bonds at the branch points3. The process of liberating glucose from glycogen is known as glycogenolysis. This process is essentially the opposite of glycogenesis with two exceptions: 1 there is no UDP-glucose step, and 2 a different enzyme, glycogen phosphorylase, is involved.
Glucosephosphate is cleaved from glycogen by the enzyme, glycogen phosphorylase, which then can be converted to glucosephosphate as shown below3. If a person is in a catabolic state or in need of energy, such as during fasting, most glucosephosphate will be used for glycolysis.
Glycolysis is the breaking down of one glucose molecule 6 carbons into two pyruvate molecules 3 carbons. During the process, a net of two ATPs and two NADHs are also produced. The following animation, using ball-and-stick models, allows you to control the 3 steps of glycolysis.
Glycolysis Animation 3 steps of Glycolysis. Thus, from a molecule of glucose, the harvesting step produces a total of four ATPs and two NADHs. Subtracting the harvesting from the investment step, the net output from one molecule of glucose is two ATPs and two NADHs. The figure below shows the stages of glycolysis, as well as the transition reaction, citric acid cycle, and electron transport chain that are utilized by cells to produce energy.
They are also the focus of the next 3 sections. If a person is in a catabolic state, or needs energy, how pyruvate will be used depends on whether adequate oxygen levels are present.
If there are adequate oxygen levels aerobic conditions , pyruvate moves from the cytoplasm, into the mitochondria, and then undergoes the transition reaction. If there are not adequate oxygen levels anaerobic conditions , pyruvate will instead be used to produce lactate in the cytoplasm. We are going to focus on the aerobic pathway to begin with, then we will address what happens under anaerobic conditions in the anaerobic respiration section.
The transition reaction is the transition between glycolysis and the citric acid cycle. The transition reaction converts pyruvate 3 carbons to acetyl CoA 2 carbons , producing carbon dioxide CO2 and a NADH as shown below. The figure below shows the transition reaction with CoA and NAD entering, and acetyl-CoA, CO2, and NADH being produced.
The acetyl is combined with coenzyme A CoA to form acetyl-CoA. The structure of CoA is shown below. Thus, for one molecule of glucose, the transition reaction produces 2 acetyl-CoAs, 2 molecules of CO2, and 2 NADHs.
Acetyl-CoA is a central point in metabolism, meaning there are a number of ways that it can be used. Under these conditions, acetyl-CoA will enter the citric acid cycle aka Krebs Cycle, TCA Cycle.
The following figure shows the citric acid cycle. The citric acid cycle begins by acetyl-CoA 2 carbons combining with oxaloacetate 4 carbons to form citrate aka citric acid, 6 carbons.
A series of transformations occur before a carbon is given off as carbon dioxide and NADH is produced. This leaves alpha-ketoglutarate 5 carbons. Another carbon is given off as CO2 to form succinyl CoA 4 carbons and produce another NADH. In the next step, one guanosine triphosphate GTP is produced as succinyl-CoA is converted to succinate.
GTP is readily converted to ATP, thus this step is essentially the generation of 1 ATP. In the next step, an FADH2 is produced along with fumarate.
Then, after more steps, another NADH is produced as oxaloacetate is regenerated. The first video and the animation do a good job of explaining and illustrating how the cycle works.
The second video is an entertaining rap about the cycle. Through glycolysis, the transition reaction, and the citric acid cycle, multiple NADH and FADH2 molecules are produced.
Under aerobic conditions, these molecules will enter the electron transport chain to be used to generate energy through oxidative phosphorylation as described in the next section. The electron transport chain is located on the inner membrane of the mitochondria, as shown below.
The electron transport chain contains a number of electron carriers. These carriers take the electrons from NADH and FADH2, pass them down the chain of complexes and electron carriers, and ultimately produce ATP. This creates a proton gradient between the intermembrane space high and the matrix low of the mitochondria.
ATP synthase uses the energy from this gradient to synthesize ATP. Oxygen is required for this process because it serves as the final electron acceptor, forming water. Collectively this process is known as oxidative phosphorylation.
The following figure and animation do a nice job of illustrating how the electron transport chain functions. ETC Animation 2. The first video does a nice job of illustrating and reviewing the electron transport chain.
The second video is a great rap video explaining the steps of glucose oxidation. Video: Electron Transport The table below shows the ATP generated from one molecule of glucose in the different metabolic pathways.
Notice that the vast majority of ATP is generated by the electron transport chain. Remember that this is aerobic and requires oxygen to be the final electron acceptor. But the takeaway message remains the same. The electron transport chain by far produces the most ATP from one molecule of glucose.
In this case, the pyruvate will be converted to lactate in the cytoplasm of the cell as shown below. Video: What happens when you run out of oxygen? Without the electron transport chain functioning, all NAD has been reduced to NADH and glycolysis cannot continue to produce ATP from glucose.
Thus, there is a workaround to regenerate NAD by converting pyruvate pyruvic acid to lactate lactic acid as shown below. However, anaerobic respiration only produces 2 ATP per molecule of glucose, compared to 32 ATP for aerobic respiration. The biggest producer of lactate is the muscle. Through what is known as the Cori cycle, lactate produced in the muscle can be sent to the liver.
In the liver, through a process known as gluconeogenesis, glucose can be regenerated and sent back to the muscle to be used again for anaerobic respiration forming a cycle as shown below.
It is worth noting that the Cori cycle also functions during times of limited glucose like fasting to spare glucose by not completely oxidizing it. Despite performing the same function, at the adipose level, the enzymes are primarily active for seemingly opposite reasons.
In the fed state, LPL on the endothelium of blood vessels cleaves lipoprotein triglycerides into fatty acids so that they can be taken up into adipocytes, for storage as triglycerides, or myocytes where they are primarily used for energy production. This action of LPL on lipoproteins is shown in the two figures below.
HSL is an important enzyme in adipose tissue, which is a major storage site of triglycerides in the body. Thus, HSL is important for mobilizing fatty acids so they can be used to produce energy.
The figure below shows how fatty acids can be taken up and used by tissues such as the muscle for energy production1. To generate energy from fatty acids, they must be oxidized. This process occurs in the mitochondria, but long chain fatty acids cannot diffuse across the mitochondrial membrane similar to absorption into the enterocyte.
Carnitine, an amino acid-derived compound, helps shuttle long-chain fatty acids into the mitochondria. The structure of carnitine is shown below. As shown below, there are two enzymes involved in this process: carnitine palmitoyltransferase I CPTI and carnitine palmitoyltransferase II CPTII.
CPTI is located on the outer mitochondrial membrane, CPTII is located on the inner mitochondrial membrane. The fatty acid is first activated by addition of a CoA forming acyl-CoA , then CPTI adds carnitine.
Acyl-Carnitine is then transported into the mitochondrial matrix with the assistance of the enzyme translocase. In the matrix, CPTII removes carnitine from the activated fatty acid acyl-CoA. Carnitine is recycled back into the cytosol to be used again, as shown in the figure and animation below.
Fatty acid transfer from cytoplasm to mitochondria Fatty Acid Activation. As shown below, the first step of fatty acid oxidation is activation. A CoA molecule is added to the fatty acid to produce acyl-CoA, converting ATP to AMP in the process.
Note that in this step, the ATP is converted to AMP, not ADP. Thus, activation uses the equivalent of 2 ATP molecules4. Fatty acid oxidation is also referred to as beta-oxidation because 2 carbon units are cleaved off at the beta-carbon position 2nd carbon from the acid end of an activated fatty acid.
The cleaved 2 carbon unit forms acetyl-CoA and produces an activated fatty acid acyl-CoA with 2 fewer carbons, acetyl-CoA, NADH, and FADH2. To completely oxidize the carbon fatty acid above, 8 cycles of beta-oxidation have to occur.
This will produce:. Adding up the NADH and FADH2, the electron transport chain ATP production from beta-oxidation and the citric acid cycle looks like this:. Compared to glucose 32 ATP you can see that there is far more energy stored in a fatty acid.
Acetyl-CoA has to first move out of the mitochondria, where it is then converted to malonyl-CoA 3 carbons. Malonyl-CoA then is combined with another acetyl-CoA to form a 4 carbon fatty acid 1 carbon is given off as CO2.
The addition of 2 carbons is repeated through a similar process 7 times to produce a 16 carbon fatty acid1. In cases where there is not enough glucose available for the brain very low carbohydrate diets, starvation , the liver can use acetyl-CoA, primarily from fatty acids but also certain amino acids , to synthesize ketone bodies ketogenesis.
The structures of the three ketone bodies; acetone, acetoacetic acid, and beta-hydroxybutyric acid, are shown below. After they are synthesized in the liver, ketone bodies are released into circulation where they can travel to the brain.
The brain converts the ketone bodies to acetyl-CoA that can then enter the citric acid cycle for ATP production, as shown below.
If there are high levels of ketones secreted, it results in a condition known as ketosis or ketoacidosis. It is debatable whether mild ketoacidosis is harmful, but severe ketoacidosis can be lethal. One symptom of this condition is fruity or sweet smelling breath, which is due to increased acetone exhalation.
Acetyl-CoA is also used to synthesize cholesterol. As shown below, there are a large number of reactions and enzymes involved in cholesterol synthesis.
Simplifying this, acetyl-CoA is converted to acetoacetyl-CoA 4 carbons before forming 3-hydroxymethylglutaryl-CoA HMG-CoA. HMG-CoA is converted to mevalonate by the enzyme HMG-CoA reductase. This enzyme is important because it is the rate-limiting enzyme in cholesterol synthesis.
A rate-limiting enzyme is like a bottleneck in a highway, as shown below, that determines the flow of traffic past it. Rate-limiting enzymes limit the rate at which a metabolic pathway proceeds.
These drugs inhibit HMG-CoA reductase and thus decrease cholesterol synthesis. Less cholesterol leads to lower LDL levels, and hopefully a lower risk of cardiovascular disease.
Macronuttients Macronutrients and metabolism required Macronuhrients order to build molecules Macronytrients larger Macronutrients and metabolism like Mindfulnessand to turn macromolecules metabolisj organelles and cells, merabolism then turn into Macronutrients and metabolism, organs, and ,etabolism systems, and finally into an organism. Your body builds new macromolecules from the Macronutrienrs in food. Energy comes from sunlight, which plants capture and, via photosynthesis, use it to transform carbon dioxide in the air into the molecule glucose. When the glucose bonds are broken, energy is released. Bacteria, plants, and animals including humans harvest the energy in glucose via a biological process called cellular respiration. In this process oxygen is required and the chemical energy of glucose is gradually released in a series of chemical reactions. Some of this energy is trapped in the molecule adenosine triphosphate ATP and some is lost as heat. Macromutrients that we have Nut Snacks for Weight Loss, taken up, Macronutriwnts, and Nut Snacks for Weight Loss the macronutrients, the next step is to learn how these macronutrients are metabolized. Alcohol is also metabolim at the end of this Citrus aurantium and antioxidant properties, even though it is not a macronutrient. Metabolism consists of all the chemical processes that occur in living cells. Anabolic means to build, catabolic means to breakdown. If you have trouble remembering the difference between the two, remember that anabolic steroids are what are used to build enormous muscle mass. This is shown in the example below of glucose and glycogen. The same is true for other macronutrients.
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