Biosynthesis of Steroids and Amino Acid Catabolism

Isoprene

Five-carbon building block for steroid synthesis.

Cholesterol

A sterol synthesized from isoprene units.

Mevalonate

Intermediate formed from acetyl-CoA in cholesterol biosynthesis.

HMG-CoA

Precursor to mevalonate in cholesterol synthesis.

HMG-CoA reductase

Key enzyme targeted by cholesterol-lowering drugs.

Squalene

30-carbon linear precursor to cholesterol.

Geranyl pyrophosphate

10-carbon intermediate formed from isoprenes.

Farnesyl pyrophosphate

15-carbon intermediate formed from two isoprenes.

Squalene monooxygenase

Enzyme that converts squalene to squalene epoxide.

Lanosterol

Cyclization product of squalene in animals.

Ergosterol

Cyclization product of squalene in plants.

Bile acids

Cholesterol derivatives that emulsify fats.

Taurocholic acid

Bile acid that aids in fat digestion.

Hypercholesterolemia

Condition of elevated cholesterol levels in blood.

LDL-cholesterol

Low-density lipoprotein associated with heart disease.

HDL-cholesterol

High-density lipoprotein protective against heart disease.

Atherosclerosis

Plaque buildup in arteries linked to high LDL.

Statins

Drugs that inhibit HMG-CoA reductase activity.

Cholesteryl esters

Cholesterol molecules esterified with fatty acids.

Biliary cholesterol

Cholesterol secreted into bile for fat digestion.

Isopentenyl pyrophosphate (IPP)

Activated isoprene intermediate in cholesterol biosynthesis.

Dimethylallyl pyrophosphate (DMAPP)

Another activated isoprene involved in synthesis.

Nitrogen Fixation

Conversion of N2 to NH3 by bacteria.

Urea Cycle

Process converting ammonia to urea for excretion.

Nitrogen Importance

Essential for nucleic acids and proteins.

Cofactors

Molecules aiding enzyme function, e.g., NAD, FAD.

Phenylketonuria (PKU)

Genetic disorder affecting phenylalanine metabolism.

Oxidation States of Nitrogen

Different forms of nitrogen with varying oxidation levels.

Nitrate

NO3-, highest oxidation state of nitrogen.

Nitrite

NO2-, intermediate oxidation state of nitrogen.

Ammonia

NH3, lowest oxidation state of nitrogen.

Nitrogen Cycle

Biochemical transformations of nitrogen in ecosystems.

Nitrification

Conversion of ammonia to nitrite and nitrate.

Assimilation

Incorporation of nitrogen into organic compounds.

Denitrification

Reduction of nitrate to N2 under anaerobic conditions.

Nitrogenase Complex

Enzyme complex facilitating nitrogen fixation.

Exergonic Reaction

Reaction releasing energy, e.g., nitrogen fixation.

ATP Consumption

About 16 ATP used per nitrogen molecule fixed.

Dinitrogenase Reductase

Enzyme transferring electrons in nitrogen fixation.

Dinitrogenase

Enzyme catalyzing reduction of nitrogen to ammonia.

Leghemoglobin

Protein binding O2 in nitrogen-fixing bacteria.

Ferredoxin

Electron carrier in nitrogen fixation process.

Flavodoxin

Alternative electron carrier in nitrogen fixation.

Mo-FeS Cage

Structure coordinating nitrogen in nitrogenase complex.

NH3 Production

Excess ammonia released to soil by organisms.

Non-essential Amino Acids

Humans synthesize 11 amino acids internally.

Essential Amino Acids

Nine amino acids must be obtained from diet.

Amino Acid Synthesis

Bacteria and plants synthesize all 20 amino acids.

Nitrogen Sources

Glutamate or glutamine provide nitrogen for synthesis.

Metabolic Precursors

Seven precursors form scaffolds for amino acid synthesis.

Amino Acid Oxidation

Carnivores meet 90% energy needs from amino acids.

Microbial Fuel Source

Microorganisms scavenge amino acids for energy.

Herbivore Energy Needs

Amino acids meet only small fraction of energy.

Plant Amino Acid Use

Plants degrade amino acids for other metabolites.

Protein Hydrolysis

Dietary proteins are broken down into amino acids.

Pepsin Function

Pepsin cuts proteins into peptides in the stomach.

Trypsin and Chymotrypsin

These enzymes further digest peptides in the intestine.

Peptide Degradation

Aminopeptidase and carboxypeptidases convert peptides to amino acids.

Gastric Gland Secretions

HCl, pepsinogen, and gastrin are secreted in stomach.

Pancreatic Enzyme Synthesis

Pancreas synthesizes many digestive enzymes for protein breakdown.

Zymogen Granules

Zymogens are stored in membrane-enclosed transport particles.

Amino Acid Absorption

Absorbed through epithelial cells in intestinal mucosa.

Amino Acid Catabolism

Fates include recycling or oxidation for energy.

Urea Cycle

Removes amino group during amino acid oxidation.

Nitrogen Excretion in Aquatic Vertebrates

Ammonia released via diffusion or active transport.

Urea Excretion

Terrestrial vertebrates excrete nitrogen as less toxic urea.

Uric Acid Excretion

Birds and reptiles excrete nitrogen as insoluble uric acid.

Human Nitrogen Excretion

Humans excrete both urea and uric acid.

Ammonia Toxicity

Ammonia must be kept at low blood concentrations.

Bacterial Ammonium Production

Bacteria in intestines produce significant NH4+.

Aminotransferases

Enzymes catalyzing amino group transfer.

Transamination

Transfer of an amine to a metabolite.

α-Ketoglutarate

Common acceptor for amino groups.

Glutamate

Trafficable amino acid generated from transamination.

PLP (Pyridoxal Phosphate)

Cofactor derived from vitamin B6 for aminotransferases.

Nucleophilic Attack

Mechanism linking PLP to enzyme via lysine.

Glutamate Dehydrogenase

Enzyme removing ammonia from glutamate.

Oxidative Deamination

Ammonia removal process in mitochondrial matrix.

Electron Acceptors

NAD+ or NADP+ used in deamination.

Urea

Final product of ammonia processing for excretion.

Transdeamination

Combination of transamination and oxidative deamination.

Glutamine

Ammonia carrier formed from glutamate.

Glutamine Synthetase

Enzyme catalyzing glutamine formation from glutamate.

Glutaminase

Enzyme liberating ammonium ion from glutamine.

Glycolysis

Anaerobic energy production pathway in muscles.

Pyruvate

Product of glycolysis, convertible to alanine.

Alanine

Transport molecule for ammonia and pyruvate.

Glucose-Alanine Cycle

Cycle transporting alanine and ammonia to liver.

Carbamoyl Phosphate

Captures free ammonia in urea cycle's first step.

Citrulline

Product formed from ornithine and carbamoyl phosphate.

Argininosuccinate

Intermediate formed in the urea cycle.

Fumarate

Released during arginine formation from argininosuccinate.

Ornithine

Regenerated in the final step of urea cycle.

Arginase

Enzyme that cleaves arginine, producing urea.

Ornithine

Substrate for the urea cycle's next round.

Carbamoyl phosphate synthase

Activated by N-acetylglutamate for urea synthesis.

N-acetylglutamate

Activator formed from glutamate and acetyl-CoA.

High-protein diet

Increases expression of urea cycle enzymes.

Starvation

Stimulates protein breakdown for energy.

Ketogenic amino acids

Converted to ketone bodies; includes Leu, Ile, Thr.