Biochemistry Review: Lipid Metabolism, Urea Cycle, and Amino Acids (Video Notes)

Vocabulary flashcards covering lipid metabolism, the urea cycle, amino acid metabolism, and related genetic disorders.

Hormone Sensitive Lipase (HSL)

Enzyme that mobilizes stored triglycerides in adipose tissue; activated when phosphorylated and regulated by insulin, glucagon, epinephrine, and cortisol.

Glycerol metabolism in the liver

Glycerol is converted to dihydroxyacetone phosphate (DHAP), which enters gluconeogenesis to form glucose.

β-oxidation

Main fatty acid oxidation pathway in mitochondria, producing acetyl-CoA, NADH, and FADH2.

Carnitine shuttle

Transports long-chain fatty acids into mitochondria for β-oxidation; rate-limiting step in mitochondrial entry.

Link between β-oxidation and gluconeogenesis

Acetyl-CoA activates pyruvate carboxylase; ATP and NADH from β-oxidation power gluconeogenesis.

ATP yield from palmitate oxidation

Net 129 ATP: 131 produced minus 2 ATP used for activation of palmitate to palmitoyl-CoA.

Minor pathways of fatty acid oxidation

α-oxidation (phytanic acid in peroxisomes) and ω-oxidation (dicarboxylic acids in liver peroxisomes).

Carnitine deficiency

Muscle necrosis, lipid-storage myopathy, hypoketotic hypoglycemia, cardiomyopathy, hepatomegaly.

CPT-II deficiency

Muscle weakness with exercise-induced myalgia and myoglobinuria (brown urine).

CPT-I deficiency

Affects liver with impaired gluconeogenesis leading to hypoketotic hypoglycemia, coma, and death.

Essential amino acids

Nine amino acids that must be obtained from the diet; others are non-essential.

Exclusively ketogenic amino acids

Leucine and Lysine; their catabolism yields ketone bodies only.

Glucogenic amino acids

Amino acids that yield pyruvate or TCA cycle intermediates for glucose synthesis.

Ketogenic amino acids

Amino acids that yield acetyl-CoA, acetoacetyl-CoA, or ketone bodies.

Major nitrogen excretion product

Urea; produced in the liver and excreted in urine.

CPS-I activator

N-acetylglutamate; allosteric activator of carbamoyl phosphate synthetase I.

Major ammonia transport forms

Glutamine and alanine serve as the primary carriers of ammonia in blood.

Alanine vs glutamate for nitrogen transport

Alanine is used because glutamate would acidify blood; alanine is neutral and safer for transport.

Liver function tests: AST and ALT

AST (aspartate transaminase) and ALT (alanine transaminase); elevated in liver injury.

Phenylketonuria (PKU) enzyme

Phenylalanine hydroxylase (or BH4 cofactor) deficiency.

PKU metabolites

Phenylacetate, phenyllactate, phenylpyruvate (phenylketones) accumulate.

PKU clinical signs

Intellectual disability, seizures, microcephaly, hypopigmentation, musty odor.

PKU dietary restriction

Limit phenylalanine, increase tyrosine, avoid aspartame.

Albinism enzyme defect

Tyrosinase deficiency; no melanin production.

Alkaptonuria enzyme defect

Homogentisate oxidase deficiency; buildup of homogentisic acid with dark urine and ochronosis.

Homocystinuria enzyme defect

Cystathionine synthase (B6-dependent) or homocysteine methyltransferase (B12 & methyl-THF dependent).

Homocystinuria clinical features

Thrombosis, stroke, Marfanoid habitus, lens dislocation, intellectual disability.

MSUD enzyme

Branched-chain α-ketoacid dehydrogenase complex; requires TLCFN cofactors (thiamine, lipoic acid, CoA, FAD, NAD).

MSUD clinical features

Sweet-smelling urine, neurotoxicity, developmental delay; may be fatal if untreated.

Fatty acids not needing carnitine

Short- and medium-chain fatty acids (≤12 carbons) can enter mitochondria without carnitine transport.

VLCFA oxidation location

Peroxisomes; oxidation does not generate ATP and yields H2O2 via oxidase reactions.

Glutamine in nitrogen transport

Major carrier of ammonia in blood; glutaminase in kidney and intestine liberates ammonia.

Glutamate dehydrogenase function

Removes amino group from glutamate, releasing NH4+ for the urea cycle.

Cofactor for all transaminases

Vitamin B6 (pyridoxal phosphate).