Chapter Five

The Proteins & Amino Acids

I. What Proteins Are Made of

Amino Acids– Amine Group– Acid Group– Side Group

All protein molecules contain nitrogen atoms.

A. Essential & Nonessential Amino Acids

Nine amino acids Cannot be made

(synthesized) by the body from other amino acids

Protein foods must be eaten daily that contain these amino acids

11 amino acids Can be made from

other parts– Nitrogen (other

proteins)– Backbone

(carbohydrates and fats)

Cont’d

Some amino acids are considered essential at different stages of life or in states of illness.

“Conditionally essential”

B. Synthesis of Proteins

Peptide bondBetween amino end &

acid end

B. Synthesis of Proteins

Different places along the chain of amino acids & side chains may be attracted to others

B. Synthesis of Proteins

Strand coils & folds back on itself, giving the protein a distinct shape.

Shapes of proteins affect how they are used in the body

C. Denaturation of Proteins

First step in its destructionExcess heat, alcohol, acids or salts of

heavy metals can damage the body’s proteins

Stomach acid opens up the protein’s structure and permits digestive enzymes to act upon the protein.

II. The Functions of Proteins

A. Growth & Maintenance Dietary protein ensures that amino

acids are available to build the proteins needed for new tissue.

Nearly all cells are constantly replaced, requiring protein.

B. Enzymes

Proteins such as enzymes are catalysts that help chemical reactions take place.

Each enzyme is specific for a particular reaction.

C. Hormones

Some hormones, but not all, are proteins

Hormones signal the appropriate enzymes to act.

D. Antibodies

Formed in response to foreign substances in the body.

Each antibody is specific to one foreign substance.

Once an antibody is made for the substance, the body develops immunity to that substance.

E. Fluid Balance

Fluid Balance:Proteins help regulate the quantity of

fluids to help maintain fluid balance.Cells and the spaces between cells

must contain a constant flux of and amount of fluid.

Water can diffuse freely in and out of a cell; proteins can not

F. Acid-Base Balance

Normal body processes produce acids & bases that must be excreted

pH: concentration of hydrogen ionsAcids: release hydrogen ions Bases: accept hydrogen ions Acid-Base Balance: equilibrium

between acids and bases in body

Cont’d

Blood proteins act as buffers for acid-base balance.– Buffers: can give up or add extra

hydrogen to maintain balance

Acidosis or alkalosis may occur if

acid-base balance is upset

G. Transport Proteins

Move nutrients and other molecules in and out of cells – Turn on and off– Hormones do the switching

Move substances from one organ to another– Lipoproteins – Vitamins and minerals

G. Energy Protein may be sacrificed to provide energy

if insufficient carbohydrate and fat are eaten.

Amino acids are degraded for energy. Amino acids will only make proteins if

carbs and fat are providing protein-sparing energy.– Protein-sparing: Leave amino acids alone

to make proteins

III. Digestion & Absorption of Protein

Begins in the stomach with denaturation

Breaks into dipeptides, tripeptides and free amino acids into small intestine

HOW BODY HANDLES PROTEIN

Amino acids taken up by cells Protein synthesis takes place If “non-essential” amino acid is missing, the

body can make it If “essential” amino acid is missing, the protein

synthesis stops and all of the amino acids in place are dismantled and the protein is not made

IV. Protein Quality

A measure of the essential amino acid content of a protein relative to the essential amino acid needs of the body

Biological value (BV) – how well supports nitrogen balance

Reference protein – egg white protein, the standard to which other proteins are compared = 100 score

Complete and Incomplete Proteins

Complete protein: contains all the essential amino acids in the right proportion relative to need

Meat, dairy products, and many soybean products

High Biological Value

Incomplete protein: lacking or low in one or more of the essential amino acids

Most plant based proteins

Low Biological Value

Complimentary Proteins

Two or more food proteins whose amino acid assortments complement each other in such a way that the essential amino acids limited in or missing from each are supplied by the others.

V. Recommended Protein Intakes

Covers the need to replace protein-containing tissue that people lose and wear out daily

TWO WAYS TO ESTIMATE

The RDA for the healthy adult is 0.8 grams of protein per kilogram of body weight

Use desirable weight 220 pounds = 100 kg 100 kg x 0.8 = 80 grams

Percentage of total calories

Dietary Goals 12-15 % of total

calories 2500 calories x .12

to .15 = 300 – 375 calories

75 – 94 grams

VI.Protein & HealthA. Too Little Protein

Protein-Energy Malnutrition (PEM) The world’s main form of malnutrition

1. Kwashiorkor: a deficiency disease caused by inadequate protein in the presence of adequate food energy

2. Marasmus: an energy-deficiency disease; starvation

B. Too Much Protein

No benefits to eating too much protein

Risks associated with overconsumption

Many protein-rich animal sources are high in fat

Zinc loss is observed in animal studies

Cont’d

If amino acids are in excess of needs, the body will remove the amine group and convert the residues to glucose and glycogen or to fat.

High protein intake also increased excretion of calcium

Extra stress on kidneys

C. Choosing Protein Wisely

More is not necessarily better Select 1/3 or less of protein from animal

sources Include legumes:

– Garbanzo beans, great northern beans, kidney beans

– Lentils, lima beans, pinto beans, split peas, white navy beans, soybeans

High quality protein, fat free, high in fiber