Biochemistry Carbohydrates, Lipids, Nucleotides, and Amino Acids

Flashcards covering carbohydrate metabolism (glycolysis, gluconeogenesis, glycogen metabolism), lipid/cholesterol synthesis and transport, amino acid deamination, the urea cycle, and essential vitamins and energy-rich bonds based on the lecture transcript.

Amylase

A digestive enzyme that breaks down starch into the disaccharide maltose through the hydrolysis of glycosidic bonds.

Glucokinase

An enzyme typical for liver tissue with a high $K_m$ (low affinity) for glucose; it catalyzes the phosphorylation of glucose and is not inhibited by glucose-6-phosphate.

Hexokinase

An enzyme present in extrahepatic tissues that catalyzes the phosphorylation of glucose with high affinity ($K_m$) and is inhibited by its product, glucose-6-phosphate.

Phosphohexoisomerase

An enzyme of glycolysis that catalyzes the reversible conversion of glucose-6-phosphate to fructose-6-phosphate.

Phosphofructokinase I

The key regulatory, irreversible enzyme of glycolysis that converts fructose-6-phosphate to fructose-1,6-bisphosphate; it is inhibited by ATP and citrate, and activated by fructose-2,6-bisphosphate.

Aldolase

An enzyme belonging to the lyases that catalyzes the cleavage of fructose-1,6-bisphosphate into glyceraldehyde-3-phosphate and dihydroxyacetonephosphate.

Triosephosphate isomerase

An enzyme that catalyzes the reversible interconversion of phosphorylated aldotriose (glyceraldehyde-3-phosphate) and phosphorylated ketotriose (dihydroxyacetonephosphate).

Glycerolphosphate dehydrogenase

An enzyme that catalyzes the reversible conversion between dihydroxyacetonephosphate and glycerolphosphate, utilizing NAD in the cytosol or FAD in the mitochondria.

Glyceraldehyde-3-phosphate dehydrogenase

An enzyme responsible for the synthesis of 1,3-bisphosphoglycerate, a compound with an acylphosphate energy-rich bond used for substrate level phosphorylation.

Phosphoglycerate kinase

An enzyme that catalyzes a reversible reaction in glycolysis and gluconeogenesis, utilizing the energy of an acylphosphate bond for the synthesis of ATP by substrate level phosphorylation.

Enolase

An enzyme that catalyzes the reversible conversion of 2-phosphoglycerate to phosphoenolpyruvate, which contains an enolphosphate energy-rich bond.

Pyruvate kinase

An irreversible glycolytic enzyme that synthesizes ATP and pyruvate from phosphoenolpyruvate; it is activated by fructose-1,6-bisphosphate and insulin, and inhibited by ATP and glucagon.

Anaerobic Glycolysis ATP Yield

The process where 1 mol of glucose produces 2 mol of ATP and 2 mol of lactate as the final product.

Aerobic Glycolysis ATP Yield

The process where 1 mol of glucose allows for the synthesis of 6 or 8 ATP, depending on the shuttle used to reoxidize cytosolic NADH2 in the mitochondria.

Lactate dehydrogenase

A reversible enzyme that catalyzes the conversion of pyruvate to lactate in the cytosol, a step required for the regeneration of oxidized NAD in anaerobic conditions.

Cori,s cycle

The metabolic pathway that connects anaerobic glycolysis in muscles with gluconeogenesis in the liver by transporting lactate.

Gluconeogenesis

A process of glucose synthesis in the liver and kidneys from non-carbohydrate precursors like lactate, amino acids (e.g., alanine), and glycerol.

Pyruvate carboxylase

A mitochondrial enzyme that requires biotin and ATP to catalyze the conversion of pyruvate to oxaloacetate, the first step of gluconeogenesis.

Glucose-6-phosphatase

An enzyme present in the liver and kidneys (but absent in skeletal muscle) that catalyzes the hydrolysis of glucose-6-phosphate to free glucose.

Glycogen synthase

The main regulatory enzyme of glycogen synthesis that catalyzes the formation of $\alpha$-1,4-glycosidic bonds using UDP-glucose as a substrate; it is activated by insulin via dephosphorylation.

Glycogenphosphorylase

An enzyme that breaks $\alpha$-1,4-glycosidic bonds into glucose-1-phosphate; it is activated by cAMP-mediated phosphorylation during starvation or stress.

Debranching enzyme

An enzyme that breaks $\alpha$-1,6-glycosidic bonds in glycogen, releasing free glucose.

HMG-CoA reductase

The main regulatory enzyme of cholesterol synthesis that catalyzes the formation of mevalonic acid in the cytosol; it is inhibited by cholesterol and glucagon.

Chylomicrons

Lipoproteins formed in the intestine that transport exogenous triacylglycerols and are secreted into the lymph before being degraded by lipoprotein lipase in the blood.

VLDL

Lipoproteins formed in the liver that transport endogenous triacylglycerols and are converted to LDL in the blood by lipoprotein lipase.

LDL

Lipoproteins formed from VLDL that transport cholesterol; high concentrations are a risk factor for atherosclerosis.

HDL

Lipoproteins formed in hepatocytes that contain the highest amount of proteins and are responsible for reverse cholesterol transport from cells to the liver.

Lipoprotein lipase

An enzyme bound to the external surface of capillary endothelial cells that hydrolyzes triacylglycerols in chylomicrons and VLDL into glycerol and fatty acids.

LCAT (Lecithine-cholesterol acyltransferase)

An enzyme formed by the liver and activated by apoprotein A that is required for the reverse transport of cholesterol by HDL.

PRPP

A compound formed from ribose-5-phosphate and ATP by PRPP synthase; it is a required substrate for de novo purine nucleotide synthesis.

PRPP-amidotransferase

The main regulatory enzyme of purine nucleotide synthesis inhibited by the final products AMP, GMP, and IMP.

Glutamate dehydrogenase

A mitochondrial enzyme that catalyzes the reversible oxidative deamination of glutamate to 2-oxoglutarate, allowing for the synthesis of 3 ATP via oxidative phosphorylation.

Glutamine

A non-toxic transport form of ammonia from tissues to the liver; its synthesis from glutamate requires ATP.

Carbamoylphosphate

An intermediate synthesized by carbamoylphosphate synthase I in liver mitochondria, through which free ammonia enters the urea cycle.

Arginase

A cytosolic hydrolase in hepatocytes that cleaves arginine to form ornithine and urea as the final step of the urea cycle.

S-adenosylmethionine

A compound used as a methyl group donor in the synthesis of creatine and adrenaline.

Ketone bodies

Compounds including acetoacetate, $\beta$-hydroxybutyrate, and acetone synthesized from acetyl-CoA in liver mitochondria during starvation or diabetes.

Succinyl-CoA transferase

An enzyme required for the extrahepatic utilization of ketone bodies (absent in the liver) that converts acetoacetate into acetoacetyl-CoA.

Thiamine pyrophosphate (TPP)

The active form of Vitamin B1 required for the formation of acetyl-CoA from glucose and the oxidative decarboxylation of pyruvate.

Riboflavine (Vitamin B2)

A component of the coenzymes FMN and FAD required for the Krebs cycle, oxidative deamination, and the reaction of aminoacid oxidases.

Creatine phosphate

A compound containing a guanidiniumphosphate energy-rich bond used for the regeneration of ATP in working muscles.