Lipid Metabolism – Vocabulary Flashcards

Vocabulary flashcards summarizing key terms, enzymes, pathways, and regulatory molecules involved in lipoprotein transport, triacylglycerol metabolism, fatty-acid synthesis and oxidation, ketone body formation, and cholesterol biosynthesis.

Chylomicron

Largest lipoprotein particle; transports dietary triacylglycerols and cholesterol from intestine to peripheral tissues (exogenous pathway).

Chylomicron Remnant

Particle left after most triacylglycerols are removed from a chylomicron; taken up by liver via ApoE-mediated receptors.

Very-Low-Density Lipoprotein (VLDL)

Liver-derived particle that exports endogenous triacylglycerols to peripheral tissues; contains ApoB-100.

Intermediate-Density Lipoprotein (IDL)

Transient particle formed after VLDL loses triacylglycerols; can be taken up by liver or converted to LDL.

Low-Density Lipoprotein (LDL)

Cholesterol-rich lipoprotein delivering cholesterol to extra-hepatic tissues via LDL receptor recognition of ApoB-100.

High-Density Lipoprotein (HDL)

Small, dense particle that collects excess cholesterol from tissues and returns it to the liver (reverse cholesterol transport).

Apolipoprotein B-48 (ApoB-48)

Structural apolipoprotein synthesized in intestine; unique to chylomicrons and their remnants.

Apolipoprotein B-100 (ApoB-100)

Structural apolipoprotein of VLDL, IDL, and LDL; ligand for LDL receptor.

Apolipoprotein E (ApoE)

Apo that mediates hepatic uptake of chylomicron remnants and IDL via ApoE receptors.

Apolipoprotein C-II (ApoC-II)

Cofactor that activates lipoprotein lipase, enabling hydrolysis of plasma triacylglycerols.

Lipoprotein Lipase (LPL)

Capillary enzyme that hydrolyzes triacylglycerols in chylomicrons and VLDL, releasing free fatty acids to tissues.

LCAT (Lecithin–Cholesterol Acyltransferase)

Plasma enzyme that esterifies cholesterol on HDL, allowing core storage of cholesteryl esters.

Exogenous Lipid Pathway

Metabolic route by which dietary lipids travel from intestine to tissues via chylomicrons.

Endogenous Lipid Pathway

Route by which lipids synthesized in the liver are distributed through VLDL → IDL → LDL sequence.

LDL Receptor (LDLR)

Cell-surface receptor that binds ApoB-100 / ApoE and internalizes LDL via clathrin-mediated endocytosis.

Cholesteryl Ester

Esterified form of cholesterol stored in lipoprotein cores or intracellular lipid droplets.

Glycerol-3-Phosphate

Three-carbon backbone precursor for triacylglycerol synthesis; derived from glycolysis (DHAP) or glycerol kinase (liver).

Acyl-CoA Synthetase (Thiokinase)

Enzyme that activates fatty acids to acyl-CoA using ATP and CoA; first step in β-oxidation and TG synthesis.

Phosphatidic Acid

Triacylglycerol precursor composed of glycerol-3-phosphate esterified with two fatty acids.

Triacylglycerol (Triglyceride) Biosynthesis

Pathway converting glycerol-3-phosphate and three acyl-CoAs into triacylglycerol for storage or VLDL export.

Hormone-Sensitive Lipase (HSL)

Cytosolic enzyme in adipocytes that hydrolyzes stored triacylglycerol; activated by phosphorylation via PKA.

Protein Kinase A (PKA)

cAMP-dependent kinase that phosphorylates HSL and perilipin, stimulating lipolysis during fasting/exercise.

β-Oxidation

Mitochondrial pathway that degrades acyl-CoA into acetyl-CoA, FADH₂, and NADH by repeating four-step cycles.

Carnitine Shuttle

Transport system using carnitine acyltransferases I & II to move long-chain acyl-CoA into mitochondria for β-oxidation.

Acyl-CoA Dehydrogenase

First enzyme of each β-oxidation cycle; introduces trans double bond and produces FADH₂.

Thiolase

β-Oxidation enzyme that cleaves β-ketoacyl-CoA, releasing acetyl-CoA and a shortened acyl-CoA.

Ketone Bodies

Acetoacetate, β-hydroxybutyrate, and acetone produced in liver mitochondria from excess acetyl-CoA during fasting.

Acetyl-CoA Carboxylase (ACC)

Rate-limiting enzyme of fatty-acid synthesis converting acetyl-CoA to malonyl-CoA; activated by citrate, inhibited by phosphorylation.

Malonyl-CoA

Three-carbon donor generated by ACC; supplies two-carbon units and inhibits carnitine palmitoyltransferase-I.

Fatty Acid Synthase (FAS)

Multi-enzyme complex that elongates acyl chain by successive addition of malonyl-CoA units, producing palmitate (C16).

Citrate Shuttle

System transporting acetyl-CoA from mitochondria to cytosol as citrate for fatty-acid synthesis.

HMG-CoA Reductase

Rate-limiting enzyme of cholesterol biosynthesis converting HMG-CoA to mevalonate; target of statins.

Mevalonate

Six-carbon product of HMG-CoA reduction; precursor to isoprenoid units in cholesterol synthesis.

Isopentenyl Pyrophosphate (IPP)

Activated five-carbon isoprene building block derived from mevalonate in cholesterol biosynthesis.

Squalene

30-carbon linear compound formed by condensation of six isoprene units; precursor to sterol skeleton.

Lanosterol

First sterol formed from squalene epoxide cyclization; undergoes multiple reactions to yield cholesterol.

Hormonal Regulation of Lipid Metabolism

Insulin promotes fatty-acid synthesis (activates ACC); glucagon/epinephrine stimulate lipolysis and inhibit ACC via cAMP-PKA phosphorylation.

cAMP

Second messenger produced by adenylate cyclase; activates PKA in response to glucagon or epinephrine.

β-Hydroxy-β-Methylglutaryl-CoA (HMG-CoA)

Intermediate in both cholesterol biosynthesis (cytosol) and ketone body production (mitochondria).

Ketoacidosis

Pathological accumulation of ketone bodies lowering blood pH, seen in uncontrolled diabetes and prolonged fasting.

LDLR-Mediated Endocytosis

Process where LDL binds LDL receptor, is internalized, fused with lysosome, releasing cholesterol for cellular use.