© 2008 Thomson - Wadsworth Chapter 6 Protein: Amino Acids

© 2008 Thomson - Wadsworth

Chapter 6

Protein: Amino Acids


The Chemist’s View of Proteins

• Proteins are made from 20 different amino acids, 9 of which are essential.

• Each amino acid has an amino group, an acid group, a hydrogen atom, and a side group.

• It is the side group that makes each amino acid unique.

• The sequence of amino acids in each protein determines its unique shape and function.




• Amino AcidsHave unique side groups that result in differences in the size, shape and electrical charge of an amino acid

Nonessential amino acids, also called dispensable amino acids, are ones the body can create. •Nonessential amino acids include alanine, arginine, asparagines, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine.




• Amino AcidsEssential amino acids, also called indispensable amino acids, must be supplied by the foods people consume. • Essential amino acids include histidine, isoleucine, leucine, lysine, methionine, phenyalanine, threonine, tryptophan, and valine.

Conditionally essential amino acids refer to amino acids that are normally nonessential but essential under certain conditions.



• Proteins Amino acid chains are linked by peptide bonds in condensation reactions.• Dipeptides have two amino acids bonded together.

• Tripeptides have three amino acids bonded together.

• Polypeptides have more than two amino acids bonded together.

Amino acid sequences are all different, which allows for a wide variety of possible sequences.





• ProteinsProtein Shapes

•Hydrophilic side groups are attracted to water.

•Hydrophobic side groups repel water.

•Coiled and twisted chains help to provide stability.




• ProteinsProtein Functions

• Some carry and store materials.• Some provide strength.• Some require minerals for activation (example: hemoglobin and the mineral iron).

Protein denaturation is the uncoiling of protein that changes its ability to function.• Proteins can be denatured by heat and acid.• After a certain point, denaturation cannot be reversed.


Digestion and Absorption of Protein

• Stomach acid and enzymes facilitate the digestion of protein.

• It is first denatured, then broken down to polypeptides.

• The small intestine continues to break down protein into smaller peptides and amino acids so it can be absorbed.




• Protein DigestionIn the Stomach

•Protein is denatured by hydrochloric acid.

•Pepsinogen (a proenzyme) is converted into its active form pepsin in the presence of hydrochloric acid.

•Pepsin cleaves proteins into smaller polypeptides.



• Protein DigestionIn the Small Intestine

•Proteases hydrolyze protein into short peptide chains called oligopeptides, which contain four to nine amino acids.

•Peptidases split proteins into amino acids.



• Protein AbsorptionUsed by intestinal cells for energy or synthesis of necessary compounds

Transported to the liverTaking enzyme supplements or consuming predigested proteins is unnecessary


Proteins in the Body

• Proteins are versatile and unique. The synthesis of protein is determined by genetic information.

• Protein is constantly being broken down and synthesized in the body.

• Researchers measure nitrogen balance to study synthesis, degradation and excretion of protein.

• Protein has many important functions in the body.

• Protein can be used for energy if needed; its excesses are stored as fat.

• The study of proteins is called proteomics.



• Protein Synthesis Synthesis is unique for each human being and is determined by the amino acid sequence.

Delivering the instructions through messenger RNA• Carries a code to the nuclear membrane and attaches to ribosomes

• Presents a list to make a strand of protein

Transfer RNA lines up the amino acids and brings them to the messenger




• Protein SynthesisSequencing errors can cause altered proteins to be made.

An example is sickle-cell anemia where an incorrect amino acid sequence interferes with the cell’s ability to carry oxygen.

Nutrients and Gene Expression - Cells regulate gene expression to make the type of protein needed for that cell.• Epigenetics refers to a nutrient’s ability to activate or silence genes without interfering with the genetic sequence.




• Roles of ProteinsBuilding Materials for Growth and Maintenance• A matrix of collagen is filled with minerals to provide strength to bones and teeth.

• Replaces tissues including the skin, hair, nails, and GI tract lining

Enzymes are proteins that facilitate anabolic (building up) and catabolic (breaking down) chemical reactions.

Hormones regulate body processes and some hormones are proteins. An example is insulin.


Fig. 6-9, p. 190

Stepped Art

Enzyme Enzyme Enzyme

AB

BA

BANew compound



• Roles of ProteinsRegulators of Fluid Balance

•Plasma proteins attract water•Maintain the volume of body fluids to prevent edema which is excessive fluid

•Maintain the composition of body fluids



• Roles of ProteinsAcid-Base Regulators

• Act as buffers by keeping solutions acidic or alkaline

• Acids are compounds that release hydrogen ions in a solution.

• Bases are compounds that accept hydrogen ions in a solution.

• Acidosis is high levels of acid in the blood and body fluids.

• Alkalosis is high levels of alkalinity in the blood and body fluids.



• Roles of ProteinsTransporters

•Carry lipids, vitamins, minerals and oxygen in the body

•Act as pumps in cell membranes, transferring compounds from one side of the cell membrane to the other




• Roles of ProteinsAntibodies

• Fight antigens, such as bacteria and viruses, that invade the body

• Provide immunity to fight an antigen more quickly the second time exposure occurs



• Roles of Proteins

Source of energy and glucose if needed

Other Roles•Blood clotting by producing fibrin which forms a solid clot

•Vision by creating light-sensitive pigments in the retina



• A Preview of Protein MetabolismProtein Turnover and the Amino Acid Pool• Protein turnover is the continual making and breaking down of protein.

• Amino acid pool is the supply of amino acids that are available.

Amino acids from food are called exogenous.

Amino acids from within the body are called endogenous.



• A Preview of Protein MetabolismNitrogen Balance

•Zero nitrogen balance is nitrogen equilibrium, when input equals output.

•Positive nitrogen balance means nitrogen consumed is greater than nitrogen excreted.

•Negative nitrogen balance means nitrogen excreted is greater than nitrogen consumed.



• A Preview of Protein Metabolism Using Amino Acids to Make Proteins or Nonessential Amino Acids – Cells can assemble amino acids into the protein needed.

Using Amino Acids to Make Other Compounds • Neurotransmitters are made from the amino acid tyrosine.

• Tyrosine can be made into the melanin pigment or thyroxine.

• Tryptophan makes niacin and serotonin. Using Amino Acids for Energy and Glucose

• There is no readily available storage form of protein.

• Breaks down tissue protein for energy if needed



• A Preview of Protein Metabolism

Deaminating Amino Acids• Nitrogen-containing amino groups are removed.

• Ammonia is released into the bloodstream.• Ammonia is converted into urea by the liver.

• Kidneys filter urea out of the blood.

Using Amino Acids to Make Fat• Excess protein is deaminated and converted into fat.

• Nitrogen is excreted.


Protein in Foods

• Eating foods of high-quality protein is the best assurance to get all the essential amino acids.

• Complementary proteins can also supply all the essential amino acids.

• A diet inadequate in any of the essential amino acids limits protein synthesis.

• The quality of protein is measured by its amino acid content, digestibility, and ability to support growth.


Protein in Foods

• Protein QualityDigestibility

•Depends on protein’s food source– Animal proteins are 90-99% absorbed.– Plant proteins are 70-90% absorbed.– Soy and legumes are 90% absorbed.

•Other foods consumed at the same time can change the digestibility


Protein in Foods

• Protein QualityAmino Acid Composition

• The liver can produce nonessential amino acids.

• Cells must dismantle to produce essential amino acids if they are not provided in the diet.

• Limiting amino acids are those essential amino acids that are supplied in less than the amount needed to support protein synthesis.

Reference Protein is the standard by which other proteins are measured. • Based on their needs for growth and development, preschool children are used to establish this standard.


Protein in Foods

• Protein Quality

High-Quality Proteins• Contains all the essential amino acids• Animal foods contain all the essential amino acids.

• Plant foods are diverse in content and tend to be missing one or more essential amino acids.

Complementary Proteins• Combining plant foods that together contain all the essential amino acids

• Used by vegetarians



Protein in Foods

• Protein Quality

A Measure of Protein Quality - PDCAAS (protein digestibility-corrected amino acid score)•Compares amino acid composition of a protein to human amino acid requirements

•Adjusts for digestibility


Protein in Foods

• Protein Regulation for Food LabelsList protein quantity in grams% Daily Values is not required but reflects quantity and quality of protein using PDCAAS.


Health Effects and Recommended Intakes of Protein

• Protein deficiency and excesses can be harmful to health.

• Protein deficiencies arise from protein-deficient diets and energy-deficient diets.

• This is a worldwide malnutrition problem, especially for young children.

• High-protein diets have been implicated in several chronic diseases.



• Protein-Energy Malnutrition (PEM) – also called protein-kcalorie malnutrition (PCM)Classifying PEM

•Chronic PEM and acute PEM•Maramus, kwashiorkor, or a combination of the two



• PEMMarasmus

• Infancy, 6 to 18 months of age• Severe deprivation or impaired absorption of protein, energy, vitamins and minerals

• Develops slowly• Severe weight loss and muscle wasting, including the heart

• < 60% weight-for-age• Anxiety and apathy• Good appetite is possible• Hair and skin problems



• PEMKwashiorkor

• Older infants and young children, 18 months to 2 years of age

• Inadequate protein intake, infections

• Rapid onset• Some muscle wasting, some fat retention

• Growth is 60-80% weight-for-age• Edema and fatty liver• Apathy, misery, irritability and sadness

• Loss of appetite• Hair and skin problems



• PEMMarasmus-Kwashiorkor Mix

•Both malnutrition and infections•Edema of kwashiorkor•Wasting of marasmus



• PEMInfections

• Lack of antibodies to fight infections• Fever• Fluid imbalances and dysentery• Anemia• Heart failure and possible death

Rehabilitation• Nutrition intervention must be cautious, slowly increasing protein.

• Programs involving local people work better.



• Health Effects of ProteinHeart Disease

•Foods high in animal protein also tend to be high in saturated fat.

•Homocysteine levels increase cardiac risks.

•Arginine may protect against cardiac risks.



• Health Effects of ProteinCancer

•A high intake of animal protein is associated with some cancers.

•Is the problem high protein intake or high fat intake?

Adult Bone Loss (Osteoporosis)•High protein intake associated with increased calcium excretion.

•Inadequate protein intake affects bone health also.



• Health Effects of ProteinWeight Control

• High-protein foods are often high-fat foods.

• Protein at each meal provides satiety.• Adequate protein, moderate fat and sufficient carbohydrate better support weight loss.

Kidney Disease• High protein intake increases the work of the kidneys.

• Does not seem to cause kidney disease



• Recommended Intakes of Protein10-35% energy intakeProtein RDA

•0.8 g/kg/day•Assumptions

– People are healthy.– Protein is mixed quality.– The body will use protein efficiently.



• Recommended Intakes of ProteinAdequate Energy

•Must consider energy intake•Must consider total grams of protein

Protein in abundance is common in the U.S. and Canada.



• Protein and Amino Acid SupplementsMany reasons for supplementsProtein Powders have not been found to improve athletic performance.•Whey protein is a waste product of cheese manufacturing.

•Purified protein preparations increase the work of the kidneys.



• Protein and Amino Acid SupplementsAmino Acid Supplements are not beneficial and can be harmful.•Branched-chain amino acids provide little fuel and can be toxic to the brain.

•Lysine appears safe in certain doses. •Tryptophan has been used experimentally for sleep and pain, but may result in a rare blood disorder.


Nutritional Genomics


Nutritional Genomics

• In the future, genomics labs may be used to analyze an individual’s genes to determine what diseases the individual may be at risk for developing.

• Nutritional genomics involves using a multidisciplinary approach to examine how nutrition affects genes in the human genome.


A Genomics Primer

• Human DNA contains 46 chromosomes made up of a sequence of nucleotide bases.

• Microarray technology is used to analyze gene expression.

• Nutrients are involved in activating or suppressing genes without altering the gene itself.

• Epigenetics is the study of how the environment affects gene expression.

• The benefits of activating or suppressing a particular gene are dependent upon the gene’s role.


Fig. H6-1, p. 208

Cell

2. Chromosome

3. DNA

4.

4. Gene

1. Nucleus

Stepped Art


Genetic Variation and Disease

• Small differences in individual genomes• May affect a disease’s ability to respond to dietary modifications

• Nutritional genomics would allow for personalization of recommendations.

• Single-Gene DisordersMutations cause alterations in single genes.

Phenylketonuria is a single-gene disorder that can be affected by nutritional intervention.


Genetic Variation and Disease

• Multigene DisordersMultiple genes are responsible for the disease.

Heart disease is a multigene disorder that is also influenced by environmental factors.

Genomic research may be helpful in guiding treatment choices.

Variations called single nucleotide polymorphisms (SNPs) may influence an individual’s ability to respond to dietary therapy.


Clinical Concerns

• An increased understanding of the human genome may impact health care by:Increasing knowledge of individual disease risks

Individualizing treatmentIndividualizing medicationsIncreasing knowledge of nongenetic causes of disease

• Some question the benefit of identifying individual genetic markers.

• Even if specific recommendation can be made based on genes, some may choose not to follow recommendations.

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© 2008 Thomson - Wadsworth Chapter 6 Protein: Amino Acids