Protein Structures and Denaturation

Protein Structures

• Protein structures are determined by weak electrical attractions within the polypeptide chain.
• Secondary Structure: Positively charged hydrogens attract nearby negatively charged oxygens
  • Contributes to strength and rigidity
• Tertiary Structure: Long polypeptide chains twist and fold into complex, tangled shapes.
  • Influenced by amino acid side groups (hydrophilic or hydrophobic)
• Quaternary Structure: Involves interactions between two or more polypeptides.
  • Hemoglobin, a protein in red blood cells, is made of four associated polypeptide chains

Protein Denaturation

• When proteins are subjected to heat, acid, or other destabilizing conditions, they undergo denaturation.
  • Quaternary, tertiary, and secondary structures are disrupted or destroyed
  • This uncoils the protein and it loses its shape and ability to function
• Denaturation is irreversible.
  • Hardening of an egg during cooking, curdling of milk with acid, stiffening of egg whites when whipped

Digestion and Absorption of Proteins

• Proteins in food are broken down into amino acids, which the body uses to make its own proteins.
• Enzymes break down polypeptides in the digestive system:
  • Long polypeptides to short polypeptides
  • Short polypeptides to tripeptides and dipeptides
  • Tripeptides and dipeptides to individual amino acids

Stomach

• The stomach partially breaks down proteins through hydrolysis.
  • Hydrochloric acid denatures proteins, allowing enzymes to attack the peptide bonds.
  • Hydrochloric acid activates pepsinogen to pepsin, which cleaves proteins into smaller polypeptides and amino acids.

Small Intestine

• Pancreatic and intestinal proteases further hydrolyze polypeptides in the small intestine.
  • They break them down into short peptide chains, tripeptides, dipeptides, and amino acids.
• Peptidases on the intestinal cell membranes split most dipeptides and tripeptides into single amino acids.

Protein Absorption

• Specific carriers transport amino acids (and some dipeptides and tripeptides) into intestinal cells.
• Some amino acids are used for energy or synthesis of needed compounds.
• Unused amino acids are transported into capillaries and travel to the liver.

Roles of Proteins

• Structural Materials: Building blocks of muscles, blood, and skin. Major structural component of cells.
• Enzymes: Catalyze chemical reactions in the body.
• Digestive enzymes break down food
• Other enzymes perform various functions throughout the body.
• Hormones: Chemical messengers that regulate body functions.
• Antibodies: Help defend the body against infections.
• Fluid Balance: Proteins help maintain fluid balance.
  • During illness or protein malnutrition, plasma proteins leak out of blood vessels, causing fluid to accumulate in tissues (edema).

Protein Quality

• Determines how well children grow and adults maintain health.
• Factors affecting protein quality:
  • Digestibility: How easily the body breaks down proteins into amino acids (animal proteins generally more digestible).
  • Amino Acid Composition: The types and proportions of amino acids in the protein source.

Complementary Proteins

• Combining foods with different amino acid profiles can create an optimized source of protein.
• While not necessary for most healthy vegetarians to precisely balance amino acids at each meal, diversity over time is important.
• Examples: beans and rice, bread and peanut butter.

Protein Deficiency

• Results from inadequate intake of protein or essential amino acids.
• Leads to slowed growth, impaired brain and kidney functions, poor immunity, and inadequate nutrient absorption.

Heart Disease and Protein

• Excess protein from certain sources can contribute to the progression of heart disease.
• Substituting vegetable protein for animal protein may improve blood pressure and blood lipids.
• Lowering homocysteine levels (an amino acid elevated in heart disease) through B vitamins has not been shown to reduce heart disease risk.
• Arginine may help protect against heart disease by lowering blood pressure and preventing blood clots.

Kidney Disease and Protein

• Restricting dietary protein, especially red meat, can slow the progression of kidney disease.

Protein Recommendations

• To replace protein losses, the body needs dietary protein.
  • Provides essential amino acids
  • Is the main source of nitrogen for building nonessential amino acids and other nitrogen-containing compounds.
• Average recommended protein intake: 10-35% of total calories.
  • Approximately 50-175 grams in a 2,000 calorie diet
• Some research suggests that a slightly higher intake may be beneficial for older adults.
• Consuming moderate amounts of high-quality protein at each meal may support protein synthesis and muscle health.

Protein RDA

• 0.8 grams per kilogram of body weight per day for adults.
• Slightly higher for infants and children.

Protein Foods

• 1 ounce of most protein foods provides about 7 grams of protein.
• USDA encourages variety in protein sources:
  • 20% from seafood
  • 70% from meats, poultry, and eggs
  • 10% from nuts, seeds, and legumes

Milk and Milk Products

• Significant source of protein per serving.

Secondary Structure of Proteins

• The secondary structure of proteins is determined by weak electrical attractions between the polypeptide chain's positively charged hydrogens and nearby negatively charged oxygens.
• These attractions give proteins strength and rigidity.

Tertiary Structure of Proteins

• The tertiary structure of proteins occurs when long polypeptide chains twist and fold into complex tangled shapes due to the characteristics of each amino acid's side group.
• Hydrophilic side groups are attracted to water molecules.
• Hydrophobic side groups are repelled by water.

Quaternary Structure of Proteins

• The quaternary structure of proteins involves interactions between two or more polypeptides.
• Hemoglobin, a large globular protein, consists of four associated polypeptide chains, each containing iron.

Denaturation of Proteins

• The quaternary, tertiary, and secondary structures of proteins can become disrupted or destroyed when exposed to heat, acid, or other destabilizing conditions.
• Denaturation is irreversible.
• Examples of denaturation include cooking an egg, curdling milk, and whipping egg whites.

Digestion and Absorption of Proteins

• Dietary proteins provide amino acids, which are used by the body for protein synthesis.
• Enzymes break down proteins in the stomach and small intestine into amino acids, tripeptides, and dipeptides.

Digestion of Proteins in the Stomach

• Hydrochloric acid denatures proteins, allowing digestive enzymes to break down peptide bonds.
• Pepsinogen is converted into pepsin, an enzyme that breaks down proteins into small polypeptides and amino acids.

Digestion of Proteins in the Small Intestine

• Pancreatic and intestinal proteases further break down polypeptides into smaller peptides and amino acids.
• Peptidase enzymes on the intestinal cell membranes split dipeptides and tripeptides into single amino acids.

Absorption of Proteins

• Amino acids are transported into intestinal cells by specific carriers.
• Amino acids may be used for energy or protein synthesis within intestinal cells.
• Unused amino acids are transported into the bloodstream and delivered to the liver.

Roles of Proteins

• Dietary protein is essential for providing essential amino acids and nitrogen for building non-essential amino acids and other nitrogen-containing compounds.
• If essential amino acids are missing, the body breaks down its own proteins to obtain them.

Protein in Foods: Reference Protein

• The reference protein is a standard used to determine the quality of a food protein by comparing its amino acid composition to essential amino acid requirements of preschool-age children.
• The limiting amino acid is the essential amino acid found in the shortest supply relative to body needs.

Protein in Foods: High-Quality Proteins

• High-quality proteins contain all essential amino acids in proportions needed for human protein synthesis.
• Animal-derived proteins are generally high-quality, except for gelatin.
• Plant-derived proteins have diverse amino acid patterns and may be limiting in one or more essential amino acids.

Protein in Foods: Complementary Proteins

• Combining plant proteins with complementary amino acid patterns can improve protein quality.
• This strategy yields complementary proteins that contain all the essential amino acids in sufficient quantities for health.

Health Effects of Protein Deficiency

• Protein deficiency can cause slowed growth, impaired brain and kidney functions, poor immunity, and inadequate nutrient absorption.

Health Effects of Excess Protein

• In developed countries, protein is abundant.
• High-protein diets can contribute to heart disease, though substituting vegetable protein for animal protein may improve blood pressure and lipids.
• The amino acid homocysteine is elevated in heart disease, and while some research suggests it is a marker, lowering it through B-vitamin supplements does not seem to lower heart disease risk.
• Arginine may help protect against heart disease by lowering blood pressure and preventing blood clots.

Recommended Protein Intake

• Dietary protein is needed to replace protein lost through breakdown.
• The protein RDA for adults is 0.8 grams per kilogram of body weight daily.
• Current recommendations suggest slightly higher intakes may be beneficial, especially for older adults.
• Including 25-35 grams of high-quality protein at each meal may support protein synthesis and muscle health.

Protein Foods in the USDA Food Patterns

• 1 ounce of most protein foods delivers about 7 grams of protein.
• The recommended intake of protein foods should be about 20% from seafood, 70% from meat, poultry, and eggs, and 10% from nuts, seeds, and legumes.

Protein from Milk and Milk Products

• Milk and milk products are the only food group besides protein foods to provide significant amounts of protein per serving.

Explore the intricacies of protein structures, including primary, secondary, tertiary, and quaternary formations. Understand the process of denaturation and how conditions like heat and acid can alter a protein's functionality. Test your knowledge on these fundamental biological concepts.