BMS208 Human Nutrition

Topic 6: Protein & Amino acids

Chris Blanchard

Learning objectives• Describe how the chemical structure of proteins differs from

the structures of carbohydrates and fats.• List the 9 essential amino acids.• Trace the digestion of protein and list the enzymes needed to

complete the process. • Explain the process used by the body to synthesize new

proteins. • List the 8 major functions of protein in the body. • Describe nitrogen balance and provide examples of positive

nitrogen balance, negative nitrogen balance, and equilibrium. • Describe deamination, where it occurs in the body, the

products produced, and the fate of these products.

Learning objectives• Discuss the factors used to evaluate protein quality. • Describe the diseases that result from inadequate intake

of protein and protein-kcalories.• Discuss the health effects of over-consumption of

protein. • Calculate the protein needed daily using the RDA for

protein.• Discuss the health risks of protein and amino acid

supplements. • Define nutritional genomics and explain its potential uses

in health care.

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 acid structure

Amino acid structure

Amino Acids• Have 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,

asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine.

Amino Acids• Essential 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.

Peptides• 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.

Peptides bonds

Protein Shapes• Hydrophilic side groups are attracted to

water.

• Hydrophobic side groups repel water.

• Coiled and twisted chains help to provide stability.

Protein Shapes

Protein Shapes

Protein Functions• Some carry and store materials.

• Some provide strength.

• Some require minerals for activation (example: hemoglobin and the mineral iron).

Protein denaturation• 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• 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.

Digestion

• In 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.

Digestion• In 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.

Absorption• Used by intestinal cells for energy or

synthesis of necessary compounds

• Transported to the liver

• Taking 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 Synthesis• Sequencing 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.

• 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 Proteins• Building 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.

Enzymes

Roles of Proteins• Regulators 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 Proteins• Acid-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 Proteins• Transporters

– 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 Proteins• Antibodies

– 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

Protein Metabolism• Protein 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.

Protein Metabolism• Nitrogen 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.

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

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 Quality• Digestibility

– 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 Quality• Amino 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 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 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 Quality

Protein in Foods• Protein Regulation for Food Labels

– List 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.

Health Effects and Recommended Intakes of Protein

• 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

Protein-Energy Malnutrition• Marasmus

– 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

Protein-Energy Malnutrition• Kwashiorkor

– 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

Protein-Energy Malnutrition• Marasmus-Kwashiorkor Mix

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

Protein-Energy Malnutrition• Infections

– 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 Protein• Heart 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 Protein• Cancer

– 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 Protein• Weight 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 Protein

• 10-35% energy intake

• Protein RDA– 0.8 g/kg/day– Assumptions

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

Recommended Intakes of Protein

• Adequate Energy– Must consider energy intake– Must consider total grams of protein

• Protein in abundance is common in first world countries

Protein and Amino Acid Supplements

• Many reasons for supplements

• Protein 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 Supplements

• Amino 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.

A Genomics Primer

Genetic Variation and Disease• Small differences in individual genomes• May affect ability to respond to dietary

modifications• Nutritional genomics would allow for

personalization of recommendations.• Single-Gene Disorders

– Mutations cause alterations in single genes.– Phenylketonuria is a single-gene disorder that can be

affected by nutritional intervention.

Genetic Variation and Disease• Multigene Disorders

– Multiple 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 treatment– Individualizing medications– Increasing 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.