Protein, Carbohydrate, and Lipid Metabolism

Comprehensive vocabulary flashcards covering key enzymes, pathways, concepts, and disorders from protein, carbohydrate, and lipid metabolism lectures.

Nitrogen Balance

The difference between nitrogen entries and exits in the body's amino acid pool, which can be neutral ($\text{Entries} = \text{Exits}$), positive ($\text{Entries} > \text{Exits}$ during growth, pregnancy, or tissue repair), or negative ($\text{Exits} > \text{Entries}$ during fasting, injury, or malnutrition).

Protein Turnover

The continuous physiological balance between the degradation and synthesis of endogenous cellular proteins, where degradation occurs via the ATP-dependent ubiquitin-proteasome pathway or ATP-independent lysosomes.

Zymogen

An inactive precursor form of an enzyme that requires proteolytic cleavage for activation, triggered by environmental factors such as low $\text{pH}$ or specific cleavage enzymes (e.g., pepsinogen to pepsin, trypsinogen to trypsin).

Essential Amino Acids

Amino acids that cannot be synthesized by human cells or metabolic intermediates and must be supplied through the diet, such as phenylalanine, valine, and leucine.

Conditionally Nonessential Amino Acids

Amino acids that can normally be synthesized endogenously but must be supplied by the diet during specific physiological or pathological states such as malnutrition, illness, or prematurity (e.g., tyrosine).

Transamination

An enzymatic reaction catalyzed by transaminases requiring Vitamin $B_6$ that transfers an amine group ($-\text{NH}_2$) from an amino acid to an $\alpha$-ketoacid (typically $\alpha$-ketoglutarate, forming glutamate).

Carbamoyl Phosphate Synthetase I (CPS I)

The rate-limiting mitochondrial enzyme of the Urea cycle that combines ammonia ($\text{NH}_3$) with carbon dioxide ($\text{CO}_2$) to form carbamoyl phosphate; it is allosterically activated by N-acetylglutamate (NAG).

Ornithine Transcarbamylase (OTC)

A mitochondrial enzyme in the Urea cycle that combines carbamoyl phosphate with ornithine to form citrulline; it is the most commonly mutated enzyme in congenital urea cycle disorders.

Blood Urea Nitrogen (BUN)

A clinical measurement of urea levels in the blood, synthesized by the liver and excreted by the kidneys, used primarily to detect and diagnose renal dysfunction.

Hyperammonemia

A pathological state characterized by elevated blood ammonia levels above normal ($5\text{--}35\text{ }\frac{\boldsymbol{\nu}\text{mol}}{\text{L}}$ or $5\text{--}35\text{ }\frac{\text{mmol}}{\text{L}}$) due to liver dysfunction or congenital urea cycle enzyme defects, leading to central nervous system damage.

Phenylketonuria (PKU)

A congenital metabolic disorder ($1:15{,}000$ births) caused by genetic defects in phenylalanine hydroxylase, leading to the accumulation of neurotoxic phenylalanine derivatives such as phenyllactate, phenylacetate, and phenylpyruvate.

Maple Syrup Urine Disease (MSUD)

A congenital disorder caused by defects in the Branched-Chain Ketoacid Dehydrogenase (BCKD) enzyme complex, causing toxic buildup of branched-chain amino acids (leucine, isoleucine, valine) and their $\alpha$-ketoacids.

Anomeric Carbon

The carbon atom in a cyclized monosaccharide (carbon-1 in aldoses) bonded to two oxygen atoms that determines the $\alpha$ or $\beta$ configuration of glycosidic bonds.

SGLT1

A sodium-dependent transport protein on the apical membrane of enterocytes that absorbs glucose and galactose via secondary active transport driven by the sodium gradient established by $\text{Na}^+/\text{K}^+$ ATPase.

Glycogenin

An autoglucosylating core protein that serves as a primer for glycogen synthesis by accepting UDP-glucose monomers at its tyrosine-194 side chain.

Glycogen Synthase

The rate-limiting enzyme of glycogenesis that catalyzes the elongation of glycogen chains by adding glucose units from UDP-glucose to form $\alpha(1 \rightarrow 4)$ glycosidic bonds at nonreducing ends.

Branching Enzyme (4:6 Transferase)

An enzyme (amylo-$\alpha(1 \rightarrow 4) \rightarrow \text{a}(1 \rightarrow 6)$-transglycosylase) that transfers $6\text{--}8$ glucosyl residues from a nonreducing end to create an $\alpha(1 \rightarrow 6)$ branch linkage on a non-terminal residue.

Glycogen Phosphorylase

The enzyme of glycogenolysis requiring pyridoxal phosphate (PLP / Vitamin $B_6$) that cleaves $\alpha(1 \rightarrow 4)$ glycosidic bonds to release glucose 1-phosphate ($\text{G1P}$).

Debranching Enzyme

A bifunctional enzyme containing glucanotransferase activity (moving outer $3$ glucosyl residues to the base chain) and amylo-$\alpha(1,6)$-glucosidase activity (cleaving the remaining branch point glucose).

Von Gierke Disease

A glycogen storage deficiency caused by a lack of glucose 6-phosphatase in the liver and kidneys, presenting with fasting hypoglycemia, hepatomegaly, renomegaly, lactic acidosis, and periodontitis.

McArdle Disease

A tissue-specific glycogen storage disorder caused by muscle glycogen phosphorylase deficiency, characterized by exercise-induced muscle weakness, cramping, and retinopathy.

Von Ebner's Glands

Serous exocrine glands located near the papillae of the tongue that synthesize and secrete lingual lipase to initiate lipid hydrolysis in the mouth and stomach.

Cholecystokinin (CCK)

A peptide hormone released by intestinal enterocytes in response to dietary lipids entering the duodenum, signaling gallbladder contraction for bile release and stimulating pancreatic lipase secretion.

Secretin

An intestinal hormone secreted in response to low $\text{pH}$ acidic chyme, stimulating pancreatic secretion of bicarbonate to neutralize intestinal digestive $\text{pH}$ to $6\text{--}8$.

Mixed Micelles

Spherical micro-aggregates of digested fatty acids, 2-monoacylglycerols, cholesterol, fat-soluble vitamins, and amphipathic bile salts that allow lipids to penetrate the unstirred intestinal water layer.

Steatorrhea

The clinical excretion of excessive undigested dietary fat in feces caused by lipid malabsorption, leading to abdominal cramping and secondary deficiencies of fat-soluble vitamins ($\text{A}$, $\text{D}$, $\text{E}$, $\text{K}$).

Chylomicrons

Large lipoproteins formed in intestinal enterocytes that package exogenous (dietary) triacylglycerols to deliver fatty acids to skeletal muscle, cardiac tissue, and adipose tissue.

Low-Density Lipoprotein (LDL)

A cholesterol-rich lipoprotein generated from VLDL in the circulation that serves as the primary transport vehicle delivering cholesterol esters to peripheral tissues via receptor-mediated endocytosis.

High-Density Lipoprotein (HDL)

A protective lipoprotein secreted by the liver and intestine that performs Reverse Cholesterol Transport by removing excess cholesterol from peripheral tissues and returning it to the liver.

$\beta$-Oxidation

A mitochondrial catabolic pathway that reduces fatty acid carbon chains by $2$ carbons per cycle, generating $1 \times \text{FADH}_2$, $1 \times \text{NADH}$, and $1$ acetyl-CoA molecule for ATP production.

HMG-CoA Reductase

The rate-limiting committed enzyme of hepatic cholesterol biosynthesis that reduces HMG-CoA to mevalonate; it is activated by insulin, inhibited by glucagon/cholesterol, and targeted by statins.

Statins

A class of lipid-lowering pharmacological agents (e.g., pravastatin, atorvastatin) that competitively inhibit HMG-CoA reductase by mimicking HMG-CoA, halting mevalonate and endogenous cholesterol synthesis.

Corneal Arcus

A gray, white, or blue ring around the corneal periphery caused by extracellular cholesterol and phospholipid deposition; its presence in individuals under $40$ years of age indicates familial hypercholesterolemia.

Ketogenesis

The metabolic process in hepatic mitochondria where excess acetyl-CoA generated during starvation is converted into acetoacetate, $\beta$-hydroxybutyrate, and acetone for peripheral tissue fuel.

Ketoacidosis

A severe metabolic condition caused by excessive accumulation of ketone bodies, leading to decreased blood $\text{pH}$, urinary excretion of ketones causing dehydration, and a characteristic fruity breath odor from acetone.