Metabolic Pathways for Lipids and Amino Acids – Key Vocabulary

Vocabulary flashcards summarizing fundamental terms and enzymes involved in lipid and amino-acid metabolism, digestion, synthesis, and associated health conditions.

Triacylglycerol (TAG)

A fat molecule composed of glycerol esterified with three fatty acids; major form of stored energy in adipose tissue.

Adipocyte

Specialized fat cell that stores unlimited quantities of triacylglycerols in adipose tissue.

Bile Salts

Amphipathic compounds that emulsify dietary fats in the small intestine, forming micelles.

Micelle

Small lipid droplet formed after bile-salt emulsification, increasing fat surface area for enzyme action.

Pancreatic Lipase

Digestive enzyme that hydrolyzes ester bonds of triacylglycerols, producing monoacylglycerols and fatty acids.

Chylomicron

Lipoprotein particle of triglycerides coated with phospholipids and proteins that transports dietary lipids through the bloodstream.

Lipase (Hormone-Sensitive)

Adipose-cell enzyme activated by glucagon or epinephrine that hydrolyzes stored TAGs to glycerol and free fatty acids.

Acetyl CoA

Two-carbon thioester intermediate central to metabolism; produced from fatty acid β-oxidation and enters the citric acid cycle.

β-Oxidation

Mitochondrial pathway that shortens fatty acyl CoA two carbons at a time, producing acetyl CoA, NADH, and FADH₂.

Fatty Acid Activation

ATP-driven formation of fatty acyl CoA from a free fatty acid and coenzyme A in the cytosol.

Carnitine Shuttle

Transport system that moves long-chain fatty acyl groups across the inner mitochondrial membrane as fatty acyl-carnitine.

Acyl CoA Dehydrogenase

First β-oxidation enzyme; catalyzes FAD-dependent oxidation forming a trans double bond between α- and β-carbons.

Enoyl CoA Hydratase

β-Oxidation enzyme that hydrates the trans double bond, adding –OH to the β-carbon.

3-Hydroxyacyl CoA Dehydrogenase

Oxidizes the β-hydroxyl group to a keto group, generating NADH during β-oxidation.

Thiolase

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

Unsaturated Fatty Acid Oxidation

β-Oxidation of fats containing cis double bonds; requires an isomerase and yields slightly less ATP because one FADH₂ step is bypassed.

Ketone Bodies

Energy molecules—acetoacetate, β-hydroxybutyrate, and acetone—formed from excess acetyl CoA during carbohydrate deprivation.

Ketogenesis

Liver pathway that condenses acetyl CoA into ketone bodies when fat breakdown exceeds citric-cycle capacity.

Ketosis

Condition of elevated ketone bodies that can lower blood pH (acidosis) during diabetes, fasting, or high-fat diets.

Lipogenesis (Fatty Acid Synthesis)

Cytosolic anabolic pathway that converts acetyl CoA into palmitate using NADPH and acyl carrier protein.

Acyl Carrier Protein (ACP)

Phosphopantetheine-containing protein that temporarily holds growing acyl chains during fatty acid synthesis.

Malonyl CoA

Three-carbon activated donor produced from acetyl CoA and bicarbonate; supplies two-carbon units for fatty acid elongation.

Transamination

Reversible transfer of an amino group from an amino acid to an α-keto acid, forming a new amino acid.

Aminotransferase (Transaminase)

Enzyme class that catalyzes transamination reactions, often using pyridoxal phosphate (PLP) as coenzyme.

Oxidative Deamination

Removal of the amino group from glutamate as NH₄⁺ with concurrent NAD(P)+ reduction, regenerating α-ketoglutarate.

Urea Cycle

Liver pathway that converts toxic ammonium ions to urea for excretion; operates across mitochondria and cytosol.

Carbamoyl Phosphate

High-energy intermediate formed from NH₄⁺, CO₂, and 2 ATP; first committed step of the urea cycle.

Ornithine Transcarbamylase

Mitochondrial enzyme transferring the carbamoyl group to ornithine, forming citrulline in the urea cycle.

Argininosuccinate Synthetase

Cytosolic enzyme that condenses citrulline with aspartate, using ATP, to form argininosuccinate.

Arginase

Final urea-cycle enzyme hydrolyzing arginine to urea and ornithine.

Glucogenic Amino Acid

Amino acid whose carbon skeleton can yield pyruvate, oxaloacetate, fumarate, or succinyl CoA for glucose synthesis.

Ketogenic Amino Acid

Amino acid degraded to acetyl CoA or acetoacetyl CoA, precursors of ketone bodies or fatty acids.

Essential Amino Acid

Amino acid that cannot be synthesized by humans in sufficient amounts and must be obtained through diet.

Nonessential Amino Acid

Amino acid that the human body can synthesize de novo from metabolic intermediates.

Phenylketonuria (PKU)

Genetic disorder lacking phenylalanine hydroxylase, leading to accumulation of phenylalanine metabolites and intellectual disability if untreated.