Lipid Metabolism, Beta-Oxidation, and Cholesterol

Vocabulary practice flashcards covering lipid synthesis, fatty acid transport, beta-oxidation, carnitine metabolism, plaque formation, and ketogenesis based on the lecture.

Cholesterol ester

The safe, non-free storage form of cholesterol created by attaching cholesterol to a fatty acid, predominantly stored within the liver.

HMG-CoA reductase

The rate-limiting enzyme in endogenous cholesterol synthesis that catalyzes production from acetyl-CoA; it is downregulated by free cholesterol and inhibited by statins.

Statins

A class of pharmaceutical drugs designed to lower cholesterol synthesis by blocking the rate-limiting enzyme HMG-CoA reductase.

Lipoprotein lipase

An enzyme produced by tissues that cleaves and removes free fatty acids from circulating triglycerides in lipoproteins (such as chylomicrons), allowing uptake into adipose and muscle cells.

Hormone-sensitive lipase (HSL)

An enzyme activated by phosphorylation via AMP-dependent protein kinase that hydrolyzes stored triglycerides preferentially at the 1 and 3 positions during fasting or stress.

Albumin

An abundant circulating blood protein possessing two specific binding sites and multiple nonspecific sites that weakly and reversibly transports free fatty acids in the blood.

Glycerol kinase

An enzyme absent in adipose tissue, preventing fat cells from utilizing glycerol for energy and necessitating its release into the bloodstream for transport to the liver.

Acyl-CoA synthetase

An enzyme that activates a fatty acid in the cytoplasm into fatty acyl-CoA, utilizing ATP and converting it into AMP and pyrophosphate ($PP_i$).

Carnitine

A compound synthesized in the liver and kidney from lysine and methionine that is essential for shuttling fatty acyl-CoA across the inner mitochondrial membrane into the matrix.

L-carnitine

The physiologically active isomer of carnitine required for fatty acid transport; in contrast, the inactive non-physiological D-form acts as an inhibitor.

Beta-oxidation

The mitochondrial pathway that sequentially removes two-carbon units from a fatty acyl-CoA chain, generating acetyl-CoA along with high-energy electron carriers $FADH_2$ and NADH.

Acyl-CoA dehydrogenase

The initial enzyme of mitochondrial beta-oxidation that oxidizes fatty acyl-CoA to form a carbon-carbon double bond and reduce $FAD$ to $FADH_2$, possessing distinct isoforms specific to chain length.

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency

The most common autosomal recessive inborn error of fatty acid oxidation (occurring in approximately 1 in 12,000 births), leading to hypoketotic hypoglycemia and liver dysfunction triggered by fasting or illness.

Plant sterols

Steroid compounds found in plant cell membranes (such as sitostanol and campesterol) that substitute for cholesterol; their poor retention and active excretion by humans also promotes cholesterol excretion.

High-density lipoprotein (HDL)

A protective lipoprotein that inhibits monocyte binding to the endothelium, prevents the oxidation of LDL, and mediates reverse cholesterol transport.

Foam cells

Subendothelial macrophages that have engulfed large amounts of oxidized and modified LDL particles, forming the core cellular component of atherosclerotic plaque.

Ketone bodies

Water-soluble alternative fuel molecules synthesized in the liver from excess acetyl-CoA during carbohydrate depletion, starvation, or diabetes, consisting of acetoacetate, 3-hydroxybutyrate, and acetone.

Acetoacetate

A ketone body synthesized from acetyl-CoA in the liver that circulates to peripheral tissues and the brain to be reconverted into acetyl-CoA for the TCA cycle, or spontaneously decarboxylated into acetone.

3-Hydroxybutyrate

A circulating ketone body formed by the reduction of acetoacetate that can cross the blood-brain barrier and serve as an energy substrate in peripheral tissues.

Acetone

A volatile, three-carbon ketone body produced during ketogenesis that is poorly oxidized for energy and is largely eliminated via the breath and urine.