Proteins and Amino Acids

©2005 Thomson-Wadsworth

Key Questions

What are proteins made of? What are the different kinds of

amino acids? What are the three important

functions of protein? What does the body need to make protein tissues? Can protein be used for energy?

Protein gets a

positive press

People know protein is important

Nearly all in U.S. get enough protein

15% of total calories in U.S. adult diet supplied by protein, far above what we need.

Image versus Reality

Term protein is derived from the Greek word protos for “first”

Indicates importance Essential structural component

of all life Involved in most biological

processes

Four Functions of Protein

Integral structural component of skeletal muscle, bone, connective tissues, organs, red blood cells and hemoglobin, hair, and fingernails Collagen is most common protein here

Basic substance that makes up digestive and other enzymes in the human body

Major component of hormones such as insulin

Also serves as an energy source

Protein Dynamics

A 154-pound man has 24 lbs of protein

All protein is broken down and re-built

Protein turnover uses 9 oz each day

We only consume 2-3 oz per day

Protein Dynamics

Most protein is recycled from protein substances being ‘turned over’

Proteins play key roles in repair of tissues and replacing tissue proteins damaged by illness or injury

Source of Energy

Protein also serves as a source of energy

Protein contains nitrogen, not stored For energy, amino acids first lose

nitrogen Free nitrogen in excess excreted in

urine Nitrogen excretion increases need for

water Amino acids converted to glucose or

fat, then used to form energy

Amino Acids

•The “building blocks” of protein are amino acids

•Protein in food is broken down by digestive enzymes and absorbed into the bloodstream as amino acids

•20 common amino acids form proteins when linked together

•Each type of protein in the body is a unique combination of amino acids linked together in chains

•DNA is genetic material that directs protein synthesis and organizes amino acids into chains

Alanine: A Simple Amino Acid

Carbon - grey

Hydrogen - white

Nitrogen - Blue

Oxygen - Red

mRNA

Protein Structures: Insulin

For each protein, its chain of amino acids fold up into a unique, complex shape

Insulin proteins function because of their unique shapes.

Collagen: main structural protein

Healthy Collagen requires Vitamin C

As a coenzyme for collagen formation

Ras protein: 3-D Structure

This molecular switch governs cell growth

Knowing its structure may enable interventions to shut off switch in cancer cells

Red blood cells and fibrin are mostly protein

Exactly which amino acids and their arrangement determine whether the protein is an enzyme, a component of red blood cells, a muscle fiber, or another tissue made from protein

Fig. 15-3, p. 3

Proteins rarely ‘solo’

Proteins usually work by assembling into protein complexes

Function like complicated machines

Execute protein synthesis and degradation, cell-to-cell signaling, and many other operations.

Amino Acids

Essential Amino Acids

Nine of the 20 common amino acids are considered essential

11 are nonessential All 20 are required to build and

maintain protein tissues Body cannot produce (or produce

enough) essential amino acids, so they must be provided by the diet

Food proteins contain essential and nonessential amino acids

Proteins Differ in Quality

How proteins support tissue construction is protein “quality”

High quality have a balance of essential amino acids to support tissue needs

If essential amino acids are missing, proteins can not be built

Proteins Differ in Quality

When required level of essential amino acid is lacking, remaining amino acids are used for energy

Amino acids are not stored in the body, so we need essential amino acids daily

We need foods that provide enough of all essential amino acids EVERY DAY.

Complete Proteins

Food sources of high-quality protein are called complete proteins

Complete proteins include those found in animal products and soy products

Incomplete proteins are deficient in one or more essential amino acids

Most plant proteins are “incomplete” Vegetarians “complement” essential

amino acid composition of plant protein by combining them to form a “complete” source of protein

More on VegetarianismNext Lecture

Amino Acid Supplements

Amino acids are natural, so people think they’re harmless

But high intakes of amino acids can harm health

High amounts disrupt protein synthesis by causing a surplus of some amino acids and a deficit of others

Amino Acid Supplements Can

Cause loss of appetite, diarrhea, and gastrointestinal upsets

Increase the workload of the liver and kidneys

Increase risk of dehydration Only known benefit of supplements:

Big Money-maker for Supplement Companies!

Build Muscle?

Amino acids and protein powders do NOT cause muscles to grow

No amino acids are delivered directly to our muscles and cause muscles to enlarge

Only exercise and a good diet (and, too frequently, steroids!) build muscles Most athletes get more than enough protein from

their usual diet

Food as a Source of Protein

Average intake in the U.S. is 75 grams per day RDA: 56 grams for men and 46 grams for women 70% comes from meats, milk, and other animal

products Dried beans and grains are good sources Most protein provides vitamins, minerals, and fat

as well Beef and pork are good sources of iron Calories from lean sirloin steak are 37% fat and

63% from protein

Food Sources of Protein

Tuna 3 oz. 24 grams Beef steak 3 oz. 26 grams Milk 16 oz. 16 grams Cereal and bread 12 grams (6 servings) ____________ Total 78 grams RDA for protein, men: 56 grams

Food Sources of Protein

Milk, 16 oz. 16 grams 1 cup oatmeal 6 grams Peanut butter sandwich 16 grams 2 bean burritos 28 grams 1 cup Spanish rice 4 grams_________________________________ 70 grams

Too Little Protein

Protein deficiency occurs alone or with deficiency of calories and other nutrients

Protein foods contain nutrients (iron, zinc, vitamin B12, niacin)

Body protein will be used for energy during starvation

Body will take protein from liver, intestines, heart, muscles, other organs and tissues

Loss of 30% of body protein causes reduced body strength for breathing, susceptibility to infection, abnormal organ functions, and death

Marasmus,Marasmus,

Protein- Protein- caloriecalorie

MalnutritionMalnutrition

Marasmus

Protein deficiency and lack of calories cause marasmus, or protein-energy malnutrition

Frequent where hunger and famine are common Occurs among people with anorexia nervosa and

diseases such as cancer and AIDS Individuals with marasmus look starved They have little body fat Must utilize protein from muscles, liver, and

tissues as an energy source

Marasmus Protein-calorie Malnutrition

Very susceptible to infection, have dry skin and brittle hair

Use of the fat pads in the cheeks (last fat used for energy during starvation)

Accompanied by vitamin and mineral deficiencies

Conditions are complicated by infection, diarrhea, and

dehydration

Fig. 15-12, p. 8

Is There Enough Protein?

Mammals: Expensive Scarce

Poultry & Fish Plant sources Insects

Abundant Traditional in many

cultures

At the Other Extreme…

Too Much Protein?

Adults can consume 35% of total calories without immediate ill effects

Consumption of 45% of total calories from protein is too high

This much protein causes nausea, weakness, and diarrhea; diets very high in protein result in death after several weeks

Disease from excess protein intake termed “rabbit fever”

Too much protein?

High Protein Intakes

Generally accompanied by high-fat and low-fiber intakes

Foods high in protein (hamburger, cheese, eggs) are often high in fat and contain little or no fiber

Lean meats provide a large part of their total calories as fat

Too Much Protein?

High protein diets implicated in weak bones, kidney stones, cancer, heart disease, and obesity.

Tolerable Upper Intake Level for protein has not been established

People are cautioned not to consume high levels of protein from foods or supplements

Trends

2/3 of U.S. protein comes from animal products

Wealth leads to meat protein increase

Increased consumption of fat, too Rates of heart disease and some

cancers increase Adequate dietary intakes without

excess fat have beans and grains mixed with lean meats and low-fat dairy products

END