Protein PDF

# Chapter 8: Proteins ## Introduction Dietary protein plays a crucial role in the body. It is essential for: - **Growth, maintenance, and repair of body tissue:** Building muscles, cells, and tissues require protein. - **Regulation of key processes:** Protein plays a significant role in various m...

# Chapter 8: Proteins ## Introduction Dietary protein plays a crucial role in the body. It is essential for: - **Growth, maintenance, and repair of body tissue:** Building muscles, cells, and tissues require protein. - **Regulation of key processes:** Protein plays a significant role in various metabolic processes. - **Source of energy:** If the body doesn't get enough energy from carbohydrates and fats, it can utilize protein as a last resort. ## Chemical Composition - **Protein synthesis:** Proteins are made up of amino acids. There are 20 different amino acids used in protein synthesis. Each protein can contain hundreds of amino acid units. - **Amino acids:** All amino acids contain carbon, hydrogen, oxygen, and nitrogen. Some amino acids, like cysteine and methionine, also contain sulfur. - **General structure of amino acids:** Amino acids consist of: - An amino group (NH₂) - A carboxyl group (COOH) - A hydrogen atom (H) - A side chain or side group specific to each amino acid. ## Properties - **Amphoteric nature:** Proteins can act as both acids and bases. - **Solubility:** Each protein has a specific solubility in different solutions. - **Colloidal nature:** Protein solutions are colloids - they do not pass through semi-permeable membranes. ## Classification ### Simple Proteins - **Globular:** - **Albumins:** Soluble in water, dilute acids, dilute bases, and salt solutions. Coagulated by heat. Example: Lactalbumin, egg albumin, serum albumin. - **Globulins:** Soluble in salt solutions, insoluble in water. Example: Serum albumin, arachin and conarchin of peanuts, myosin. - **Histones:** Basic proteins, soluble in most common solvents. Example: Nucleoprotein. - **Fibrous:** - **Collagen:** Resistant to digestive enzymes, insoluble, converted to digestible proteins like gelatins on boiling. Found in skin, tendons, and bones. - **Elastins:** Partially resistant to digestive enzymes, contain less hydroxyproline. Found in arteries, tendons, and elastic tissues. - **Keratins:** Highly insoluble and resistant to digestive enzymes, high cystine content. Found in skin, hair, and nails. ### Conjugated Proteins - **Nucleoproteins:** Salts or basic protein or polypeptide and nucleic acids. Found in chromosomes and nucleoli. - **Mucoproteins:** Protein or small polypeptide containing muco-polysaccharide; hexosamine less than 4%. Found in the glycoid of serum alpha globulin, submaxillary and gastric mucoids. - **Glycoproteins:** Protein or small polypeptide containing muco-polysaccharide; hexosamine less than 4%. Found in serum alpha, beta, and gamma globulins. ### Derived Proteins - **Primary derived proteins:** Denatured proteins produced by heat, acids, and alkalis. Example: coagulated egg white. - **Coagulated proteins:** Example: Fibrin from fibrinogen. - **Proteans:** Earliest products of protein hydrolysis, insoluble in water. - **Meta proteins:** Second stage products of protein hydrolysis, obtained by treatment with slightly stronger acids. - **Secondary derived proteins:** Progressive hydrodynamic products of protein; hydrolysis. Example: Proteoses, peptones, polypeptides, and peptides. ## Nutritional Classification of Proteins - **Complete proteins:** Contain all ten essential amino acids in the required proportion for the human body to promote good growth. Example: egg albumin, milk casein. - **Partially incomplete protein:** Lack one or more essential amino acids, promoting moderate growth. Example: wheat and rice proteins (limiting Lys, Thr). - **Incomplete proteins:** Completely lack one or more essential amino acids, not promoting growth. Example: gelatin (lacks Trp), zein (lacks Trp, Lys). ## Nutritional Classification of Amino Acids - **Essential amino acids:** Cannot be synthesized by the body at a rate sufficient to meet the needs for growth and maintenance. **MTVILPALY**: Methionine, Threonine, Valine, Isoleucine, Leucine, Phenylalanine, Alanine, Lysine, Tryptophan. - **Non-essential amino acids:** The body can make adequate amounts if nitrogen is available in the diet. - **Conditionally essential amino acids:** Require precursors to be synthesized, may be needed in the diet in the absence of these precursors. ## Functions ### Growth and Maintenance of Tissue - **Cell division:** Protein is essential for cell division, which is the foundation of growth and repair of tissues. - **Structural integrity:** Collagen, a protein found in bones, tendons, and ligaments, provides structural support and connects cells. - **Muscle contraction**: Proteins actin and myosin are responsible for muscle contraction. - **Protein turnover:** Continuous breakdown and synthesis of proteins, maintaining a dynamic equilibrium in the body. ### Formation of Essential Body Compounds - **Enzymes:** Proteins that catalyze virtually all biochemical reactions in the body. - **Hormones:** Proteins that regulate various metabolic processes. Example: insulin, gastrin, and growth hormone. - **Hemoglobin:** Protein that transports oxygen in the blood. - **Antibodies:** Proteins essential for the body's immune system. ### Transport of Nutrients - **Membrane-bound carrier proteins:** Transport specific nutrients across membranes. Example: retinol binding protein, metallothione, lipoproteins. ### Regulation of Water Balance - **Oncotic pressure:** Protein in blood helps maintain fluid balance by pulling water back into blood. - **Hydrostatic pressure:** Pressure created by the heart, pushing fluid out of the blood, causing edema if protein levels are low. ### Maintenance of Appropriate pH - **Buffers:** Proteins in blood help maintain the correct pH levels in the body. ### Defense and Detoxification - **Antibodies::** Specific proteins that target specific antigens, providing immunity. - **Detoxification:** Enzymes in the liver help detoxify toxins from food. ### Source of Energy: - Protein provides 4kcal per gram, but it is usually used only as a last resort for energy when carbohydrate and fat intake is inadequate. ## Specific Functions of Amino Acids - **Glycine:** Involved in the synthesis of purines, glutathione, creatine and creatinine, bile acids, hippuric acid, and serine. - **Glutamic acid:** Component of glutathione, involved in ammonia metabolism, a precursor for GABA (a neurotransmitter). - **Arginine:** Cleavage of arginine leads to urea production in the liver, a precursor for neurotransmitters. - **Lysine:** Parent substance for carnitine, which transports fatty acids. - **Methionine and Cysteine:** Involved in transmethylation, glutathione synthesis, and protection against toxins. - **Phenylalanine and Tyrosine:** Required for brain development, precursors for epinephrine and thyroxine, involved in the synthesis of melanin. - **Histidine:** Present in muscles and hemoglobin, involved in allergic reactions and histamine synthesis. - **Tryptophan:** Precursor for niacin and serotonin, a neurotransmitter, and involved in vasoconstriction. - **Proline and Hydroxyproline:** Present in collagen and connective tissues. - **Leucine, Isoleucine, and Valine:** Branched-chain amino acids, used for alanine formation. ## Digestion and Absorption ### Gastric digestion - **Pepsin:** Proteolytic enzyme in gastric juice, breaks down proteins into polypeptides. Optimal pH for pepsin: 2.0. ### Proteolysis in the intestinal tract - **Trypsin, chymotrypsin, carboxy peptidase:** Enzymes released from pancreatic juice, further hydrolyze polypeptides. - **Aminopeptidases:** Enzymes present in the intestinal mucosa, complete the hydrolysis of peptides into amino acids. ### Absorption of amino acids - **Carrier proteins:** Transport amino acids across the intestinal wall into the blood. ### Digestibility of protein - **Protein quality:** Affects the efficiency of protein absorption. Animal proteins are generally more digestible than plant proteins (60-90% vs 60-90%). ## Metabolism - **Liver:** Main site for the uptake of amino acids after absorption. ## Factors Affecting Protein Utilization - **Amino acid balance:** The ratio of essential amino acids in a protein determines its quality. - **Calorie intake:** If calorie intake is inadequate, protein is used for energy rather than building tissues. - **Immobility:** Protein synthesis greatly decreases in immobile individuals. - **Injury:** Loss of nitrogen (protein) increases after injury. - **Emotional stability:** Stress increases protein loss. ## Quality of Proteins - **Biological value (BV):** Reflects the percentage of absorbed nitrogen retained for growth or maintenance. - **Net protein utilization (NPU):** Similar to BV, but accounts for nitrogen losses in digestion. - **Protein efficiency ratio (PER):** Measures weight gain per gram of protein consumed. - **Net protein ratio (NPR):** A modification of PER that accounts for maintenance requirements. - **Amino acid score:** Compares the amount of limiting amino acid in a test protein to a reference protein. ## Complementary Value of Proteins - **Combined protein sources:** Combining different protein sources can help overcome deficiencies in essential amino acids. ## Protein Requirements - **Infants:** Protein requirements are based on the amino acid profile of breast milk. - **Children:** Factorial method is used to calculate protein requirements, accounting for growth and endogenous losses. - **Adults:** Nitrogen balance method is used to determine requirements. ## Nitrogen Balance - **Positive nitrogen balance:** Intake exceeds loss, indicating growth. - **Negative nitrogen balance:** Loss exceeds intake, indicating protein breakdown. - **Nitrogen equilibrium:** Intake equals loss, indicating maintenance. **Nitrogen loss**: Primarily through urine, faeces, and sweat. ## Recommended Dietary Allowances (RDA) - **Average daily protein requirement:** 0.5 g/kg for adults consuming high-quality protein. This can be adjusted based on protein quality (NPU) and individual needs, such as pregnancy or lactation. ## Protein Requirements for Athletes: - **Strength and speed athletes:** May need 1.2 to 2.0 g/kg of protein per day. - **Resistance training athletes:** Increased protein needs due to increased muscle growth and protein breakdown. - **Endurance athletes:** May need 1.2 to 1.4 g/kg of protein per day. ## Sources of Protein: - **Good sources:** Animal products (meat, fish, eggs), legumes, nuts, seeds. - **Moderate sources:** Cereals, millets. - **Poor sources:** Fruits, vegetables, roots, tubers.