Protein Digestion and Denaturation Quiz

Proteins

• Essential nitrogenous component for living organisms
• Named by Jöns Jakob Berzelius (1838)
• Derived from the Greek word "proteios" – meaning "primary" or "of first importance"
• Its importance was understood after 1926
• 1926: James B. Sumner demonstrated that urease is a protein enzyme
• 1958: Frederick Sanger determined the sequence of the first identified protein – insulin
• 1958: Structure of the first proteins identified: Hemoglobin and Myoglobin
• Unlike carbohydrates and fats, proteins contain nitrogen in their structure
• 16% of the protein's weight is nitrogen
• Cells and enzymes are composed of proteins
• Cells constantly change and renew themselves, requiring proteins
• Without sufficient protein intake, cells cannot renew themselves – essential!
• Proteins act as mediators in almost all processes within a cell
• Proteins are the most abundant macromolecules in the body
• 50% of our body's dry weight consists of proteins
• Proteins have large molecular weights
• The source of body proteins is the protein obtained through food
• The body does not have a protein reserve, only a short-term protein backup for emergency situations
• Proteins are large and complex molecules
• Broken down into simpler structures known as amino acids
• Formed by the combination of numerous and diverse amino acids
• All proteins are composed of 20 amino acids
• The unique sequence of amino acids gives a protein its specific characteristics and determines its function
• Approximately 130 additional amino acids exist with significant functions but do not form part of proteins
• Amino acids are composed of carbon, oxygen, hydrogen, and nitrogen
• Two amino acids (methionine, cysteine) also contain sulfur, called sulfur-containing amino acids
• These amino acids differ in structure and properties
• Amino acids are building blocks of protein
• Amino acids are an essential part of protein synthesis

Amino Acids

• Contain an amino group (NH2), carboxyl group (COOH), carbon (C), hydrogen (H), oxygen (O), and nitrogen (N)
• Lysine and methionine also contain sulfur (S)
• Animal cells cannot synthesize the amino group (NH2)
• Plants, however, can synthesize the amino group from atmospheric nitrogen and produce amino acids
• In plants: Nitrogen (NH2) from soil, CO2, and H2O combine to form amino acids (AA) and proteins
• In animals: Proteins from plants broken down into amino acids, converted, rearranged, and combined to synthesize the animal's own tissue proteins
• Amino acids are categorized based on side chains and pH, and based on their essential (indispensable) nature
• Listed are Aliphatic, Aromatic, Hydroxyl Group, Sulfur-Containing, Acidic, and Basic
• Essential amino acids cannot be produced by the human body—must be obtained through diet.
• Non-essential amino acids can be synthesized from other amino acids
• Also, there are semi-essential amino acids
• Aliphatic Amino Acids: Glycine, Alanine, Valine, Leucine, Isoleucine
• Aromatic Amino Acids: Phenylalanine, Tyrosine, Tryptophan
• Hydroxyl Group Amino acids Serine, Threonine
• Sulfur-Containing Amino Acids: Methionine, Cysteine
• Acidic Amino Acids Glutamic acid, Glutamine, Aspartic acid, Asparagine
• Basic Amino Acids Lysine, Arginine, Histidine, Ornithine
• Amino acids released through protein digestion are utilized in various metabolic purposes, including Protein biosynthesis, Carbohydrate biosynthesis, Energy production, Endogenous amino acid synthesis, Biosynthesis of nitrogen-containing organic biomolecules (purine and pyrimidine bases in nucleic acids), Ammonia and urea biosynthesis
• Amino acids exist freely in red blood cells, intracellular and extracellular fluids, and transported throughout the body via circulation
• Key reactions of amino acid metabolism: Transamination, and Deamination

Transamination

• Involve both catabolism and anabolism of amino acids
• Crucial in energy production, Glucose synthesis (gluconeogenesis), Synthesis of fats or ketone bodies, and Synthesis of non-essential amino acids
• Primarily occur in hepatocytes (liver cells) and, to a lesser extent, in heart and skeletal muscle cells, taking place in the cytoplasm
• Examples of reactions: Alanine + α-Ketoglutarate → Pyridoxal Phosphate → Glutamate + Pyruvate), Aspartate + α-Ketoglutarate → Pyridoxal Phosphate → Glutamate + Oxaloacetate, Valine + α-Ketoglutarate → Valine Aminotransferase → Pyridoxal Phosphate → Glutamate + Keto-isovalerate
• Pyruvate, oxaloacetate are substrates for glucose in the TCA cycle
• Alanine's hepatic gluconeogenesis rate is much higher than that of all other amino acids
• The amino acids alanine, aspartate, and glutamate most contribute to the TCA cycle

Deamination

• Removal of amino groups (NH3 ammonia) from amino acids
• Ammonia is then used for urea synthesis in the urea cycle
• Simultaneously, the carbon skeleton of the amino acid is converted into keto acids, which can be used for energy production or other metabolic pathways
• Transamination and deamination often occur simultaneously and are interconnected through a central molecule, glutamate

Structure of Protein

• Proteins are formed when one amino acid (AA) binds to another through a specific covalent bond
• First discovered AA: Asparagine (1806)
• Last discovered AA: Threonine (1938)
• Amino acids are named in accordance with sources
• Arginine: From asparagus
• Glutamate: From gluten
• Tyrosine: From tyros (in Greek) - cheese
• Glycine: From glykos (in Greek) - sweet
• For synthesis of body proteins (tissues, hemoglobin, enzymes, hormones), all 20 amino acids must be present simultaneously in sufficient amounts
• Body proteins contain these amino acids in specific proportions
• One type of amino acid cannot be substituted for another
• The sequence of amino acids determines a protein's unique characteristics
• Primary, Secondary, Tertiary, and Quaternary structure
• Primary structure: linear chain of amino acids
• Secondary structure: hydrogen bonds form helix or pleated sheet shape
• Tertiary Structure;Folding & bending, spherical shape, Disulfide bonds important
• Quaternary Structure: multiple polypeptides, with tertiary structure, held together, functional protein

Protein Utilization in the Body

• Every cell synthesizes large amounts of specific proteins
• For synthesis, essential amino acids (EAAs) must be obtained from dietary sources
• For non-essential amino acids, amino groups and α-keto acids are provided
• Synthesis of amino acids occurs through transamination reactions
• The amino group is removed via oxidative deamination (in the liver)
• The amino group is converted into urea and excreted in the urine
• A small amount of the amino group is used for the synthesis of non-essential amino acids
• If a meal contains a large amount of protein, more than half of the absorbed amino acids undergo deamination, while the remainder circulates as free amino acids
• Branched-chain amino acids mostly remain in their free form in circulation
• Further, amino acids are converted into pyruvic acid, oxaloacetic acid, acetic acid, and α-ketoglutaric acid – these intermediates enter the Krebs cycle;
• The conversion of some amino acids into others occurs in the liver (mediated by amino transferase and vitamin B6)
• Amino acids are categorized as Glycogenic or Ketogenic;
• Interconvertible amino acids (cysteine, tyrosine, arginine, proline, histidine, glycine, phenylalanine)
• Amino acids are used in synthesis of new proteins (ex: tryptophan, methionine, tyrosine, glycine, histidine, arginine, glutamine, asparagine, glutamic acid)

Protein Quality

• Proteins from plant sources often contain limited numbers of essential amino acids and lower digestibility, decreasing utilization rate
• Amount and type varies among food sources
• Low-Quality Protein: Contain fewer essential amino acids than body needs or have decreased digestibility
• High-Quality Protein; Contain all essential amino acids in sufficient amounts, and are easily digestible
• Complete protein: fully utilized by the body
• Combining grains and legumes (example: beans and rice, or traditional dishes like aşure (a Turkish dessert)); Consuming grains with dairy group foods (example: milk-based desserts, or soups with milk/yogurt)
• Protein from animal sources (eggs, milk, meat) commonly has higher digestibility (91–100%). Cereal proteins (79-90%), Legume proteins (69-90%)
• Removing bran from grains increases digestibility
• Chickpeas and lentils are more digestible than other legumes
• Limiting essential amino acids: legumes (methionine and cysteine), grains (lysine and threonine) corn (methionine, threonine, and tryptophan) wheat lysine

Protein Balance and Requirements

• To determine the body's protein requirements, nitrogen (N) balance studies are conducted
• Nitrogen balance: consumed nitrogen through food equals excreted nitrogen by the body
• Negative nitrogen balance: nitrogen intake is less than excretion leading to increased protein break down
• Positive nitrogen balance: nitrogen intake is more than excretion leading to increased protein synthesis
• Protein requirements vary with age, body size, gender, pregnancy, lactation, illnesses, injuries, and activity levels
• Pregnancy: Protein requirements increase (15-20 g/day additional on animal based diet, 25 g/day additional on plant-based diet)
• Lactation: Additional protein needed for milk production
• Illnesses: protein breakdown in the body increases
• Infants and children with growth and development delays: Additional protein required
• Athletes and laborers: Also require more protein to support strenuous physical activity

Deficiencies and Excesses

• Causes of protein deficiency: inadequate intake, incorrect nutrition/diet practices due to lack of knowledge, digestive/absorption disorders, protein loss, losses from chronic conditions (bleeding), losses from burns, impaired liver function
• Protein deficiency effects: slowed or stunted growth, weakness, delays in mental development, reduced academic performance, blood and hemoglobin disruption, anemia, reduced liver enzyme activity, delayed wound healing, decreased appetite, Kwashiorkor/Marasmus diseases
• Excessive protein intake: increases calcium excretion leading to osteoporosis, increased workload in the kidneys (kidney stone formation), high consumption of saturated fat, cholesterol, elevated risks of cardiovascular diseases and cancer, constipation, can result in constipation

Vegetarian Diets

• Lacto-vegetarian: Dairy products
• Ovo-vegetarian: Eggs
• Lacto-ovo vegetarian: Eggs and dairy products
• Semi-vegetarian (white vegetarian): Exclude some animal-based foods (usually red meat)
• Vegan: Do not consume any animal-derived foods
• Deficiencies may occur in vegans regarding iron, calcium, riboflavin, zinc, and vitamin B12
• Plant-based proteins should be combined in a variety of ways to balance essential amino acids (EAAs).