Protein Metabolism Flashcards

Comprehensive practice flashcards covering protein metabolism, nitrogen balance, digestion, zymogens, urea cycle, transamination, hyperammonemia, amino acid catabolism, and specialized nitrogenous compounds.

Amino Acid Pool

The total supply of free amino acids available throughout the body, maintained without a dedicated storage molecule (unlike lipids in adipose tissue or carbohydrates as glycogen).

Neutral Nitrogen Balance

A state of nitrogen equilibrium where total nitrogen intake (entries) equals total nitrogen excretion (exits).

Positive Nitrogen Balance

A metabolic state where nitrogen entries exceed exits, characteristic of anabolism such as growth, tissue repair, and pregnancy.

Negative Nitrogen Balance

A metabolic state where nitrogen exits exceed entries, characteristic of catabolic states such as fasting, severe burns, tissue injuries, fever, or malabsorption.

Protein Turnover

The continuous coupled process of intracellular protein degradation and synthesis, occurring at a rate of approximately $400\text{ g/day}$.

Ubiquitin-Proteasome System

An ATP-dependent intracellular pathway responsible for targeted protein degradation.

Lysosomal System

An ATP-independent cellular degradation pathway that utilizes acid hydrolases to break down proteins.

Zymogen

An inactive precursor form of an enzyme that requires biochemical modification (such as cleavage by other enzymes or pH change) to become active.

Pepsin

A stomach protease activated from pepsinogen at low pH ($1\text{--}3$) that initiates protein digestion via denaturation and cleavage into polypeptides.

Celiac Disease

An autoimmune digestive disorder triggered by gluten (glutelin and prolamin) in wheat, rye, and barley, causing damage to the small intestine.

Essential Amino Acids

The 10 amino acids that cannot be synthesized by the human body and must be provided by the diet: Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, and Lysine.

Nonessential Amino Acids

Amino acids that can be synthesized endogenously from other amino acids or metabolic intermediates: Alanine, Asparagine, Aspartate, Glutamate, and Serine.

Conditionally Nonessential Amino Acids

Amino acids that can normally be synthesized endogenously, but require dietary supplementation during illness, stress, preterm birth, or nutritional insufficiency: Cysteine, Glutamine, Glycine, Proline, and Tyrosine.

Transamination

The reversible transfer of an amino group ($\text{-NH}_2$) from an amino acid to an $\text{α}$-ketoacid, catalyzed by aminotransferases requiring Vitamin $\text{B}_6$ as a cofactor.

Glutamate

The central amino acid that collects nitrogen groups during transamination reactions, acting as the immediate source for nitrogen release or excretion pathways.

Alanine Transaminase (ALT)

An aminotransferase enzyme that converts alanine and $\text{α}$-ketoglutarate to pyruvate and glutamate; elevated blood levels serve as a specific marker for liver damage.

Aspartate Transaminase (AST)

An aminotransferase enzyme involved in nitrogen transfer between aspartate and glutamate; used clinically as a marker of tissue and liver injury.

Glutamine

A non-toxic transport molecule synthesized by glutamine synthetase that safely carries ammonia through the bloodstream from peripheral tissues to the liver.

Oxidative Deamination

The liver mitochondrial reaction catalyzed by glutamate dehydrogenase that cleaves the amino group from glutamate, releasing free toxic ammonia ($\text{NH}_3$).

Carbamoyl Phosphate Synthetase I (CPS I)

The rate-limiting mitochondrial enzyme of the urea cycle that combines free ammonia ($\text{NH}_3$) and $\text{CO}_2$ using 2 ATP; allosterically activated by N-acetylglutamate (NAG).

Ornithine Transcarbamylase (OTC)

A mitochondrial enzyme in the urea cycle that condenses carbamoyl phosphate with ornithine to yield citrulline; its genetic deficiency is the most common congenital defect of the urea cycle.

Blood Urea Nitrogen (BUN)

A clinical blood test measuring urea concentration; high BUN levels typically indicate impaired renal clearance or kidney dysfunction.

Hyperammonemia

A condition defined by pathologically elevated blood ammonia levels ($>35\text{ }μ\text{mol/L}$ up to $\text{∼}1000\text{ }μ\text{mol/L}$), causing central nervous system toxicity, tremors, slurred speech, cerebral edema, and potentially death.

Glucogenic Amino Acids

Amino acids whose carbon skeletons are degraded into metabolic intermediates (e.g., pyruvate, oxaloacetate) that can enter gluconeogenesis.

Ketogenic Amino Acids

Amino acids whose carbon skeletons are degraded into acetoacetate or precursors like acetyl CoA for ketone body and lipid synthesis (specifically Leucine and Lysine).

Phenylketonuria (PKU)

An autosomal recessive error of amino acid metabolism caused by deficiency in phenylalanine hydroxylase or its cofactor $\text{BH}_4$, leading to accumulation of toxic phenylketones, a musty urine odor, and severe mental impairment if untreated.

Maple Syrup Urine Disease (MSUD)

An autosomal recessive defect in the branched-chain $\text{α}$-ketoacid dehydrogenase (BCKD) complex, leading to accumulation of branched-chain amino acids (leucine, isoleucine, valine) and a characteristic maple syrup odor in urine.

Creatine

A specialized nitrogenous compound synthesized from glycine and arginine that forms high-energy creatine phosphate reserves in muscle and brain tissue.

Catecholamines

Specialized neurotransmitters and hormones (dopamine, norepinephrine, epinephrine) derived from tyrosine that regulate central nervous system function and fight-or-flight responses.

Melanin

A skin and eye pigment produced from tyrosine in epidermal melanocytes; metabolic defects in its pathway cause albinism.