Biochemistry: Protein Turnover and Amino Acid Catabolism

A comprehensive set of flashcards covering key concepts related to Chapter 23: Protein Turnover and Amino Acid Catabolism in biochemistry.

What is protein turnover?

The degradation and resynthesis of proteins in cells.

What are the essential amino acids?

Amino acids that cannot be synthesized by the body and must be acquired from the diet.

What is the primary proteolytic enzyme of the stomach?

Pepsin.

At what pH is pepsin maximally active?

pH 2.

What must happen to excess amino acids in the body?

They must be used as metabolic fuel.

What organ is primarily responsible for amino acid degradation in mammals?

The liver.

What type of molecule is ubiquitin?

A small protein (76 amino acids) that tags proteins for destruction.

What is required for ubiquitin to attach to a target protein?

ATP hydrolysis.

How many different transporters exist for absorbing amino acids in the small intestine?

At least seven.

What is a degron?

A specific sequence of amino acids that signals a protein should be degraded.

What can lead to an increase in protein aggregation diseases?

Defective E3 proteins.

What disease is associated with a defect in an E3 protein?

Angelman syndrome.

What is the role of the proteasome?

It digests ubiquitin-tagged proteins.

What components make up the 26S proteasome?

One 20S catalytic unit and two 19S regulatory units.

What is the function of the 19S regulatory unit?

Binds ubiquitin chains, unfolds polyubiquitinated proteins, and directs them into the catalytic core.

What determines the specificity of proteasome active sites?

The types of β subunits present.

How does the urea cycle relate to amino acid metabolism?

It converts excess ammonium ions into urea for excretion.

What is the significance of glutamate dehydrogenase?

It catalyzes the conversion of glutamate into ammonium ions.

What is pyridoxal phosphate's role in amino acid metabolism?

It is a coenzyme required for aminotransferase reactions.

Which amino acid is directly converted into pyruvate during metabolism?

Alanine.

What is the connection between aspartate and oxaloacetate?

Aspartate is transaminated to form oxaloacetate.

What metabolic intermediate is formed from branched-chain amino acids?

Acetyl CoA, acetoacetate, or propionyl CoA.

What are the products of phenylalanine degradation?

Tyrosine, acetoacetate, and fumarate.

What is the effect of defects in the urea cycle?

Elevated levels of NH4+ leading to hyperammonemia.

What is the disease resulting from a deficiency of phenylalanine hydroxylase?

Phenylketonuria.

How does the body excrete excess nitrogen?

As urea through the urea cycle.

How do branched-chain ketoaciduria symptoms manifest?

Urine has a maple syrup odor due to accumulated ketoacids.

What are two ways the body can dispose of excess nitrogen?

Via urea (ureotelic) or as ammonia (ammoniotelic) in aquatic organisms.

What is the fate of the carbon skeleton from amino acids?

They can be converted into major metabolic intermediates like glucose or enter the citric acid cycle.

What amino acid can only yield acetyl CoA when degraded?

Leucine.

What is unique about tryptophan degradation?

It requires multiple oxygenases and multiple steps.

What is the final product of the cleavage of arginine in the urea cycle?

Urea and ornithine.

Which enzyme catalyzes the initial step of the urea cycle?

Carbamoyl phosphate synthetase I.

What effect do high phenylalanine levels have on amino acid transport?

They inhibit the transport of tyrosine and tryptophan into the brain.

What is required for methionine degradation to proceed?

The formation of S-adenosylmethionine (SAM).

What is a common intermediary formed from threonine degradation?

α-Ketobutyrate.

How are muscle tissues involved in nitrogen transport?

Muscles transport nitrogen to the liver as alanine via the glucose-alanine cycle.

How do the branched-chain amino acids enter metabolic pathways?

Through conversion into succinyl CoA or other CoA derivatives.

What role does isovaleryl CoA dehydrogenase play in metabolism?

It catalyzes the conversion of isovaleryl CoA during branched-chain amino acid degradation.

Which enzyme hydrolyzes arginine to yield urea?

Arginase.

What process is linked with argininosuccinate in the urea cycle?

The synthesis of urea.

What condition results from excessive levels of NH4+ in the blood?

Hyperammonemia.

What is a symptom of homocystinuria?

Scoliosis or muscle weakness.

What is alkaptonuria?

A condition caused by an absence of homogentisate oxidase leading to dark urine.

What dietary changes can manage argininosuccinate deficiency?

Restricting protein intake and supplementing with arginine.

What contributes to the complexity of eukaryotic proteasomes?

Different isoforms of α and β subunits provide distinct substrate specificity.

What are uricotelic organisms?

Organisms that excrete nitrogen as uric acid.

What is the product of the degradation of tyrosine?

Acetoacetate and fumarate.

How do amino acids enter metabolism from the α-ketoglutarate pathway?

Through glutamate which is deaminated to form α-ketoglutarate.

What is the role of tetrahydrobiopterin in amino acid metabolism?

It acts as a cofactor in the hydroxylation of phenylalanine.

What can excessive protein degradation provide during fasting?

Glucose and citric acid cycle intermediates.

What does the glucose-alanine cycle indicate about muscle metabolism?

Muscle degrades branched-chain amino acids for energy and transports nitrogen to the liver.

What do PLP-dependent enzymes commonly catalyze?

Decarboxylations and transaminations.

What is the relationship between pyridoxal phosphate and amino acid substrates in transamination?

PLP forms covalent Schiff-base intermediates with amino acids.

Which common amino acid can be converted directly into serine?

Glycine.

What is the significance of the hydroxyl group on the Thr residue in proteasome active sites?

It attacks the carbonyl groups of peptide bonds.

Which amino acids can be categorized as ketogenic?

Leucine and lysine.

What is the primary metabolic fate of branched-chain amino acids?

They yield acetyl CoA or propionyl CoA.

How is the degradation of aromatic amino acids initiated?

By the action of dioxygenases.

How are unusual biochemical products historically linked to disease diagnosis?

Through urine analysis.

What can high levels of ammonia lead to in the nervous system?

Disruption of neurotransmitter systems and cellular swelling.

What are the consequences of a defective urea cycle?

Increased ammonia levels leading to neurotoxicity.

What enzyme catalyzes the conversion of aspartate to argininosuccinate?

Argininosuccinate synthetase.

What is the overall process of amino acid degradation?

Amino acids are broken down into metabolic intermediates after removal of nitrogen.

What is the significance of the urea cycle to nitrogen metabolism?

It allows the safe excretion of nitrogen by converting ammonia into urea.

What major metabolic intermediates can carbon skeletons from amino acids convert into?

Pyruvate, acetyl CoA, or citric acid cycle intermediates.

How does degradation of serotonin relate to tryptophan?

Tryptophan is the precursor for serotonin, which is also degraded.

What role do branched-chain α-ketoacid dehydrogenase complexes play in protein metabolism?

They facilitate the oxidative decarboxylation of branched-chain keto acids.

What is a common treatment strategy for amino acid metabolism disorders?

Dietary modifications to restrict certain amino acids.

Which metabolic cycle involves both amino acids and carbohydrates?

The gluconeogenesis pathway.

What is the effect of injury or damage to liver cells regarding aminotransferases?

Aminotransferases leak into the blood, indicating liver damage.

What is the significance of the glucose-alanine cycle during intense exercise?

It involves muscle protein degradation to produce energy and transport nitrogen to the liver.

How are intermediary products of amino acid catabolism directed into central metabolic pathways?

By conversion into key metabolites like pyruvate and acetyl CoA.

What does the presence of alanine and aspartate aminotransferases in blood indicate?

Possible liver damage.

What role does the structure of protein play in its turnover rate?

The half-lives of proteins vary widely depending on their stability and functional necessity.

Which amino acids are known for their role in the intermediary metabolism of nitrogen?

Glutamate and aspartate.

How do enzymes regulate nitrogen assimilation and excretion?

By catalyzing specific reactions in the amino acid degradation and urea cycles.

What defines the regulatory mechanism of the urea cycle?

The balance of substrates and the activity of key enzymes such as carbamoyl phosphate synthetase I.

Which process stabilizes catalytic intermediates in PLP catalysis?

The formation of a Schiff base.

What metabolic processes are influenced by the actions of the proteasome?

Cell cycle regulation and apoptosis.

What happens to excess nitrogen in mammals?

Converted to urea for excretion.

What diseases could arise from mutations in the genes coding for the urea cycle enzymes?

Various metabolic disorders related to amino acid metabolism.

What pathway allows for the utilization of amino acids as precursors for gluconeogenesis?

The interconversion of metabolic intermediates derived from amino acids.

What process converts asparagine into aspartate?

Hydrolysis by asparaginase.

What happens to ammonium ions in most terrestrial vertebrates?

They are converted to urea for excretion.

What can the presence of elevated phenylalanine levels indicate?

Phenylketonuria or other metabolic disruption.

What dietary modifications can help manage nitrogen metabolism disorders?

Modify protein intake or supplement specific amino acids.

How does oxidative deamination of glutamate produce ammonium ions?

By removing the α-amino group in the liver.

Why is the degradation of amino acids crucial during fasting?

To provide glucose and fuel for essential metabolic processes.

How does dietary restriction of branched-chain amino acids affect maple syrup urine disease?

It prevents the buildup of toxic metabolites.

What enzyme initiates the urea cycle by catalyzing the formation of carbamoyl phosphate?

Carbamoyl phosphate synthetase I.

What mechanism immortalizes the degradation of proteins in terms of covalent modification?

Ubiquitination.

What substrates are used in the urea cycle?

Free ammonium ions and bicarbonate.

What physiological factors influence protein degradation in cells?

Damage, misfolding, or metabolic demands.

What triggers protein turnover in the cellular context?

Cellular signals and environmental changes.