Amino acid catabolism Pt. 1

Question 1

Nitrogen Metabolism

Answer 1

Environment: Nitrogen exists as N2 in the atmosphere (78% of air) and as nitrate (NO3-) in the soil or ocean.

Reduction: Nitrate (NO3-) reduced to NH4+ in plants, fungi, and bacteria in a process called nitrate assimilation.

Dependence: Animals depend on plants and microorganisms for the synthesis of organic nitrogenous compounds, such as amino acids.

Question 2

Nitrogen Fixation

Answer 2

Process: N2 gas converted to NH4+ in a prokaryotic process called nitrogen fixation.

Reaction: N2 + 8 e– + 8 H+ ↔ 2 NH3 + H2

Components: Involves Fe protein (reductase) and MoFe protein (nitrogenase) with complex chemistry.

Question 3

Nitrogen in the Body

Answer 3

Composition: Nitrogen makes up about 3% of the human body.

Found in: Nucleic acids (DNA and RNA) and proteins.
Storage: Largest cache of nitrogen is in amino acids, with the largest store in skeletal muscle.

Dietary Sources: Meats, eggs, grains, legumes, and dairy products.

Intake: Minimum suggested intake is about 50 grams/day, with exercise increasing the requirement.

Question 4

Amino Acid Oxidation

Answer 4

Occurrence: When the diet is rich in protein or ingested amino acids exceed the body’s synthesis needs.

Energy Source: Carnivores derive up to 90% of energy from amino acids, while herbivores derive only a small fraction.

Conditions: Occurs during starvation, uncontrolled diabetes mellitus, or low-carbohydrate diets (e.g., South Beach, Atkins).

Question 5

Acute Pancreatitis

Answer 5

Condition: Disease where the pancreatic duct is blocked.

Result: Proteolytic zymogens are converted to active enzymes, degrading the pancreas lining.

Symptom: Very painful condition.

Question 5

Amino Acid Uptake in Digestive System

Answer 5

Liver Function: Amino groups are metabolized in the liver.

Transport to Liver: Amino groups are shuttled to the liver as glutamine from most tissues and as alanine from muscle.

Conversion: Amino groups are ultimately converted to urea in mammals for excretion in urine.

Question 6

Amino Group Catabolism

Answer 6

Excretory Forms: Excess NH4+ is excreted as ammonia (microbes, bony fishes), urea (most terrestrial vertebrates), or uric acid (birds and terrestrial reptiles).

Carbon Oxidation: Urea and uric acid have highly oxidized carbon atoms, discarded after extracting most available energy.

Question 7

Digestive Tract Overview

Answer 7

Components: Includes parietal cells and chief cells in gastric glands.

Hormonal Response: Gastrin stimulates parietal and chief cells.

Protein Degradation: Pepsin (aspartate protease) initiates protein degradation in the stomach.

Question 8

Zymogen Granules and Exocrine Cells

Answer 8

Synthesis Site: Rough endoplasmic reticulum in exocrine cells.

Zymogen Granules: Membrane-enclosed transport particles containing digestive enzyme zymogens.

Release Mechanism: Exocytosis releases zymogens into the collecting duct, leading to the pancreatic duct and small intestine.

Question 9

Amino Acid Absorption

Answer 9

Location: Occurs through the epithelial cell layer of the villi in the intestinal mucosa.

Entry: Amino acids enter capillaries for circulation.

Comparison: Lipid products enter the lymphatic system after absorption by the intestinal mucosa.

Question 10

Pancreas Structure

Answer 10

Head: Positioned toward the center of the abdomen, where the stomach meets the first part of the small intestine.

Central Section: Known as the neck or body.

Thin End (Tail): Extends to the left side.

Question 11

Metabolic Fates of Amino Groups

Answer 11

Energy Derivation: Humans derive a small fraction of oxidative energy from amino acids, unless starving, diabetic, or on a low-carb diet.

Protease Action: Proteases degrade ingested proteins in the stomach and small intestine, most as zymogens.

Absorption: Individual amino acids are absorbed.

Catabolism Steps: Early steps separate amino groups from carbon skeleton in a transaminase reaction involving pyridoxal phosphate, producing alpha keto acids.

Major Transporters: Glutamate, glutamine, and alanine are major transporters of amino groups from tissues to the liver.

Question 12

Transamination Reaction

Answer 12

Definition: Transamination is a chemical reaction transferring an amino group from one molecule to another, crucial in amino acid metabolism.

Enzyme: Catalyzed by a transaminase or aminotransferase.

Coenzyme: Pyridoxine (Vitamin B6) acts as a crucial coenzyme, facilitating amino group transfer.

Question 13

Pyridoxine’s Role

Answer 13

Function: Pyridoxine acts as a coenzyme in the transamination reaction.

Essential Nutrient: Vitamin B6 is essential for proper functioning of transaminases.

Question 14

Comparison: Glutamate and α-Ketoglutarate

Answer 14

Similarities: Glutamate and α-ketoglutarate have five carbons and are dicarboxylic acids.
Difference
: Glutamate has an alpha-amino group, while α-ketoglutarate has an alpha-keto group.

Interchangeability: Transamination reactions can interchange these functional groups.

Question 14

Enzyme-Catalyzed Transaminations

Answer 14

Amino Group Acceptor: In many reactions, α-ketoglutarate is the amino group acceptor.

Cofactor: All aminotransferases have pyridoxal phosphate (PLP) as a cofactor.

Reversibility: The transamination reaction is readily reversible.

Question 15

Transamination Reaction Catalyst

Answer 15

Catalyst: Transamination reactions are catalyzed by transaminases, also known as amino transferases.

Reversible Reaction: The direction of the reaction, whether transferring amino group to α-ketoglutarate or vice versa, is readily reversible.

Question 16

Glutamate’s Role in Amino Group Metabolism

Answer 16

Process: Most amino acids degraded in the liver undergo transamination, transferring the a-amino group to a-ketoglutarate.

Result: Glutamate is generated, along with corresponding a-keto acids.

Enzyme: Glutamate dehydrogenase in liver mitochondria removes the a-amino group from glutamate, releasing ammonia and regenerating a-ketoglutarate.

Question 17

Glutamine’s Function in Detoxification

Answer 17

Detoxification: Ammonia in the blood is detoxified by conjugation to glutamate by glutamine synthetase, generating glutamine.

Degradation: Glutamine is degraded by glutaminase in the mitochondria of liver cells, producing glutamate and ammonia.

Question 18

Alanine in Muscle Cells

Answer 18

Generation: In muscle cells during exercise or fasting, pyruvate from glycolysis accepts amino groups from other amino acids, generating alanine and corresponding a-keto acids.

Transport: Alanine travels through the blood.

Degradation: In the liver, alanine undergoes transamination with a-ketoglutarate, regenerating pyruvate.

Utilization: Pyruvate can be used to generate glucose, supplying muscle cells for glycolysis.