Chapter 6: Proteins

Overview of Protein

• Body is made up of thousands of protein substances

• Contains nitrogen

• Regulates and maintains body functions

Amino Acid

R group Acid group

Amine group

NH2 O

R C C OH

H

Tyrosine

Serine

ProlineValine

GlycineTryptophan

GlutamineThreonine

Glutamic acidPhenylalanine

CysteineMethionine

Aspartic AcidLysine

AsparagineLeucine

ArginineIsoleucine

AlanineHistidine

Nonessential AAEssential AA

Dietary Protein

• High-quality– complete

• Low-quality– incomplete

• All-or-none principle in protein synthesis

• Limiting amino acid

• Complementary protein

Limiting Amino Acid

CCCCC

AAAAAAAA CAR CAR CAR

RRRRRR CAR CAR

R

A A A

C is the limiting amino acid in this example

Complementary Protein

Food 1 Food 2

CC CCCC

AAAA AA CAR CAR CAR

RRR RRR CAR CAR CAR

Complementary ProteinBeans (legumes)

Grains Nuts/seeds

Vegetables

Protein Synthesis

• DNA contains coded instructions

• Copies of codes are transferred to the cytoplasm (via mRNA)

• Amino acids added one at a time with aid of tRNA

Protein Synthesis

Protein Turnover

• Protein undergo breakdown and synthesis

• Responds to change

• Amino acid can be recycled

Protein Organization

• Peptide bond• Dipeptide• Tripeptide• Oligopeptide

SH SH

CH2 CH2

H O CH H2O H O CH

H N C OH H N C OH H N C N C OH

CH H O CH H O

CH3 H2O CH3

Denaturation of Proteins

Heat/acid/alkaline/enzymes

Results in alteration of the protein’s three

dimensional structure

Protein in Foods

Plant Protein

• Provide protein, minerals, and dietary fiber

• Contain no cholesterol

• Limited saturated fats

• Allow a few weeks for the GI tract to adjust to the higher fiber

• Availability of Beno®

Digestion of Protein in the Stomach

• Proteins are denatured by cooking and acid in the stomach

• Gastrin stimulates the release of acid and pepsin• Pepsin is activated and breaks down proteins into

peptones

Digestion of Protein in the Small Intestine

• Stimulates the release of CCK• Pancreas release the protein splitting

enzymes: trypsin, chymotrypsin, and carboxypeptidase into the duodenum

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

• Peptides and amino acids are ready for absorption

Protein Absorption• Active absorption• Whole proteins 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

Functions of Proteins

• Building blocks of body components

• Maintain fluid balance

• Contribute to acid/base balance

• Building blocks for hormones and enzymes

• Immune function

• Gluconeogenesis

• Energy yielding

RDA for Protein

• Promotes 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 ~10-15 gm /day for pregnancy• Endurance athletes may need 1.5 - 2 gm/kg

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

protein• Excess protein cannot be stored as protein• New DRI for protein coming

Is a High-Protein Diet Harmful?

• 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• Increase calcium loss• National Academy of Sciences recommend no more

than 2 x RDA for protein

Individual Amino Acid Supplement

• Supplement may cause imbalances and toxicities

• Body is designed to handle whole proteins

• Supplement can overwhelm the absorptive mechanism

• Excess of one AA can hamper absorption of other AAs

Malnutrition

• Protein-Energy Malnutrition

• Marasmus– Seen in hospitalized patients

• Kwashiorkor

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• Reduce brain growth

Soy

• Soy protein is similar to animal protein (used in school lunches)

• High in linoleic acid and some linolenic acid

• Calcium source and bone health• Isoflavones• Heart disease• Cancer

Vegetarian Diets

• Why become a vegetarian?

• Vegans

• Fruitarians

• Lactovegetarians

• Lactoovovegetarians

The Vegetarian Diet Pyramid