Proteins

Chapter 6: Proteins

ObjectivesUpon completion of this chapter, the student will be able to:• describe how amino acids make up protein.• distinguish between essential and nonessential amino acids.• explain why adequate amounts of each of the essential amino acids

are required for protein synthesis.• list the primary functions of protein in the body.• calculate the RDA for protein for an adult when a healthy weight is

given.• describe what is represented by positive protein balance, negative

protein balance, and protein equilibrium.• distinguish between high-quality and low-quality protein and the

sources of each• describe how protein-energy malnutrition can eventually lead to

disease in the body.• develop vegetarian diet plans that meet the body’s nutritional needs.

Introduction

• Focus on protein in diet in developing world – crucial, less so in developed world.

• 17% body weight.• Amino acids are building blocks of protein.• Dietary proteins provide N for protein

synthesis.• Proteins in the body perform a variety of

essential functions.• Proteins can be used to produce energy.

Functions of Proteins

• Building blocks of body components• Maintain fluid balance• Contribute to acid/base balance• Building block for hormones and enzymes• Immune function• Gluconeogenesis• Energy yielding

Generic amino acid

R group Acid group Amine group

NH2 O

R C C OH

H

Classification of amino acids

• 20 different amino acids found in balanced diet.

• All important.• 11 non essential

(dispensable).– Can be synthesized in the

body.– N coming from other amino

acids.• 9 essential amino acids

cannot be synthesized.– Must be obtained from

foods

Semi-essential amino acids

ess. AA: methionine --> (semi AA): cysteine

ess. AA: phenyalanine --> (semi AA): tyrosine

If cysteine and tyrosine are not consumed in thediet, methionine and phenyalanine will be used tomake them

Making of a nonessential amino acid

Phenylalanine Tyrosine

(Essential AA) phenylalanine hydroxyase (Nonessential AA)

Phe derived from the diet

Phenylketonuria

Phenylalanine Tyrosine(Essential AA) phenylalanine hydroxyase (Nonessential AA)

“ MISSING; LACKING”

Control Intake Becomes essential

Tyrosine cannot be synthesized and BOTH amino acids must be derived from foods. BOTH amino acids are now essential. Phe intake must be carefully monitored to prevent accumulation of toxic products.

Dietary proteins

• It is the purpose of proteins in our diet to:– Provide the 9 essential amino acids– Provide the nonessential amino acids or

nitrogen to make the nonessential amino acids

• Animal and plant proteins vary in the proportion of essential and nonessential amino acids.

• Important to meet amino acid needs over the course of a day.

Quality of protein• High quality protein or

complete protein– Contains ample amount of

all 9 essential AA– Usually of animal origin.– Important in children’s diet.– Essential amino acids =

30% total protein– Milk is a good idea.

• Low quality protein– Generally of plant origin

(soybeans are good).– Deficient or low in one or

more essential AA.

Limiting amino acid

CCCCC AAAAAAAA CAR CAR CAR RRRRRR CAR CAR R A A A

C is the limiting amino acid in this example

The remaining amino acids are used for energy, converted to fat or CHO – All or none principle

Complementary Protein

Food 1 Food 2

CC CCCC AAAA AA CAR CAR CAR RRR RRR CAR CAR CAR

Complementary Protein

Beans (legumes)

Grains Nuts/seeds

Vegetables

Protein synthesis • DNA mRNA Protein

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Protein turnover

• Constant synthesis and degradation.• Allows cells to adapt to changing circumstances

by producing needed proteins.• GI tract lining is continually replaced.• Amino acids are recycled - Essential

– Daily an adult makes and degrades 250 – 300g of protein – many amino acids recycled.

– Only about 65 – 95g of protein consumed daily.

Protein Organization

• Primary structure– Sequence of AA

• Secondary structure– Bends and folds held

together by bonds• Tertiary structure

– 3-D shape• Quaternary structure

– 2 or more proteins interacting together

Sickle Cell Anemia


Denaturation of proteins

Heat/acid/alkaline/enzymes Alteration of the protein’s three dimensional structure –

dysfunctional – peptide bonds intact.

Protein in foods

• Water packed tuna – nutrient dense protein source.

• Top 5 sources of protein in U.S.– Beef– Poultry– Milk– White bread.– Cheese.

The value of plant protein

• Plant foods can provide energy, fiber, Mg and protein.

• NO CHOLESTEROL.• Little saturated fat.• Legumes can be

beneficial particularly since they contain soluble fiber.– May cause gas.– This can be reduced by

appropriate cooking and soaking.

Interest in Soy protein is growing

• Soybeans harvested and fed to cattle.• Soy protein is similar to animal protein (used in school

lunches).• High in linoleic acid and some linolenic acid.• Calcium source and bone health.

– Reduces risk of osteoporosis.• Contains isoflavones.

– Act like plant estrogens.• May lower cholesterol by 10%.

– Reducing risk of CVD.• Reduce risk of breast cancer?• Reduced risk of kidney stones.

Digestion of Protein in the Stomach

• Proteins are denatured by the cooking (aids digestion) and the stomach acid.

• Gastrin stimulates the release of pepsinogen from the chief cells in the stomach.– Gastrin also stimulates acid secretion

• Pepsinogen is converted to pepsin by the acid in the stomach.

• Pepsin (an enzyme produced in the stomach) breaks down proteins into smaller peptides.

• Breakdown products of protein digestion then enter the small intestine.

Digestion of protein in the small intestine

• Stimulates the release of GIP, CCK from the walls of the intestine.

• Pancreas releases the protein splitting enzymes: trypsin, chymotrypsin, and carboxypeptidase into the duodenum.

• The enzymes will break peptides into smaller peptides and amino acids.

• Peptides and amino acids are ready for absorption.

Protein Absorption

• Active absorption.• Peptide are broken down at the microvilli surface and

within the absorptive cells.• Whole proteins are eventually broken down to amino

acids. • Many different amino acid transport mechanisms. • Amino acids are sent to the liver via portal circulation

where they are combined into proteins, converted into glucose or fat, used for energy or released into the circulation.

• Few whole proteins are absorbed.

Putting proteins to work in the body

• Producing vital body constituents.

• Contributing to acid – base balance.

• Forming hormones and enzymes.

• Contributing to immune function.

• Forming glucose.• Providing energy.• Maintaining fluid balance.

– See figure

Protein needs

• It is important to balance protein intake with protein losses – protein equilibrium.

RDA for Protein

• Protein required to maintain equilibrium

0.8 gm of protein / kg of healthy body weight 154 lb. = 70 kg 2.2 kg/lb.

70 kg x 0.8 g protein = 56 g protein kg healthy body wt

RDA for Protein

• Increased by ~10-15g/day for pregnancy• Endurance athletes may need another 1.2 – 1.4g/kg

healthy weight.– No added advantage to exceeding 2.5x RDA.

• About 8-10% of total kcals• Most of us eat more than the RDA for protein.

– Men in U.s. consume ~ 95g and women 65g daily.

• Excess protein cannot be stored as protein.– Converted into glucose or fat and then stored or metabolized.

• Older adults should try to eat more.

Is a high-protein diet harmful?• These diets are rich in vitamin

B-6, Fe and Zn.• However, low in plant foods

(fiber), vitamins, phytochemicals

• Intake of animal protein increases risk for heart disease (high in saturated fat)

• Excessive intake of red meat is linked with colon cancer.

• Burden on the kidney.– Significant in diabetes.

• Increase calcium loss in urine.• National Academy of Sciences

recommends no more than 2 x RDA for protein.

Individual amino acid supplement

• Supplement may cause imbalances and toxicities.– Methionine and tyrosine most likely to cause harm.

• Body is designed to handle whole proteins.• Supplement can overwhelm the absorptive

mechanism.• Excess of one amino acid can hamper

absorption of other amino acids.• Stick to whole foods for amino acids

Malnutrition

• Kwashiorkor– Inadequate intake of protein combined with an

existing condition.• Marasmus

– Protein-Energy Malnutrition.– Protein calorie malnutrition.

• Individuals consume too little protein and energy.– Seen in hospitalized patients

Kwashiorkor • Low protein density diet• Energy needs are

marginally met• Signs and symptoms:

– Apathy, listlessness, failure to grow, poor weight gain, change in hair color, nutrient deficiency, flaky skin, fatty infiltration in the liver, massive edema in the abdomen and legs


Marasmus

• Starving to death• Insufficient protein,

energy, nutrients.• “skin and bones”

appearance• Little or no

subcutaneous fat• Reduced brain growth


Vegetarian diets• From a necessity to

an option.• Linked to specific

philosophies and religions.

• 12 million vegetarians in U.S.

• Increasing in popularity.

• Healthy option.

Why do people become vegetarian

• Against animal killing.• Religious beliefs.• Cheaper option.• Realization that animals are fed roughage

that we cannot digest.• Promotes high CHO, vitamin and dietary

fiber intake coupled with low cholesterol and saturated fat.

Food planning for vegetarians • Vegans.

– Eat only plant food.

• Fruitarians.– Fruits, nuts and honey.

• Lactovegetarians.– Include dairy products and

plant food.

• Lactoovovegetarians.– As above plus eggs.

• Almost vegetarians– Eat fish.

The Vegan

• Requires creative planning.• Grains and legumes used to high quality protein.

– Important in children.• Variety is key to providing all essential amino

acids.• Combining vegetables.• Fortified breakfast cereals.• Milk is omitted from vegan diet.

– Fortified soy milk a good alternative•

Concerns with vegan diet

Vitamin B-12IronCalciumComplete proteinChildren vegetarians

kcal needsprotein needsnutrient needs

Proteins

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