Protein Lancelot McLean, PhD lmclean@llu.edu

Proteins More complex than carbohydrates or fats Twenty amino acids Different characteristics Essential amino acids Cannot be synthesized (or produced in sufficient amounts) by the body Must be obtained from diet Nonessential amino acids Can be synthesized in sufficient amounts by the body

20 Amino acids

Essential and Nonessential Amino Acids

Primary Structure The linear sequence of amino acids in a protein is called the primary structure of the protein.

Secondary Structure The structure brought about by the formation of hydrogen bonds between the C=O and N-H groups of the amino acids in the polypeptide chain. Consists of several repeating patterns.

Tertiary structure A description of the way the polypeptide chain folds itself into its final 3-dimensional shape. Held together by interactions between the side chains ( R groups). 1) Hydrogen bonds 2) Disulfide bonds 3) Ionic interactions 4) Hydrophobic interactions

Quaternary structure The 3D structure formed by interaction of more than one polypeptide chain Subunits are held together by noncovalent interactions (e.g. H-bonds, ionic bonds and hydrophobic interactions Subunits may either function independently or cooperatively

Proteins Denaturation Disruption of stability Uncoil and lose shape Stomach acid as well as heat can cause denaturation

Protein Digestion Mouth Stomach Small intestine Crushed and moistened Hydrochloric acid denatures proteins Pepsin cleaves proteins to smaller polypeptides and some free amino acids Small intestine Hydrolysis reactions breaks down polypeptides to shorter tripeptides, dipeptides and amino acids Peptidase enzymes split tripeptides and dipeptides into single amino acids

Protein Digestion

Breaking Down Nutrients for Energy Amino acids can be used for a) energy b) to synthesize needed compounds

Protein Synthesis

Roles of Proteins Growth and maintenance Building blocks for most body structures Muscles, blood, skin, bone, teeth etc. Replacement of dead or damaged cells Skin cell lifespan is about 30 days GI tract cells about a few days Enzymes Break down, build up, and transform substances Catalysts

Roles of Proteins Hormones Messenger molecules Transported in blood to target tissues Regulatory molecules Acid base regulators Proteins have negative charges, attract H+ ions thereby maintaining acid base balance Fluid balance regulators Proteins attract water – plays role in edema

Roles of Proteins Transporters Hemoglobin carries O2 Lipoproteins transport lipids Special transport proteins carry vitamins and minerals

Roles of Proteins Antibodies Defend body against disease Specificity Immunity – memory Source of energy and glucose During starvation and insufficient carbohydrate intake

Nitrogen Balance Nitrogen balance occurs when the amount of nitrogen consumed equals that of the nitrogen excreted in the urine, sweat, and feces. Most healthy adults are normally in nitrogen balance Positive nitrogen balance Negative nitrogen balance

Nitrogen Balance Adequate protein intake in adults yields a Nitrogen Balance In children or in individuals undergoing healing, there is a net increase in the amount of nitrogen in the body. Positive Nitrogen Balance In wasting or degeneration there is a Negative Nitrogen Balance

Quality of protein The quality of a dietary protein is a measure of its ability to provide the essential amino acids required for tissue maintenance Proteins from animal sources have a high quality because they contain all the essential amino acids in proportions similar to those required for synthesis of human tissue proteins Meat, poultry, milk and fish Proteins from plant sources have a lower quality than do animal proteins. However, proteins from different plant sources may be combined in such a way that the result is equivalent in nutritional value to animal protein Soy protein, legumes, nuts, grains, vegetables

Quality of protein Proteins from plant sources Combing 2 incomplete proteins that have complementary amino acid deficiencies results in a mixture with a higher biologic value Isoleucine Lysine Methionine Tryptophan Legumes + - Grains Together

Recommended intakes for protein 10-35% of energy intake Some individuals may require more People who exercise strenuously on a regular basis e.g. athletes Women who are pregnant or lactating Infants to support growth

Consumption of excess protein Protein consumed in excess of the body’s needs is deaminated, and the resulting carbon skeletons are metabolized to provide energy or acetyl CoA for fatty acid synthesis When excess protein is eliminated from the body as urinary nitrogen, it is often accompanied by increased urinary calcium, increasing the risk of nephrolithiasis (kidney stones) and osteoporosis

Negative Health Effects of High-Protein High-protein diets Osteoporosis Calcium excretion increases when protein intake is high Some research suggests that animal protein may be more detrimental than plant protein

Negative Health Effects of High-Protein High-protein diets Heart disease Animal-protein intake Rich in saturated fats Homocysteine levels Associated with increased oxidative stress and inflammation Coffee’s role in heart disease has been controversial Research suggests it is among the most influential factors in raising homocysteine levels Homocysteine

Negative Health Effects of High-Protein High-protein diets Cancer Protein-rich foods Red meat and processed meats Cancer of the colon Kidney disease Acceleration of kidney deterioration Due to excretion of end products of protein metabolism Especially in people with kidney disease

Negative Effects of too little protein When carbohydrate intake is low, amino acids are deaminated to provide carbon skeletons for the synthesis of glucose that is needed as a fuel by the CNS When carbohydrate intake is low, substantial amounts of protein are metabolized to provide precursors for gluconeogenesis

Negative Effects of too little protein Protein energy malnutrition (PEM) In developed countries PEM is most frequently seen in patients with medical conditions that decrease appetite or alter how nutrients are digested or absorbed, or in hospitalized patients with major trauma or infections In developing countries, an inadequate intake of protein and/or energy is the primary cause of PEM

Negative Effects of too little protein Protein Energy Malnutrition (PEM) Affected individuals show a variety of symptoms, including a depressed immune system with a reduced ability to resist infection Death from secondary infection is common Two extreme forms of PEM are kwashiorkor and marasmus

Protein Energy Malnutrition (PEM) Kwashiorkor Results from a sudden and recent deprivation of food (acute) Mainly caused by Protein deficiency Illness such as infection Protein deprivation is associated with severely decreased synthesis of visceral protein Frequently seen in children after weaning between 18 months and 2 years of age, when their diet consists predominantly of carbohydrates

Protein Energy Malnutrition (PEM) Kwashiorkor Typical symptoms include stunted growth, edema, skin lesions, depigmented hair, anorexia Edema results from the lack of adequate plasma proteins to maintain the distribution of water between blood and tissues

Protein Energy Malnutrition (PEM) Marasmus (chronic) Results from severe deprivation of food over a long time (chronic) Occurs mostly in children from 6 – 18 months of age in impoverished areas of the world Learning disability Brain normally grows to almost full adult size within first 2 years of life

Protein Energy Malnutrition (PEM) Marasmus (chronic) Typical symptoms include arrested growth, extreme muscle wasting, weakness, and anemia