Chapter 6 Proteins and Amino Acids: Function Follows Form

Why Is Protein Important? Major component of all plant and animal issues Vital to many aspects of health Provide energy and help keep skin, hair, and nails healthy Deficiency leads to wasting Excess protein intake a greater problem in the U.S.

Amino Acids Are the Building Blocks of Proteins Proteins are sequences of amino acids (AA) 20 different amino acids 9 essential amino acids Body cannot make them Must get them through diet 11 nonessential amino acids Do not need to be supplied by diet If body cannot make them, categorized as conditionally essential amino acids

Amino Acids as Building Blocks Amino acid structure One central carbon atom One hydrogen atom (H) One carboxylic acid group (-COOH) One amino group (-NH2) One side group unique to each amino acid Side group gives the amino acid its identity

Protein Structure Amino Acid Sequence Each amino acid is joined to the next by a peptide bond Peptide bonds Dipeptide (2 AA) Oligopeptide (3 AA) Polypeptide (4-10 AA)

Protein Structure Protein Shape Sequence and properties of AA determines protein shape Shape determines protein’s function

Protein Denaturation Denaturation Disrupts shape Protein unfolds and loses its shape (denature) Caused by acidic or alkalinity, high temperature, alcohol, oxidation, and agitation

Functions of Body Proteins Each protein has a specific function determined by its unique shape

Functions of Body Proteins Structural and mechanical functions Collagen Most abundant Gives skin and bone their elastic strength Keratin Primary constituent of hair and nails Motor proteins Turn energy into mechanical work

Functions of Body Proteins Immune functions Antibodies Blood proteins that attack and inactivate bacteria and viruses Part of the body’s immune response

Functions of Body Proteins Enzymes Catalyze, or speed up, chemical reactions Every cell contains thousands of types of enzymes

Functions of Body Proteins Hormones Regulate body processes Acid-base balance Proteins help maintain stable pH levels by acting as buffers Acidosis or alkalosis occurs when proteins unable to fulfill buffer function

Functions of Body Proteins Transport functions Proteins act as Channels and pumps, allowing substances to flow through membranes Carriers, transporting important substances in the blood stream

Functions of Body Proteins Fluid Balance Fluids found inside and outside cells Inside cells: intracellular fluid Outside cells: extracellular Two types: Interstitial fluid (between cells) Intravascular fluid (in the blood)

Functions of Body Proteins Fluid Balance Blood proteins maintain appropriate fluid levels in vascular system Edema results when diet lacks enough protein to maintain normal levels of blood proteins Edema: Swelling caused by buildup of fluid between cells

Functions of Body Proteins Source of Energy and Glucose Carbohydrates and fat are protein-sparing Body prefers burning them for energy Will resort to burning protein for energy if necessary Body readies protein for use as energy through deamination Removal of amino group from an amino acid

Protein Digestion and Absorption In the stomach Proteins are denatured by hydrochloric acid Pepsin begins digestion 10–20% of digestion In the small intestine Proteases and intestinal lining cells break down large peptides into smaller peptides

Protein Digestion and Absorption Undigested Protein Pass out of the body in feces Diseases of the intestinal tract cause problems with digestion by decreasing absorption efficiency Celiac disease Cystic fibrosis

Protein Digestion and Absorption Amino Acid and Peptide Absorption Travel via portal vein to liver  released into general circulation

Protein in the Body Protein Synthesis AA assembled in specific sequence Draws on AA pool as needed Dispensable AA missing Cell will make that AA or obtain it from the liver

Protein in the Body Protein Synthesis Draws on AA pool as needed Indispensable AA missing Body may break its own protein down to supply the missing AA Indispensable AA unavailable Protein synthesis halts and partially completed protein used elsewhere in the body

Protein in the Body Amino Acid Pool and Protein Turnover The AA pool Available AA Protein synthesis; produce energy and glucose Protein turnover Constant recycling of protein

Protein in the Body Synthesis of Nonprotein Molecules Amino acids as precursors of DNA, RNA, and coenzymes Also used to make neurotransmitters: Chemicals that send signals from nerve cells to other parts of the body

Protein in the Body Protein and Nitrogen Excretion Breakdown of amino acids Removes nitrogen (amino) group Amino groups converted to urea for excretion

Protein in the Body Nitrogen Balance Nitrogen intake vs. nitrogen output Nitrogen equilibrium Nitrogen intake = nitrogen output Healthy adults

Protein in the Body Positive nitrogen balance Nitrogen intake > nitrogen output Growth; recovery from illness Negative nitrogen balance Nitrogen intake < nitrogen output Injury and illness

Protein in the Diet Recommended Intakes of Protein Adult RDA = 0.8 gram per kilogram of body weight Severe physical stress can increase body’s need for proteins Infections, burns, fevers, surgery increase protein losses Diet must replace lost protein

Protein in the Diet Protein Consumption in the United States Generally higher than recommended range

Protein in the Diet Protein Quality Complete Proteins Supply all indispensable amino acids Animal proteins; soy proteins Incomplete and Complementary Proteins Low in one or more indispensable amino acids Most plant proteins

Protein in the Diet Protein Quality Complementary proteins Two incomplete proteins can equal a complete protein One makes up for the other’s lack of specific essential amino acids Combination then provides sufficient amounts of all essential amino acids

Protein in the Diet Protein Quality Evaluating Protein Quality and Digestibility Provides all the indispensable amino acids at amounts the body needs Provides enough other amino acids to serve as nitrogen sources for making dispensable amino acids Should be easy to digest Consider the protein digestibility-corrected amino acid score (PDCAAS)

Protein in the Diet Proteins and AAs as Additives and Supplements Protein and AA additives Structure, texture, and taste of foods Protein and AA supplements Used for a variety of reasons: dieters, athletes, certain diseases Risks unknown

Vegetarian Eating Why People Become Vegetarians Various reasons, including Religious beliefs Environmental concerns Aversion to eating another living creature Concerns about animal cruelty concerns In some cases a necessity

Vegetarian Eating Health Benefits of Vegetarian Diets Less fat, saturated fat, and cholesterol intake More magnesium and folate More antioxidants More fiber and phytochemicals Reduces risk for heart disease, obesity, hypertension, and cancer

Vegetarian Eating Health Risks of Vegetarian Diets Vegan diets may be low in some nutrients Calcium, iron, zinc, vitamin D, and vitamin B12 Higher intake of phytates More restrictive food choices = less nutrients Careful planning needed for growth and overall health

Vegetarian Eating Dietary Recommendations for Vegetarians Choose a variety of foods Choose whole, unrefined foods Choose a variety of fruits and vegetables Choose lower-fat dairy products and eggs in moderation Consume a regular source of vitamins B12 and D Fortified foods or supplements

Health Effects of Too Little or Too Much Protein Protein-Energy Malnutrition Kwashiorkor Edema and other signs Between ages of 18 and 24 months Associated with extreme poverty

Health Effects of Too Little or Too Much Protein Protein-Energy Malnutrition Marasmus Chronic PEM Infants and 6- to 18-month-old children Adults with cancer or starvation

Health Effects of Too Little or Too Much Protein Health Effects of Excess Dietary Protein Kidney function Increases kidney filtration rate, straining function Mineral losses Link between high-protein diet and osteoporosis

Health Effects of Too Little or Too Much Protein Health Effects of Excess Dietary Protein Obesity Correlation with body fat Heart disease Increased saturated fat and cholesterol intake Cancer Increased risk for certain types of cancer