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Danny M.Pincivero

133 LECTURE - PROTEIN WHAT ARE PROTEINS? • Protein, Greek derivative “protos”, meaning “of prime importance” • Organic molecules • 2nd most abundant component of plants and animals (2d to water) • Proteins provide structure; amino acids form proteins. Building blocks of life. Food sources: • Animal-based food (meat, fish, milk) – highest quality protein • Plant sources: dried beans, peas, nuts, seeds, some vegetables WHAT ARE PROTEINS? Assemblies of amino acids What is an amino acid? How many amino acids? • 20 (same 20 in all animals), 9 are essential, 11 are non-essential Essential amino acid: • Refers to an amino acid that the body CANNOT synthesize….must be ingested. Non-essential amino acid: • Refers to an amino acid that the body CAN synthesize. Conditionally essential amino acids Examples: • Tyrosine is made from phenylalanine • Cysteine is made from methionine KIN 1F03 – Human Nutrition and Health D.M. Pincivero, 2012 134 Conditionally essential amino acids (cont'd) Branch chain amino acids: Isoleucine, leucine, valine 1/3 of all muscle amino acids are these three branch chains Conditionally essential – bodies can make conditionally essential on the condition that we form them from essential amino acids How are amino acids unique? • Side chains • Dictates size, shape, composition, electric charge (dictates whether that amino acid will be bound to a lipid or open to the aqueous medium of a cell, referring to protein channels), pH • Eg sodium channel buried in cell membrane, some maino acids have to be able to form chem bonds with fatty acid tails of phospholipid molecules • Some amino acids are lipophilic, others are hydrophilic ; middle portion, pore of sodium channel is an aqueous medium.. amino acids that are hydrophilic are oriented in this direction 4 levels of protein structure • Primary • Secondary • Tertiary • Quartenary Two amino acids form a peptide bond KIN 1F03 – Human Nutrition and Health D.M. Pincivero, 2012 135 Primary structure: formation of peptide bonds • Amino (NH 2 to carboxylic acid (COOH) connection Example: Glycine + Phenylalanine Amino aicds themselves are a stimulus for making new protein which starts by linking two amion acids together to form a peptide bond which forms a dipeptide Primary structure: formation of a DIPEPTIDE Tripeptide = 3 amion acids Oligopeptide = 4-10 amino acids Polypeptide = >10 amino acids Most natural polypeptides = 50-200 amino acids…..called a PROTEIN KIN 1F03 – Human Nutrition and Health D.M. Pincivero, 2012 136 PROTEIN STRUCTURE • So how many different proteins can we form? Example: • Polypeptide length = 200 amino acids (most common length) • How many amino acids to choose from? 20.. # of combinations: 200 20 Primary structure • Cross-linking by disulfide bridges Examples: • Insulin (sequenced in 1953 by Frederick Sanger) Two chains connected by disulphide bridges.. amino acid containing sulfide portion is cysteine KIN 1F03 – Human Nutrition and Health D.M. Pincivero, 2012 137 Secondary structure • Additional formation of chemical bonds…..2 resulting shapes: • α-helix – hydrogen bonds between N-H and C=O groups • 3.6 amino acids/turn • Example: myosin molecule….coiled coil – polypeptide chain that forms an alpha helix.. polypeptide with addition chem bonds formed btwn every 3.6 amino acids • Cell membrane proteins often have alpha helices that move through central part of membrane • B pleated sheets fold upon each other … still linear chain of amino acids that form bonds with each other • β-pleated sheet • Hydrogen bonds….parallel “layering” of polypeptide portions • Example: albumin KIN 1F03 – Human Nutrition and Health D.M. Pincivero, 2012 138 Tertiary structure: • Protein structure loses symmetry • Involves complex protein folding • Same link of amino acids as secondary just bonded and folding on itself Quartenary structure: • Proteins that contain more than 1 polypeptide chain….each chain is referred to as a “sub-unit” of the protein • DIMER – protein with 2 identical sub-units Example: • Hemoglobin: 2 sub-units of 1 type (α-unit) and 2 of another type (β-unit)… called an α2 2 tetramer. • Protein responsible for transporting oxygen in blood • Each hemoglobin carries 4 oxygens.. huge oxygen binding / carrying capacity Protein denaturation • Have to break down dietary protein through digestive system, cooking process denatures proteins so it loses its chemical structure – good to make protein more bioavailable.. enzymes can get amino acids more easily, bad where core temp starts to denature proteins in body • Eating raw eggs will reduce vitamin B availability • Loss of protein “shape”….energy input overcomes dissociation energy of chemical bonds • Factors: pH, heat, alcohol, oxidation, mechanical agitation Example: egg whites (approx. 15% protein, no CHOL, 40 different proteins) Most abundant protein in them: Ovalbumin = 54% (385 amino acids, glycoprotein) Next most common protein: Ovotransferrin = 12% Heat: denatures proteins • 63 deg C, ovotransferrin denatures • 80 deg C, ovalbumin denatures…..makes egg white firm Mechanical agitation: same effect as heat, but mixes air with water portion to produce “foam-like” structure KIN 1F03 – Human Nutrition and Health D.M. Pincivero, 2012 139 Raw egg whites • Avidin (egg white protein) binds to biotin (B-vitamin) • Produces vitamin B deficiency… no advantage to raw eggs. Protein functions: 1) Mechanical • Collagen: provides structures to all tissues • Biomolecular motors: myosin (when it moves, it results in force production), it’s an enzyme; kinesin, dynein - these proteins use ATP for movement; ATP consumers, found in neurons, found in neurotransmitter movement through the neuron 2) Enzymes • Catalyzes chemical reactions 3) Hormones • Insulin, TSH (thyroid stimulating hormone), leptin (secreted by adipocytes); all proteins, have an endocrine function – secreted by one tissue, have an effect on another tissue 4) Immune function • Antibodies….blood proteins….neutralizes action of antigens, combats foreign bodies 5) Fluid balance • Albumin (most common blood protein) and globulins in blood; • Too big to leave CV system….but attracts fluid out of interstitial space (space surrounding cells) into CV, which helps maintain fluid balance between cardiovasc system and fluid that’s existing outside of all of cells / in the rest of body – interstitial fluid, prevents swelling • Decrease in blood proteins directly tied to accumulation of fluid in tissues due to fact that drawing water out of interstitial space isn`t there.. osmosis factor isn`t there – no pressure to draw fluids out of interstitial space which results in tissue swelling.. fluid accumulates in tissues.. edema.. common sign of protein/energy malnutrition 6) Acid-base balance • Balance between acids and alkaline molecules in blood • Blood pH = 7.4 KIN 1F03 – Human Nutrition and Health D.M. Pincivero, 2012 140 • Blood proteins bind acidic or alkaline atoms/molecules eg. Hemoglobin.. protein found in RBCs, can bind protons produced by tissues and buffer a decreasing pH; functions to transport O2 to tissues; where Hb is delivering O2, it unloads O2 and tissues unload protons which are picked up by Hb which has a buffering function in the blood which prevents pH of blood from dropping b/c proteins in RBCs can pick up those protons, protons produced during the natural course of breathing but blood doesn’t become acidic b/c of Hb picking up protons 7) Transport functions • 70-80% of energy expenditure at rest… ATPases (Na+/K+ and Ca++); pump resestablishes presence of K outside cell and Na inside cell, costs ATP, ATPases are always active, most of O2 is going here when not exercising • With exercise, we introduce another ATPase – myosin; when we exercise, myosin becomes predominant consumer of ATP b/c so many are active with exercise • Lipoproteins (transport mechanisms for triglycerides and cholesterol), maintain structure of outer shell 8) Energy • Oxidative deamination (using proteins for energy) – wants proteins LAST of macronutrients to be used for ATP • Protein is broken down into amino acids, amino group gets ripped off, Removal of nitrogen group from amino acid (amino acid deamination) • C, H, and O used to make glucose (gluconeogenic amino acid) or ketone bodies (ketogenic) ….ketone bodies (ketoacids) converted to Acetyl CoA by neurons • Excess protein converted to fatty acids (happens when carb intake is low and protein intake is high) SUMMARY OF PROTEIN DIGESTION: • Digestion begins in the stomach where HCl is secreted by certain cells in gastric pits of stomach • When pepsinogen is secreted by other gastric cells, reacts with HCl STOMACH HCl: denatures proteins (pH approximately 0.8) Pepsinogen + HCl = pepsin (active at pH = 2.5)…accounts for 10-20% of proteins digested (protein loses its structure b/c we want access to amino acids) SMALL INTESTINE KIN 1F03 – Human Nutrition and Health D.M. Pincivero, 2012 141 • Amino acids and dipeptides enter, small intestine only has to deal with smaller, less complicated molecules Proteases: Catalyzes large to small peptides, breaks down chains Trypsin and chymotrypsin: comes from pancreas • Catalyzes large to small peptides Peptidases (microvilli in intestinal wall) secreted by intestinal wall Catalyzes tri-, and di-peptides into amino acids PROTEIN SYNTHESIS: How is new protein made? • DNA, mRNA, tRNA = new protein • Muscle cells are multinucleated.. have a good capacity for making new proteins by getting sequence of new proteins and DNA, transcribing it.. mRNA, tRNA.. new protein. • Encoding a new protein from blueprint….amino acids taken up by ribosomes (rRNA) o Ribosomal DNA ensures amino acids are ordered in the right sequence in order to make the new complete protein • 2 things to make new protein o 1. “building” supplies  amino acids (dietary sources)  Protein supplements: providing building materials… protein powders made from milk or soy.. o 2. Activate the “process”  Hormones need to provide the signal for cells to grow, drugs affect it too  Exercise (muscle contraction) very potent stimulus for protein production  Dietary protein supports exercise; without exercise, increase in dietary protein just adds calories… taking supplements is only effective if coupled with exercise • Turned on by hormones and signaling molecules KIN 1F03 – Human Nutrition and Health D.M. Pincivero, 2012 142 PROTEIN SYNTHESIS: • Materials necessary?? Amino acids • Non-essential amino acids: obtained from cell and/or CV system • Essential amino acids: obtained from body… protein degradation; breaking down other protein that have these essential amino acids • Protein deficient diets – cells always in process of remodeling but still need supply of proteins from somewhere so low energy diet sees a net breakdown in proteins Other molecules: Amino acid Used to make • Tryptophan Serotonin (regulates sleep), niacin • Tyrosine Norepinephrine and epinephrine • Tyrosine Thyroxine (main active hormone secreted by thyroid gland regulates metabolism) • Histidine Histamine (CV vasodilator) PROTEIN IN FOOD: SOURCES • Animal-based foods, eggs, milk, legumes, grains, some vegetables • Plant based foods don’t supply as rich a supply of protein as animal based foods RECOMMENDATIONS (RDA’s) Years Quantity (grams/kg body mass) 0-6 months 2.2 6 mo. – 1 year 1.6 1-3 years 1.2 4-6 years 1.1 7-10 years 1.0 11-14 years 1.0 KIN 1F03 – Human Nutrition and Health D.M. Pincivero, 2012 143 15-18 years (males) 0.9 15-18 years (females) 0.8 - RDA for protein – infants have the highest need for dietary protein / kg body mass.. decreases with age INCREASED PROTEIN NEEDS: • Infections, burns, illness, surgery – conditions where human tissue has been damaged.. damaged human tissue needs to be regenerated.. can be fixed when dietary intake of protein increases • Pregnancy and lactation – protein needed to support grown fetus, lactation - protein is important for nursing infant Athletes “Strength/power” athletes • 1.7-1.8 grams/kg body mass • Due to accelerated protein turnover resulting from muscle “damaging” exercise • Resistance exercise breaks down muscle protein – causes structural damage to muscle tissue.. eg weight lifting – really sore day or two layer due to structural damage due to loading / tearing of proteins in muscle • Protein is an energy source as well, proteins broken down to use amino acids as energy • Exercise activates intracellular “signaling molecules” for protein synthesis • Dietary protein supplementation contributes to gain in fat free mass and muscle strength.. actually a benefit due to provision of extra amino acids. Useless if not exercising. • Anabolic steroids – doesn’t provide material supplies, goes right to DNA and turns them on, directly to nucleus which activates enzymes and unzips DNA and ramps up new protein production very quickly  accelerates growth of other cells as well eg. Cancer cells “Endurance” athletes • 1.2-1.3 grams/kg body mass (above avg adult but below strength training athlete) • Protein used as an energy substrate during long duration exercise • Exercising for 2 – 3 hours, going to burn down carb supply pretty well, conserves that buy breaking down muscle tissue and using amino acids for energy • Protein used as a substrate during long exercises KIN 1F03 – Human Nutrition and Health D.M. Pincivero, 2012 144 AMINO ACID SUPPLEMENTATION: • Is it helpful?….not necessary if diet meets energy needs with high quality protein sources….just adding extra energy. • BCAA drinks: 7g BCAA’s per serving (less bioavailable than other protein sources), food is typically the best source, most bioavailable protein we have • Because so much of 3 specific amino acids are provided, can saturate the protein transporters… if supplement is taken with food or another protein source, then : Individual amino acids • Can interfere with absorption of other amino acids Excess protein intake • Excess protein intake leads to production of urea.. • Enters urea cycle…..leads to dehydration • Conversion to fat PROTEIN QUALITY: Food sources that contain high or low quality protein Criteria establishing high-quality protein in food: 1. Provides ALL the essential amino acids 2. Provides enough amino acids to serve as nitrogen sources to synthesize non-essential amino acids 3. Easy to digest and absorb Food sources: Animal-based products and soybean-based foods Soybeans: low in cysteine (but still has some so its still high quality), but ok…..more volume can be consumed without risk of CHOL (since it’s plant based) and saturated fats…..reducing cardiovascular risk. Low quality protein • Considered an “incomplete” protein-based food item KIN 1F03 – Human Nutrition and Health D.M. Pincivero, 2012 145 • Lacks adequate amount of amino acids • Plant-based food (except soy)…..but, plant-based proteins are considered COMPLIMENTARY KIN 1F03 – Human Nutrition and Health D.M. Pincivero, 2012 146 PROTEIN QUALITY: What is a complimentary protein-based food item? • Combining plant-based foods to achieve adequate amino acid digestion and absorption Examples: • Beans and rice • Rice and lentils • Pasta and beans….pasta is low in lysine, but high in methionine and cysteine and beans are the opposite. RULE OF THUMB: combine grains and legumes OR legumes and nuts/seeds. PLANT SOURCES OF PROTEIN: • Yeast, breads, pasta, (most common), lentils • What are lentils? Beans that by definition are not high quality protein, don’t deliver all amino acids but are pretty close, low in methanine and cysteine but high in other ones • Proteins in eggs are very bioavailable and high quality, more
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