People afflicted with PKU lack sufficient activity of the enzyme phenylalanine hydroxylase and are therefore unable to break down phenylalanine into tyrosine adequately. Because of this, levels of phenylalanine rise to toxic levels in the body, which results in damage to the central nervous system and brain. Symptoms include delayed neurological development, hyperactivity, mental retardation, seizures, skin rash, tremors, and uncontrolled movements of the arms and legs.
Pregnant women with PKU are at a high risk for exposing the fetus to too much phenylalanine, which can cross the placenta and affect fetal development. The earlier a modified diet is begun, the less severe the symptoms will be. The person must closely follow a strict diet that is low in phenylalanine to avoid symptoms and damage. Phenylalanine is found in high concentrations in artificial sweeteners, including aspartame.
Therefore, these sweeteners must be avoided. Some animal products and certain starches are also high in phenylalanine, and intake of these foods should be carefully monitored. Digestion of proteins begins in the stomach, where HCl and pepsin begin the process of breaking down proteins into their constituent amino acids. As the chyme enters the small intestine, it mixes with bicarbonate and digestive enzymes.
The bicarbonate neutralizes the acidic HCl, and the digestive enzymes break down the proteins into smaller peptides and amino acids. Digestive hormones secretin and CCK are released from the small intestine to aid in digestive processes, and digestive proenzymes are released from the pancreas trypsinogen and chymotrypsinogen.
Enterokinase, an enzyme located in the wall of the small intestine, activates trypsin, which in turn activates chymotrypsin. These enzymes liberate the individual amino acids that are then transported via sodium-amino acid transporters across the intestinal wall into the cell. The amino acids are then transported into the bloodstream for dispersal to the liver and cells throughout the body to be used to create new proteins.
When in excess, the amino acids are processed and stored as glucose or ketones. The nitrogen waste that is liberated in this process is converted to urea in the urea acid cycle and eliminated in the urine. In times of starvation, amino acids can be used as an energy source and processed through the Krebs cycle. Answer the question s below to see how well you understand the topics covered in the previous section.
Skip to main content. Module 8: Metabolism and Nutrition. Search for:. All 20 amino acids are needed for protein synthesis. For example, lack of essential amino acid in the diet can stop peptide chain formation and protein synthesis and affect body weight gain and animal performance.
Protein turnover is a dynamic process involving continuous and simultaneous protein synthesis and protein degradation. The net rate of protein gain or loss is governed by the balance of synthesis and degenerative processes. Constant turnovers of proteins in the body and the loss of proteins, mainly in feces, are the basis for protein requirement.
Even when an animal is not growing, it still has a protein requirement. The amount of protein needed in the diet depends on age, physiological e. Skip to content All proteins in the body are in a state of constant flux, the size of the amino acid pool depends on a balance between synthesis and degradation. Chapter Objectives To introduce the fate of absorbed proteins and synthesis of nonessential amino acids To discuss the process of detoxification of ammonia produced through nitrogen metabolism.
Amino acid synthesis and degradation are brought about by two reactions called transamination and deamination that occur in the liver. Transamination is a chemical reaction that transfers an amino group to a keto acid to form new amino acids. Some amino acids can be glucogenic or ketogenic. Detoxification of ammonia to urea is through the urea cycle.
Key Points The liver is the major site of amino acid metabolism. Nonessential amino acids are synthesized through the process of transamination. Degradation of amino acid involves two processes deamination and transamination. Deamination is the removal of amino groups from the C skeleton and the release of ammonia.
Toxic ammonia is disposed through the urea cycle for detoxification of NH3 into urea mammals or uric acid birds. Two important functions of the urea cycle are detoxification of ammonia liver and provisioning of arginine kidney to form urea and ornithine. Two nonprotein amino acids ornithine and citrulline are involved in the urea cycle. Therefore, what makes life possible is the transformation of the potential chemical energy of fuel molecules through a series of reactions within a cell, enabled by oxygen, into other forms of chemical energy, motion energy, kinetic energy, and thermal energy.
Energy metabolism is the general process by which living cells acquire and use the energy needed to stay alive, to grow, and to reproduce.
How is the energy released while breaking the chemical bonds of nutrient molecules captured for other uses by the cells? The answer lies in the coupling between the oxidation of nutrients and the synthesis of high-energy compounds, particularly ATP , which works as the main chemical energy carrier in all cells.
There are two mechanisms of ATP synthesis: 1. The latter occurs in both the mitochondrion, during the tricarboxylic acid TCA cycle, and in the cytoplasm , during glycolysis. In the next section, we focus on oxidative phosphorylation, the main mechanism of ATP synthesis in most of human cells.
Later we comment on the metabolic pathways in which the three classes of nutrient molecules are degraded. B Scheme of the protein complexes that form the ETS, showing the mitochondrial membranes in blue and red; NADH dehydrogenase in light green; succinate dehydrogenase in dark green; the complex formed by acyl-CoA dehydrogenase, electron transfer flavoprotein ETFP , and ETFP-ubiquinone oxidoreductase in yellow and orange; ubiquinone in green labeled with a Q; cytochrome c reductase in light blue; cytochrome c in dark blue labeled with cytC; cytochrome c oxidase in pink; and the ATP synthase complex in lilac.
On the left is an electron micrograph showing three oval-shaped mitochondria. Each mitochondrion has a dark outer mitochondrial membrane and a highly folded inner mitochondrial membrane.
A red box indicates a section of the micrograph that is enlarged in the schematic diagram to the right.
The schematic diagram illustrates the electron transport chain. Two horizontal, mitochondrial membranes are depicted. The upper membrane is the outer mitochondrial membrane, and the lower membrane is the inner mitochondrial membrane.
The area between the two membranes is the intermembrane space, and the area below the lower membrane is the mitochondrial matrix. Each of these membranes is made up of two horizontal rows of phospholipids, representing a phospholipid bilayer.
Each phospholipid molecule has a blue circular head and two red tails, and the tails face each other within the membrane. A series of protein complexes are positioned along the inner mitochondrial membrane, represented by colored shapes.
The proteins that make up the electron transport chain start on the left and continue to the right. At the far left, NADH dehydrogenase is represented by a light green rectangular structure that spans the membrane. Next, succinate dehydrogenase is represented by a dark green bi-lobed shape embedded in the half of the inner membrane and facing the matrix.
Next, acyl-CoA dehydrogenase, electron transfer flavoprotein ETFP , and ETFP-ubiquinone oxidoreductase form a complex, and are represented by three yellow and orange ovals on the matrix-facing side of the inner membrane. Next, ubiquinone is represented by a lime green circle labeled with a Q located in the side of the inner membrane facing the intermembrane space. Next, cytochrome c reductase is represented by a light blue oval-shaped structure that spans the membrane.
Next, cytochrome c oxidase is represented by a pink oval-shaped structure that spans the inner membrane. Next, the ATP synthase complex is represented by an upside-down lollipop-shaped structure that traverses the inner membrane and contains a channel through the membrane; the round, purple head enters the mitochondrial matrix, and the lilac-colored stem spans the membrane.
These electrons are transferred to ubiquinone. Succinate dehydrogenase converts succinate to fumarate and transfers additional electrons to ubiquinone via flavin adenine dinucleotide FAD.
During this reaction, additional electrons are transferred to ubiquinone by the FAD domain in this protein complex. Next, the electrons are transferred by ubiquinone to cytochrome c reductase, which pumps protons into the intermembrane space.
The electrons are then carried to cytochrome c. Next, cytochrome c transfers the electrons to cytochrome c oxidase, which reduces oxygen O 2 with the electrons to form water H 2 O. During this reaction, additional protons are transferred to the intermembrane space. The electrons are "transported" through a number of protein complexes located in the inner mitochondrial membrane, which contains attached chemical groups flavins, iron-sulfur groups, heme, and cooper ions capable of accepting or donating one or more electrons Figure 2.
These protein complexes, known as the electron transfer system ETS , allow distribution of the free energy between the reduced coenzymes and the O 2 and more efficient energy conservation. Electron transport between the complexes occurs through other mobile electron carriers, ubiquinone and cytochrome c. FAD is linked to the enzyme succinate dehydrogenase of the TCA cycle and another enzyme, acyl-CoA dehydrogenase of the fatty acid oxidation pathway. These observations led Peter Mitchell, in , to propose his revolutionary chemiosmotic hypothesis.
The reaction catalyzed by succinyl-CoA synthetase in which GTP synthesis occurs is an example of substrate-level phosphorylation. Acetyl-CoA enters the tricarboxylic acid cycle at the top of the diagram and reacts with oxaloacetate and water H 2 O to form a molecule of citrate and CoA-SH in a reaction catalyzed by citrate synthase. Next, the enzyme aconitase catalyzes the isomerization of citrate to isocitrate. Fumarate combines with H 2 O in a reaction catalyzed by fumerase to form malate.
Then, oxaloacetate can react with a new molecule of acetyl-CoA and begin the tricarboxylic acid cycle again. The diagram shows the molecular structures for citrate, isocitrate, alpha-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and oxaloacetate.
The enzymes that act at each of the eight steps in the cycle are shown in yellow rectangles. In aerobic respiration or aerobiosis, all products of nutrients' degradation converge to a central pathway in the metabolism, the TCA cycle. In this pathway, the acetyl group of acetyl-CoA resulting from the catabolism of glucose, fatty acids, and some amino acids is completely oxidized to CO 2 with concomitant reduction of electron transporting coenzymes NADH and FADH 2.
Consisting of eight reactions, the cycle starts with condensing acetyl-CoA and oxaloacetate to generate citrate Figure 3. In this case, the hydrolysis of the thioester bond of succinyl-CoA with concomitant enzyme phosphorylation is coupled to the transfer of an enzyme-bound phosphate group to GDP or ADP.
Also noteworthy is that TCA cycle intermediates may also be used as the precursors of different biosynthetic processes. The number of calories someone burns in a day is affected by how much that person exercises , the amount of fat and muscle in his or her body, and the person's basal metabolic rate BMR.
BMR is a measure of the rate at which a person's body "burns" energy, in the form of calories, while at rest. The BMR can play a role in a person's tendency to gain weight. For example, someone with a low BMR who therefore burns fewer calories while at rest or sleeping will tend to gain more pounds of body fat over time than a similar-sized person with an average BMR who eats the same amount of food and gets the same amount of exercise.
BMR can be affected by a person's genes and by some health problems. It's also influenced by body composition — people with more muscle and less fat generally have higher BMRs. But people can change their BMR in certain ways. For example, a person who exercises more not only burns more calories, but becomes more physically fit, which increases his or her BMR. Reviewed by: Larissa Hirsch, MD. Larger text size Large text size Regular text size.
What Is Metabolism?
0コメント