Protein Metabolism Quiz

Proteins in the Diet and Nitrogen Metabolism

• Dietary protein intake was once believed to lead to greater muscle deposition and strength, but current understanding suggests adequate protein intake alone is sufficient for health. Excess intake is potentially unhealthy.
• Protein turnover is a constant process, involving synthesis and degradation of body protein (1-2% daily).
• Protein excretion is unavoidable, even without dietary protein.
• Excess amino acids are metabolized and excreted when protein intake is high.
• The rate of protein synthesis and degradation varies depending on physiological demands and the protein type. Half-lives of proteins range from 30 seconds to many days.
• 75-80% of liberated amino acids are reutilized for new protein synthesis, while the remainder becomes urea and is used in other metabolic processes.
• Daily nitrogen loss is approximately 30-40 grams or 5-7 grams of nitrogen.
• A 70 kg adult loses about 24 grams daily protein.
• Adequate daily protein intake for a 70 kg adult should be 34 grams (assuming 70% utilization),
• Foods vary in their protein content, with some being rich in protein and others being poor.
• Biological value assesses protein value based on net protein content of available nitrogen and excreted nitrogen. Higher values mean better utilization.
• Chemical score measures essential amino acids ratio to an ideal protein.
• Essential amino acids include: Arginine*, Histidine*, Isoleucine, Leucine, Lysine, and Valine. * denotes semi-essential or essential only in young individuals or developing individuals.
• Positive nitrogen balance occurs when nitrogen intake exceeds excretion; this indicates tissue growth or repair.
• Negative nitrogen balance occurs when nitrogen excretion exceeds intake; this indicates tissue breakdown or loss.
• High protein intake can induce protein catabolizing enzymes in the liver, and amino acids are metabolized when carbohydrates are unavailable (starvation or diabetes).
• Separate amino groups are removed from amino acids.
• Most amino acids are metabolized in the liver.
• Several pathways and reactions occur in amino acid catabolism, including transamination, oxidative deamination, and urea synthesis.

Nitrogen is Important

• Only a few microorganisms incorporate N₂ into organic molecules.
• NH₃ is toxic to the nervous system.
• Many plants can incorporate NH₃ into their structures.

Paths and Reactions in Amino Acid Catabolism

• Most amino acids are metabolized in the liver.
• Some NH₃ is used; the rest is excreted.
• NH₃ from other tissues is transported to the liver.
• Important reactions are transamination, oxidative deamination, & urea synthesis.

Protein Turnover

• Dietary proteins are broken down into amino acids that enter the blood.
• Amino acids are used for synthesis of various compounds or tissue in the body.
• Excess amino acid is excreted as urea.
• All proteins are synthesized and degraded constantly in the body.

Rate of Synthesis and Degradation

• Protein turnover involves constantly degrading and synthesizing proteins in the body.
• 1-2% of the body's protein is degraded daily.
• Protein degradation and synthesis rates vary between proteins in the body.
• The rates can change based on the body's needs or demands.

Replacing the Lost Amount

• Proteins contain 16% nitrogen by weight.
• Daily nitrogen loss can be determined by multiplying total body nitrogen by 6.25 to calculate the total body protein.

Quality of Protein

• Proteins are measured by biological value.
• Chemical score measures essential amino acids.

Essential Amino Acids

• Essential amino acids are broken down in the body and the carbon skeleton is either used for gluconeogenesis or ketogenesis, or used as oxidized as CO2 and H2O.

Evaluation of Protein Nutrition

• Positive nitrogen balance: nitrogen intake is greater than excretion (indicates growth, repair).
• Negative nitrogen balance: nitrogen excretion is greater than intake (indicates breakdown, loss).

Effects of Dietary Intake

• High intake induces protein-catabolizing enzymes in the liver.
• Amino acid catabolizing enzymes have a high Km and low affinity.

Oxidation of Amino Acids

• During normal protein turnover or a protein-rich diet, excess amino acids are catabolized and excreted.
• Oxidized when carbohydrates are unavailable (starvation or diabetes).

Urea Cycle

• The nitrogen is removed and combined with carbon dioxide to produce urea which is excreted from the body.
• This cycle occurs in the liver.

Glutamate Channels NH₃ Flow

• In the liver, NH₃ is converted to glutamate.
• Glutamate carries the amino group to the mitochondria, where it's removed.
• Excess NH₃ in muscle is converted to alanine.

Oxidative Deamination

• Conversion of glutamate to a-ketoglutarate, releasing ammonia.
• Oxidative deamination is catalyzed by glutamate dehydrogenase in the mitochondria.

Fates of NH₃

• NH₃ is converted to urea in the liver.
• NH₃ can be used for synthesis of other molecules, such as amino acids.

Glucose Alanine Cycle

• The cycle exchanges amino groups between the liver and the muscles.

Amino Acid Oxidation and the Catabolism of the Carbon Skeleton of Amino Acids

• During protein degradation and a high protein diet, amino acids oxidize.
• Excess amino acids provide carbon skeletons for the synthesis of other molecules. -Some amino acids are oxidized to CO2 and H20.

Amino Acid Oxidation

• During normal protein turnover, amino acids oxidize to generate energy and other molecules.
• A high protein diet can lead to amino acid oxidation.
• Starvation can also cause amino acid oxidation.

Amino Acid Oxidation

• Amino acids lose their amino groups to form a-keto acids.
• The a-keto acids are oxidized to produce carbon dioxide and water or converted into other molecules.

Amino Acids and Their Carbon Skeletons

• Amino acids lose their amino groups and form a-keto acids.
• The a-keto acids are oxidized to create carbon dioxide and water, used for the synthesis of other molecules.

The Catabolism of the Carbon Skeleton of Amino Acids

• The carbon skeletons of amino acids are further metabolized through several pathways including gluconeogenesis, ketogenesis, or complete oxidation to CO2 and H2O.

Glucogenic and Ketogenic Amino Acids

• Some amino acids can be converted to glucose, whereas others are converted into ketone bodies.

Enzyme Cofactors

• PLP is used in transamination reactions.
• Biotin transfers carbon units.
• Tetrahydrofolate transfers intermediate carbon units.

Pyridoxine (Vit B6)

• Pyridoxal phosphate (PLP) is a coenzyme important in amino acid metabolism.

Biotin

• Biotin is a coenzyme for carboxylation reactions and transfers one-carbon units (CO2)

Role of Folic Acid in Amino Acid Metabolism

• Folic acid is important for transferring one-carbon units in different oxidation states.
• THF acts as a carrier for several oxidation states for one-carbon units.
• Folic acid is important in the transfer of methyl groups.

p-Aminobenzoate

• p-Aminobenzoate is a compound used to produce some amino acids.

Methionine

• Methionine is an essential amino acid and important for the transfer of methyl groups in several biological processes.

Pyruvate Carboxylase

• The enzyme catalyzes the carboxylation of pyruvate to oxaloacetate, a reaction important in gluconeogenesis.

Role of Folic Acid and Amino Acid Metabolism

• Single-carbon atoms can exist in varied oxidation states and can be incorporated into various molecules.
• Tetrahydrofolate (THF) carries these carbon molecules.

Other Amino Acid Pathways

• Various amino acids are catabolized through specific pathways, which vary greatly, and can form the carbon skeletons for gluconeogenesis, ketogenesis, or complete oxidation to CO2 and H2O.

Genetic Disorders of Amino Acid Catabolism

• Several genetic disorders result from deficiencies or mutations in enzymes involved in amino acid breakdown.

Biosynthesis of Amino Acids

• Higher animals have lost the ability to synthesize some amino acids.
• Biosynthesis of amino acids frequently occurs through short pathways from amphibolic intermediates.

Number of Enzymes Required for Synthesis

• The number of enzymes required for the synthesis of essential and nonessential amino acids differs.

Problems with Amino Acid Catabolism

• Various problems can arise from impaired pathways, and the diseases are specific to affected pathways and can have varied symptoms and impacts

Tyrosinemia

• Defective tyrosine degradation pathways can create an excess of tyrosine or its byproducts
• These compounds can lead to disease and health problems.

Maple Syrup Urine Disease

• Branched-chain a-keto acids accumulate in the blood. This condition is caused by the deficiency of an enzyme, and it leads to the toxic accumulation of these acids, which causes adverse effects on the body. The symptoms include mental retardation, vomiting, and convulsions.

Phenylketonuria

• Defect in phenylalanine hydroxylase, the enzyme that converts phenylalanine to tyrosine.
• Phenylalanine accumulates, leading to intellectual disability and other adverse effects.

(Additional notes on Urea Cycle and other pathways):

• Multiple enzymatic pathways contribute to the process.

• Regulations and pathways can affect production and activity.

(Additional notes on various disorders in Amino Acid Catabolism):

• This involves specific defective steps of breakdown.

• The genetic disorders are usually caused by enzyme deficiencies, which can cause toxic compounds to accumulate in the body and present as various symptoms leading to potentially severe health issues.

Test your knowledge on protein metabolism with this quiz covering key concepts such as nitrogen intake, biological value of proteins, and the urea cycle. Answer questions about daily protein needs, nitrogen loss, and metabolic processes related to protein utilization.