Chapter 18: Amino Acid Oxidation And The Production Of Urea

Pyridoxal phosphate is a cofactor in transamination reactions. Transamination is a process where an amino group is transferred from one amino acid to a keto acid, resulting in the formation of a new amino acid and a new keto acid. Pyridoxal phosphate acts as a coenzyme by binding to the amino acid and facilitating the transfer of the amino group. This reaction is important for the synthesis of nonessential amino acids and the conversion of amino acids into energy or other metabolites.

Pyridoxal phosphate (PLP) is the correct answer because it is a coenzyme that plays a crucial role in transaminase reactions. Transaminases are enzymes that transfer an amino group from an amino acid to a keto acid, forming a new amino acid and a new keto acid. PLP acts as a cofactor for transaminases by forming a Schiff base with the amino acid substrate, allowing the transfer of the amino group to occur. This coenzyme is derived from vitamin B6 and is essential for amino acid metabolism.

Urea synthesis in mammals primarily occurs in the liver. The liver plays a crucial role in the metabolism of nitrogenous waste products, such as ammonia, which are produced during the breakdown of proteins. The liver converts ammonia into urea through a series of enzymatic reactions in a process called the urea cycle. Urea is then transported to the kidneys for excretion in urine. While other organs may also play a minor role in urea synthesis, the liver is the primary site for this process.

The conversion of ornithine to citrulline is a step in the synthesis of urea. Urea is a waste product that is formed in the liver from the breakdown of proteins. Ornithine is an amino acid that is involved in the urea cycle, which is the process by which ammonia is converted into urea for excretion. Citrulline is an intermediate in this cycle and is converted further to form arginine, another amino acid involved in the urea cycle. Therefore, the correct answer is urea.

Pyridoxine, also known as vitamin B6, is the correct answer because it is required as a coenzyme for all transaminations. Transaminations are a group of chemical reactions that involve the transfer of an amino group from one molecule to another. Pyridoxine is necessary for the proper functioning of enzymes involved in these reactions, making it essential for amino acid metabolism. Niacin, riboflavin, thiamin, and vitamin B12 are not specifically involved in all transaminations, making them incorrect choices.

A person's urine contains urea, which is a waste product of protein metabolism. If a person's urine contains unusually high concentrations of urea, it suggests that they have been consuming a diet that is very low in carbohydrates and very high in protein. This is because the body breaks down protein into urea during metabolism. Therefore, the correct answer is "Very low carbohydrate, very high protein."

Transamination is a process in which an amino group is transferred from an amino acid to a keto acid. In this case, the amino group from alanine is transferred to alpha-ketoglutarate to form glutamate. This reaction requires the presence of a coenzyme, which is pyridoxal phosphate (PLP). PLP acts as a cofactor and facilitates the transfer of the amino group. Biotin, NADH, and thiamine pyrophosphate (TPP) are not involved in this specific reaction.

Malate is not involved in the production of urea from NH4+ via the urea cycle. The urea cycle is a series of biochemical reactions that occur in the liver to convert toxic ammonia into urea, which can be safely excreted by the body. Malate is not one of the substrates or intermediates involved in this process. The correct substances involved in the urea cycle are aspartate, ATP, carbamoyl phosphate, and ornithine.

The correct answer is transamination requiring pyridoxal phosphate (PLP). Transamination is the process by which an amino group is transferred from one amino acid to a keto acid, resulting in the formation of a new amino acid and a new keto acid. This reaction is catalyzed by enzymes known as transaminases, which require the coenzyme pyridoxal phosphate (PLP) to function. PLP acts as a carrier for the amino group, facilitating its transfer between the amino acid and the keto acid. Therefore, transamination is the first reaction in amino acid catabolism for many amino acids.

The conversion of glutamate to an alpha-ketoacid and NH4+ is catalyzed by glutamate dehydrogenase. This enzyme facilitates the transfer of an amino group from glutamate to alpha-ketoglutarate, forming an alpha-ketoacid and releasing ammonia. Glutamate dehydrogenase does not require any cofactors for this reaction and does not involve ATP hydrolysis. Therefore, the correct answer is that the conversion is catalyzed by glutamate dehydrogenase.

Serine, alanine, and cysteine can undergo catabolism to produce pyruvate. Catabolism refers to the breakdown of molecules to release energy. In this case, these amino acids can be broken down through various metabolic pathways, ultimately leading to the production of pyruvate. Pyruvate is a crucial molecule in cellular metabolism as it can be further metabolized to produce ATP, the main energy currency of the cell. Therefore, the correct answer is pyruvate.

Ornithine transcarbamoylase is an enzyme involved in the urea cycle. It catalyzes the formation of citrulline from ornithine and another reactant. The urea cycle is a series of biochemical reactions that occur in the liver and kidneys to remove toxic ammonia from the body. Ornithine transcarbamoylase specifically helps in the conversion of ornithine, an amino acid, into citrulline, which is further metabolized in the urea cycle to produce urea. This enzyme plays a crucial role in the detoxification of ammonia and the production of urea.

Glutamic acid can be directly converted into a citric acid cycle intermediate by transamination. Transamination is a process in which an amino group is transferred from one amino acid to a keto acid, forming a new amino acid and a new keto acid. Glutamic acid can donate its amino group to form α-ketoglutarate, which is an intermediate in the citric acid cycle. Therefore, glutamic acid can directly enter the citric acid cycle through transamination.

Isoleucine and tyrosine are both ketogenic and glucogenic amino acids.

Phenylketonuria (PKU) is a genetic disease that occurs due to the inability to convert phenylalanine to tyrosine. Phenylalanine is an amino acid that is normally converted to tyrosine by the enzyme phenylalanine hydroxylase. However, individuals with PKU lack this enzyme or have a defective form of it, leading to a buildup of phenylalanine in the body. This buildup can cause intellectual disability, developmental delays, and other health problems. Therefore, the inability to convert phenylalanine to tyrosine is the correct explanation for PKU.

Aspartate directly donates a nitrogen atom for the formation of urea during the urea cycle. The urea cycle is a series of biochemical reactions that occur in the liver, and it is responsible for removing toxic ammonia from the body. Aspartate is an amino acid that plays a crucial role in this process by providing a nitrogen atom that is incorporated into urea. This nitrogen atom comes from the amino group of aspartate, which is then transferred to other molecules in the urea cycle to ultimately form urea.

Serine or cysteine can be converted to pyruvate through a series of enzymatic reactions. Pyruvate is an intermediate molecule in the citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid cycle. It is formed from the breakdown of glucose during glycolysis and serves as a substrate for the citric acid cycle. Therefore, serine or cysteine can be metabolized to pyruvate and then enter the citric acid cycle as acetyl-CoA.

Glutamate is metabolically converted to alpha-ketoglutarate and NH4+ through the process of oxidative deamination. This process involves the removal of an amine group from glutamate, resulting in the formation of alpha-ketoglutarate and the release of ammonia. This process is important in the metabolism of amino acids and is catalyzed by the enzyme glutamate dehydrogenase.

Krebs' contribution to the elucidation of the pathway involved in mammalian synthesis of urea is true. The statement about the amino acid arginine being the immediate precursor to urea is also true. Additionally, the statement that the carbon atom of urea is derived from mitochondrial HCO3– is true. The precursor to one of the nitrogens of urea being aspartate is also true. However, the statement that the process of urea production is an energy-yielding series of reactions is false. The synthesis of urea requires the input of energy, specifically in the form of ATP.

Threonine is one of the essential amino acids for humans. Essential amino acids cannot be synthesized by the body and must be obtained through diet. Threonine is important for the synthesis of proteins and the production of other amino acids. It is also involved in the formation of collagen and elastin, which are important for the structure and elasticity of skin and connective tissues. Threonine is necessary for the proper functioning of the immune system and the maintenance of overall health.

The conversions that require more than one step are 4 and 5. This can be inferred because the other options only include one conversion each, while options 4 and 5 include multiple conversions.

The reaction catalyzed by glutamate dehydrogenase is not similar to transamination in that it involves the coenzyme pyridoxal phosphate (PLP).

Tetrahydrofolate (THF) and its derivatives shuttle one carbon units between different substrates. This means that THF acts as a carrier of single carbon atoms, transferring them from one molecule to another during various metabolic processes. This is important for the synthesis of important molecules such as nucleotides and amino acids.

Secretin is not a protease that acts in the small intestine. It is a hormone that is released by the duodenum in response to acidic chyme from the stomach. Secretin stimulates the pancreas to release bicarbonate ions into the small intestine to neutralize the acidity of the chyme. It does not have any proteolytic activity and does not directly break down proteins like the other enzymes listed (chymotrypsin, elastase, enteropeptidase, and trypsin).

Enteropeptidase is the enzyme that is critical in the activation of zymogens during protein digestion in the small intestine. Zymogens are inactive forms of enzymes that need to be activated in order to carry out their functions. Enteropeptidase is responsible for activating trypsinogen, which is a zymogen produced by the pancreas. Once trypsinogen is activated into trypsin by enteropeptidase, it can then activate other pancreatic zymogens, such as chymotrypsinogen and procarboxypeptidases, to further break down proteins into smaller peptides. Therefore, enteropeptidase plays a critical role in the digestion of proteins in the small intestine.

Trypsinogen is a zymogen that can be converted to an endopeptidase called trypsin. Trypsin is known for its ability to hydrolyze peptide bonds adjacent to lysine (Lys) and arginine (Arg) residues. In its inactive form, trypsinogen serves as a precursor molecule that is activated by cleavage to produce trypsin. Once activated, trypsin can cleave peptide bonds in proteins, specifically those next to Lys and Arg residues, leading to the hydrolysis of these bonds. Therefore, trypsinogen is the correct answer as it can be converted to the endopeptidase trypsin.

In maple syrup urine disease, the metabolic defect involves oxidative decarboxylation. This means that there is a problem with the process of removing a carboxyl group from a molecule through oxidation. This defect leads to the buildup of certain amino acids, specifically the branched chain amino acids, in the body. This buildup can have toxic effects and result in the characteristic symptoms of the disease, including the sweet-smelling urine that gives the condition its name.