Protein Metabolism Practice Flashcards

Comprehensive set of study flashcards covering all major concepts, pathways, enzymes, and clinical conditions in protein metabolism.

What are the three primary physiological uses of amino acids in the human body?

Amino acids are used to build proteins (e.g., collagen), serve as metabolic fuel for energy production ($ATP$), and synthesize specialized nitrogenous molecules (e.g., dopamine).

What toxic compound is generated when the amine group ($-\text{NH}_2$) is removed during nitrogen metabolism?

Ammonia ($\text{NH}_3$), which is toxic to tissues and must be converted into urea for urinary excretion.

How do storage capacities for lipids and carbohydrates differ from those for amino acids?

Lipids are stored as triglycerides in adipose tissue and carbohydrates as glycogen in the liver and skeletal muscle, whereas amino acids have no storage form in the body.

What are the daily rates of cellular protein degradation and protein synthesis in normal protein turnover?

Cellular protein degradation and protein synthesis both occur at a rate of approximately $400\,\text{g/day}$.

What formula defines Nitrogen Balance?

$$\text{Nitrogen Balance} = \text{Entries} - \text{Exits}$$

What physiological states lead to a positive nitrogen balance?

Anabolic states where entries exceed exits, including growth, tissue repair, and pregnancy.

What physiological or pathological conditions cause a negative nitrogen balance?

Catabolic states where exits exceed entries, including fasting, severe burns, tissue injuries, fever, and malabsorption.

What are the two major intracellular protein degradation systems, and how do they differ in ATP requirement?

The Ubiquitin-Proteasome system is ATP-dependent, whereas Lysosomes operate via ATP-independent pathways.

What are the primary site, $\text{pH}$, and key enzyme involved in gastric protein digestion?

Digestion begins in the stomach at $\text{pH } 1\text{--}3$ where denaturation occurs alongside enzymatic digestion by pepsin.

Which pancreatic digestive enzymes act in the small intestine, and at what $\text{pH}$ do they function?

Trypsin, chymotrypsin, carboxypeptidase, and elastase, which operate in the small intestine at $\text{pH } 6\text{--}8$.

What are zymogens, and why are digestive proteases secreted in this form?

Zymogens are inactive enzyme precursors (e.g., pepsinogen, trypsinogen, chymotrypsinogen) secreted to prevent enzymatic autodigestion of secretory tissues.

What is Celiac Disease and what dietary component triggers it?

Celiac Disease is an autoimmune disorder triggered by gluten (composed of glutelin and prolamin) in wheat, rye, and barley, causing immune-mediated mucosal damage to the small intestine.

What defines an essential amino acid, and which 10 amino acids are essential?

Essential amino acids cannot be synthesized by the body and must be provided by the diet. They are Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, and Lysine.

What are conditionally nonessential amino acids, and when do they become essential?

Amino acids (such as Cysteine, Glutamine, Glycine, Proline, and Tyrosine) that can normally be synthesized, but become essential during periods of severe illness, stress, inadequate nutrition, or in preterm infants.

From which metabolic intermediates/pathways are Glutamate, Aspartate, and Serine synthesized?

Glutamate is synthesized from $\alpha\text{-ketoglutarate}$, Aspartate from oxaloacetate (both TCA cycle intermediates), and Serine from 3-phosphoglycerate (3-PG, from glycolysis).

What vitamin cofactor is required by transaminases (amino acid transaminases)?

Vitamin $\text{B}_6$ (pyridoxal phosphate).

Which amino acid acts as the central collector of amino nitrogen during transamination reactions?

Glutamate.

How is excess nitrogen safely transported from peripheral tissues (such as muscle) to the liver via the non-alanine pathway?

Glutamate combines with free ammonia via Glutamine Synthetase to form Glutamine, which travels through the blood to the liver where Glutaminase releases the ammonia.

How is nitrogen transported from skeletal muscle to the liver via the Alanine pathway?

Transamination of pyruvate by Alanine Transaminase ($ALT$) produces Alanine, which travels through blood to the liver and is converted back to pyruvate and Glutamate by $ALT$.

What do elevated serum levels of Alanine Transaminase ($ALT$) and Aspartate Transaminase ($AST$) indicate clinically?

Serum elevation of $ALT$ and $AST$ serves as a clinical marker of liver damage.

In what two molecular forms does nitrogen enter the Urea Cycle in the liver?

As free ammonia ($\text{NH}_3$) in the mitochondria and as the amino acid Aspartate in the cytosol.

What enzyme catalyzes the rate-limiting step of the Urea Cycle, and where is it located?

Carbamoyl Phosphate Synthetase I ($CPS\text{ I}$), located in the mitochondrial matrix of hepatocytes.

What molecule is the required allosteric activator for Carbamoyl Phosphate Synthetase I ($CPS\text{ I}$)?

N-acetylglutamate ($NAG$), synthesized from Glutamate and Acetyl CoA in a reaction stimulated by Arginine.

What is the most common inherited genetic disorder of the Urea Cycle?

Ornithine Transcarbamylase ($OTC$) deficiency, an X-linked disorder.

What is Blood Urea Nitrogen ($BUN$), and what does an elevated level indicate?

$BUN$ measures the concentration of urea nitrogen in blood; high levels indicate renal (kidney) dysfunction.

What are normal blood ammonia levels compared to concentrations found in hyperammonemia?

Normal blood levels are $5\text{--}35\,\mu\text{mol/L}$, whereas hyperammonemia exhibits levels reaching up to $\sim 1000\,\mu\text{mol/L}$.

What clinical symptoms are characteristic of hyperammonemia?

Tremors, slurring of speech, drowsiness, vomiting, cerebral edema, blurred vision, coma, and death due to ammonia toxicity to the central nervous system.

Differentiate between glucogenic and ketogenic amino acids.

Glucogenic amino acids are catabolized into intermediates of glucose metabolism, while ketogenic amino acids are broken down into acetoacetate or Acetyl CoA for lipid/ketone body synthesis.

Which two amino acids are exclusively ketogenic?

Leucine and Lysine.

What enzyme converts Phenylalanine to Tyrosine, and what cofactor is required?

Phenylalanine hydroxylase, requiring tetrahydrobiopterin ($\text{BH}_4$) as a cofactor.

What toxic metabolites accumulate in Phenylketonuria ($PKU$), and what sign do they cause?

Accumulation of phenyllactate, phenylacetate, and phenylpyruvate, which leak into urine and produce a characteristic musty odor.

What dietary management is required for individuals with Phenylketonuria ($PKU$)?

Strict dietary restriction of Phenylalanine paired with Tyrosine supplementation.

Which three amino acids are categorized as Branched-Chain Amino Acids ($BCAAs$)?

Leucine, Isoleucine, and Valine.

What enzyme deficiency causes Maple Syrup Urine Disease ($MSUD$)?

Deficiency in the Branched-Chain $\alpha\text{-ketoacid}$ Dehydrogenase complex ($BCKD$).

What diagnostic urine odor is associated with Maple Syrup Urine Disease ($MSUD$), and why?

A maple syrup odor caused by toxic accumulation of Isoleucine and other $BCAAs$ along with their corresponding $\alpha\text{-ketoacids}$.

Which three amino acids donate carbon and nitrogen atoms during purine and pyrimidine nucleotide synthesis?

Glutamine, Aspartate, and Glycine.

Which amino acid is the primary precursor for Heme/Porphyrin biosynthesis?

Glycine (four Glycine residues combine with succinyl CoA).

Which two amino acids combine to form Creatine?

Glycine and Arginine.

What amino acid serves as the precursor for Catecholamines (dopamine, epinephrine, norepinephrine) and Melanin?

Tyrosine.

What condition is caused by defects in Tyrosine metabolism leading to impaired melanin synthesis?

Albinism, presenting with pale skin, hair, and eyes, sun sensitivity, and visual disturbances.