Biochemistry Urea Cycle and Citric Acid Cycle Quiz

Biochemistry of GIT

• This book covers the biochemistry of the Gastrointestinal Tract (GIT).
• The content is divided into three main sections: Protein Metabolism, Nucleotide Metabolism, and Vitamins and Minerals.
• The book is authored by Dr. Mohamed Agha for the academic years 2024-2025.

Protein Metabolism

• Blood amino acid levels range from 3-6 mg/dL in the fasting state, rising to 2-3 mg/dL after a protein meal.
• Approximately 300 grams of protein are broken down daily into amino acids in the body.
• Amino acids participate in both anabolic (building) and catabolic (breaking down) pathways.
• Anabolic pathways involve the formation of tissue proteins, plasma proteins, hemoglobin, enzymes, hormones, and non-protein nitrogenous compounds.
• Catabolic pathways involve the conversion of proteins to Urea, a primary breakdown product in the liver.
• Energy supply from protein is 4.1 kcal/g, only if there is a lack of carbohydrates or fats.
• Nitrogen balance is the difference between total nitrogen intake (from dietary protein) and total nitrogen losses (in stools, urine and other ways).
• Nitrogen equilibrium occurs when nitrogen intake matches nitrogen losses.
• Positive nitrogen balance occurs if nitrogen intake surpasses losses, aiding growth, pregnancy or recovery from illness.
• Negative nitrogen balance happens when intake is less than losses, usually due to starvation, illness or malnutrition causing increased losses.

General Metabolism of Proteins

• The liver is the main site for removing amino groups.
• Major process of amino acid breakdown involves transamination followed by oxidative deamination.
• Minor process happens via oxidative deamination by D- and L-amino acid oxidase.
• Amino acids undergo transamination with a-ketoglutarate to transfer and amino group forming Glutamate- this is a reversible reaction
• Vitamin B6 (PLP) is required as a coenzyme during transamination.

Metabolic Fate of Carbon Skeleton of Amino Acids

• The carbon skeletons of amino acids, after the removal of their amino groups, are further metabolized into intermediates in the citric acid cycle (TCA cycle). These intermediates can be converted to:
• Glucogenic amino acids
• Ketogenic amino acids
• Mixed (glucogenic and ketogenic) amino acids

Metabolism of Ammonia

• Ammonia is primarily removed from the body as urea, through the urea cycle which happens in the liver.
• 90% of Ammonia is converted into Urea.
• 60% of the urea is formed through the conversion of Glutamine and Glutamate into ammonia- then urea.

Ammonia Transport in Circulation

• Ammonia is transported in the blood predominantly as non-toxic forms like urea and glutamine.
• Glutamine is an amide of glutamic acid, acting as a storage and transport form for ammonia.

Krebs Urea Cycle

• The Krebs Urea Cycle (Krebs Hensleit Cycle) is a metabolic process that converts ammonia into urea in the liver.
• Urea is the major end product of amino acid nitrogen catabolism, making up 90% of excreted nitrogen
• The Urea cycle comprises five reactions, with two happening in the mitochondria and the remaining three in the cytoplasm.
• Fumarate is a link between the Krebs Urea Cycle and the Krebs TCA Cycle (Citric Acid Cycle).

Short-Term and Long-Term Regulation of Urea Cycle

• The urea cycle is regulated to match the body's need for urea production.
• In short-term regulation, enzymes like Carbamoyl Phosphate Synthetase I are directly affected by N-acetylglutamate, helping the rate-limiting step.
• Long-term regulation manages enzyme levels based on protein intake and other factors.

Ammoniacal Encephalopathy

• Elevated ammonia levels in the blood can be toxic to the brain.
• This can cause a spectrum of disorders, from mental alterations to coma. This toxicity is due to the conversion to Glutamate, which in turn alters GABA production and decreases ATP synthesis.
• There are both congenital and acquired reasons for these conditions.

Metabolic Disorders of Phenylalanine and Tyrosine

• Phenylketonuria (PKU) is a genetic disorder affecting phenylalanine metabolism.
• Tyrosinemia is another genetic disorder affecting tyrosine metabolism.

Tyrosine

• Tyrosine is a non-essential amino acid synthesised from phenylalanine.
• It is a precursor to several important compounds for the body.

Melanin

• Melanin is a pigment synthesised from tyrosine.
• Melanin protects the skin from harmful UV rays.
• Melanin production is affected by a number of illnesses or disorders including disorders in amino acid metabolism

Metabolism of Glycine

• Glycine is a non-essential amino acid.
• It is converted into serine, then pyruvate through different steps of reaction.
• It has major roles in the body including its role in protein synthesis, purine synthesis, and bile salt synthesis

Metabolism of Tryptophan

• Tryptophan is a glucogenic and ketogenic amino acid, with important roles in both energy creation and structure in the body
• It is an essential amino acid, converted through a specific pathway to NAD+ and NADP+
• Tryptophan through the Kynurenine pathway gets converted to important cofactors which can be converted to niacin.

Water Soluble Vitamins

• Vitamins like Thiamine (B1), Riboflavin (B2), Niacin (B3), and Pantothenic acid (B5) are all crucial in important energy releasing reactions.
• They play major roles in the cellular reactions by helping in converting nutrients into energy.
• Vitamin B6, biotin and folic acid also fulfil important functions in the body.

Mineral Metabolism

• Discusses elements that play vital roles in body functions, such as:
• Sodium-osmotic balance, muscle functions
• Potassium-muscle tone, nerve impulses
• Calcium-bone formation, blood clotting
• Magnesium-enzyme activation
• Phosphorus-energy storage, bone formation
• Sulphur-amino acid structure
• Chloride-regulation of body fluids
• Other trace elements also mentioned: copper, zinc, iodine, manganese, selenium, etc

Nucleotide Metabolism

• Nucleotides are vital building blocks of DNA and RNA, carrying energy and acting as coenzymes.
• Their digestion involves enzymes that break down nucleoproteins, nucleic acids into nucleotides
• Nucleosides are absorbed from the digestive system.
• The degradation and synthesis of purine and pyrimidine nucleotides is covered.

Purine Salvage Pathway

• This recycling pathway saves energy by reusing the nitrogenous bases from the breakdown of nucleotides, mainly those from purines.

Purine Catabolism

• Explains the breakdown of purines, leading to the production of uric acid, the final product of this breakdown.

Clinical Disorders of Purine Catabolism

• Discusses hyperuricemia and gout, conditions resulting from abnormalities in uric acid metabolism.

Test your knowledge on the urea cycle and citric acid cycle with this quiz. Explore key concepts like ATP consumption, enzyme activity, and the significance of metabolic intermediates. Perfect for biochemistry students and professionals alike.