Biochemistry · Short notes

Lipid and Protein Metabolism: Short Notes & Cheat Sheet

Fatty acid oxidation and synthesis, ketone bodies, cholesterol and lipoproteins, amino acid metabolism, the urea cycle and inborn errors.

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Short notes

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Lipoproteins and what they carry

From chylomicron to HDL, particles become smaller and denser as triglyceride is removed.

Fatty acids and ketones

β-oxidation occurs in mitochondria. Fatty acids enter via the carnitine shuttle; CPT-I is rate-limiting and is inhibited by malonyl-CoA.
Each β-oxidation cycle yields 1 FADH₂, 1 NADH and 1 acetyl-CoA; one palmitate gives about 106 ATP.
Fatty acid synthesis occurs in the cytosol. Acetyl-CoA carboxylase (needs biotin) is rate-limiting, NADPH is required and the product is palmitate.
Ketone bodies (acetoacetate, β-hydroxybutyrate, acetone) are made in liver mitochondria in fasting and DKA. The liver cannot use them; brain, heart and muscle can.
Essential fatty acids are linoleic acid (omega-6) and α-linolenic acid (omega-3).

Cholesterol and lipoproteins

HMG-CoA reductase is the rate-limiting step of cholesterol synthesis and is the target of statins.
Chylomicrons carry dietary triglyceride; VLDL carries triglyceride made in the liver; LDL delivers cholesterol to tissues; HDL returns cholesterol to the liver.
Apolipoproteins: ApoB-48 (chylomicrons), ApoB-100 (VLDL and LDL; binds the LDL receptor), ApoA-I (HDL; activates LCAT), ApoC-II (activates lipoprotein lipase) and ApoE (remnant uptake).
Lipid storage diseases: Gaucher (glucocerebrosidase), Tay–Sachs (hexosaminidase A), Niemann–Pick (sphingomyelinase).

Amino acids and the urea cycle

Essential amino acids: phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, leucine and lysine (arginine is semi-essential).
Transamination (ALT, AST; needs PLP from vitamin B6) moves amino groups; glutamate dehydrogenase releases ammonia.
The urea cycle in the liver turns toxic ammonia into urea. Carbamoyl phosphate synthetase I is rate-limiting and is activated by N-acetylglutamate. Making one urea costs 3 ATP.
Leucine and lysine are purely ketogenic; most other amino acids are glucogenic.
Amino acid products: tyrosine → catecholamines, thyroid hormones, melanin; tryptophan → serotonin, melatonin, niacin; histidine → histamine; arginine → nitric oxide; glutamate → GABA.
Inborn errors: phenylketonuria (phenylalanine hydroxylase; low-phenylalanine diet, avoid aspartame), alkaptonuria (dark urine), albinism (tyrosinase), maple syrup urine disease, homocystinuria.

Cheat sheet

High-yield facts in one place. Cover the right-hand column and test yourself.

Lipoproteins

Lipoprotein	Main job · apolipoprotein
Chylomicron	Dietary TG from gut · ApoB-48
VLDL	Liver TG to tissues · ApoB-100
IDL	VLDL remnant · ApoB-100, ApoE
LDL	Cholesterol to tissues · ApoB-100
HDL	Reverse cholesterol transport · ApoA-I

Inborn errors

Disorder	Deficient enzyme
Phenylketonuria	Phenylalanine hydroxylase
Alkaptonuria	Homogentisate oxidase
Albinism	Tyrosinase
Maple syrup urine disease	Branched-chain keto acid dehydrogenase
Homocystinuria	Cystathionine β-synthase
Gaucher disease	Glucocerebrosidase
Tay–Sachs disease	Hexosaminidase A

Memory aids

PVT TIM HaLL: Essential amino acids: Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Leucine, Lysine.

Exam traps

The liver makes ketone bodies but cannot use them.
Fatty acid synthesis is in the cytosol; β-oxidation is in mitochondria.
Leucine and lysine are the only purely ketogenic amino acids.
People with phenylketonuria must avoid aspartame, which contains phenylalanine.

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