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

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.

Tryptophan Synthesase Monomer  Some proteins are made of a few amino acids, others contain hundreds

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 Vegetarianism Next 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 B 12, 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, Protein- calorie Malnutrition

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 , 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

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