week 8 METABOLISM 1/2

What are amino acids, and what are their main sources and physiological roles?

Amino acids are the building blocks of proteins, essential for protein synthesis, neurotransmitter, and hormone production. They come from dietary proteins or breakdown of cellular proteins.

How are dietary proteins broken down and amino acids absorbed?

Dietary proteins are broken down by enzymes like pepsin (stomach) and trypsin (intestine) into amino acids and small peptides. Absorption occurs via active transport, facilitated diffusion, sodium-dependent transport, and peptide transport across the gut wall.

What is transamination, and why is it important in amino acid metabolism?

Transamination transfers an amino group from an amino acid to a keto acid, forming new amino acids and carbon skeletons. It’s a key step in amino acid catabolism and nitrogen metabolism.

What happens during oxidative deamination, and why must ammonia be managed carefully?

Oxidative deamination removes amino groups from amino acids, producing ammonia (NH₄⁺) and a carbon skeleton. Ammonia is toxic, so it must be converted to less harmful substances for excretion.

What is the urea cycle and its role in nitrogen metabolism?

The urea cycle converts toxic ammonia into urea, which is safely excreted in urine. It maintains nitrogen balance and prevents ammonia toxicity.

What is the difference between glucogenic and ketogenic amino acids?

Glucogenic amino acids can be converted into glucose via gluconeogenesis. Ketogenic amino acids are converted into ketone bodies or fatty acids. Both feed carbon skeletons into energy production or biosynthesis pathways.

What metabolic stages occur during starvation?

• Early (0–18 hrs): Glycogenolysis – glycogen breakdown to glucose for energy.

• Short-term (18–48 hrs): Gluconeogenesis – glucose made from amino acids, lactate, glycerol.

• Prolonged (>48 hrs): Lipolysis – fats broken to fatty acids; ketone bodies produced as alternate brain fuel.

What is glycolysis and what are its key steps?

Glycolysis breaks down 1 glucose (6C) into 2 pyruvate (3C) in the cytoplasm.

Key enzymes: Hexokinase, Phosphofructokinase-1 (PFK-1, major regulator), Aldolase, Pyruvate Kinase.

Produces a net of 2 ATP and 2 NADH per glucose.

What happens in the Citric Acid Cycle and which steps are key regulatory points?

Takes place in the mitochondrial matrix; acetyl-CoA oxidized to CO₂.

Produces 3 NADH, 1 FADH₂, 1 ATP per cycle.

Key regulatory enzymes: Citrate Synthase, Isocitrate Dehydrogenase, α-Ketoglutarate Dehydrogenase.

How is the Citric Acid Cycle regulated?

Citrate Synthase: inhibited by ATP, NADH, succinyl-CoA.

Isocitrate Dehydrogenase: inhibited by ATP, NADH; activated by ADP, calcium.

α-Ketoglutarate Dehydrogenase: inhibited by ATP, NADH, succinyl-CoA; activated by calcium.

What is the Electron Transport Chain (ETC) and its role?

Located in the inner mitochondrial membrane.

Transfers electrons from NADH/FADH2 to oxygen via complexes I-IV.

Pumps protons, creating a gradient used by ATP synthase to make ATP.

Produces the majority of ATP during aerobic respiration.

Why are glycolysis, the citric acid cycle, ETC, and starvation mode important to understand?

They represent core pathways of energy production and regulation.

Understanding key regulatory steps helps explain metabolic control and adaptation.

Starvation mode illustrates how metabolism shifts to maintain energy homeostasis in stress.