Proteins and Amino Acids (Chapter 6)

Protein Excess

Adequate protein intake is absolutely essential to life, but too much protein has been hypothesized to affect the health of the kidneys and bones.

Monosodium Glutamate (MSG)

Monosodium glutamate (MSG) is a flavor enhancer that is not tolerated by some individuals.

Essential or indispensable amino acids

-Of the 20 amino acids commonly found in protein, 9 cannot be made by the adult human body. -Indispensable amino acids must be consumed in the diet

Amino Acid Absorption

- Amino acids and di- and tri-peptides enter the mucosal cells of the small intestine using one of several active transport systems. - Amino acids pass from the mucosal cell into the blood and travel to the liver.

Why People with Celiac Disease Must Avoid Gluten

Celiac disease is a disorder that causes damage to the intestines when the protein gluten is eaten. -Gluten intolerance, also called celiac disease, celiac sprue, or gluten-sensitive enteropathy, is a form of food intolerance. Individuals with celiac disease cannot tolerate gluten, a protein found in wheat, rye, and barley. Celiac disease is an autoimmune disease in which gluten triggers the immune system to attack the villi in the small intestine, causing symptoms such as diarrhea, abdominal bloating and cramps, weight loss, and anemia

How Exercise Affects Protein Needs

Certain types of physical activity may increase protein needs.

Using Food Labels to Make Healthy Choices

Food labels provide a ready source of information about the protein content of packaged foods; however, since the labeling of raw meats and fish is voluntary, many of the greatest sources of protein in the diet do not carry food labels.

Protein in Our Food

Our typical protein intake is about 90 grams daily with little change over the past 30 years.

Complementary Proteins

Protein complementation is the process of combining proteins from different sources so that they collectively provide the proportions of amino acids required to meet the body's needs.

Protein Molecules

Proteins are comprised of amino acids which contain a central carbon atom bound to a hydrogen atom, an amino group (which contains nitrogen), an acid group, and a side chain that will vary to give the different 20 amino acids.

Proteins That Regulate Acid-Base Balance

Proteins both in the blood and within the cells act as buffers by attracting or releasing a hydrogen ion to raise or lower the pH. -The chemical reactions of metabolism require a specific level of acidity, or pH, to function properly -The digestive enzyme pepsin works best in the acid environment of the stomach, whereas the pancreatic enzymes operate best in the more neutral environment of the small intestine.

Why Protein Needs Are Increased by Illness and Injury

The body needs extra protein during times of illness and injury to help the body heal and rebuild. -Protein consumed in the diet must supply amino acids to replace losses that occur during protein turnover, to repair damaged tissues, and to synthesize new body proteins for growth -Extreme stresses on the body such as infections, fevers, burns, or surgery increase the amount of protein that is broken down. For the body to heal and rebuild, these losses must be replaced by dietary protein.

Kidney Stones

The increase in urinary calcium excretion associated with high-protein diets has led to speculation that a high protein intake may increase the risk of kidney stones in the urinary tract.

Protein and Amino Acid Intolerances

While some higher protein foods can cause food allergies, some individuals may also experience food intolerances.

When Protein Intake Exceeds Need

[remember: protein is not stored as protein]. If the diet is adequate in energy and high in protein, the amino acids from the excess protein are deaminated and used to produce ATP or converted into acetyl-CoA to synthesize fatty acids for storage.

Enzymes and Protein Hormones

- Enzymes are protein molecules that speed up metabolic reactions but are not used up or destroyed in the process. - Hormones are chemical messengers that are secreted into the blood and act on target cells in other parts of the body. -Hormones made from amino acids are classified as protein or peptide hormones. For instance, insulin and glucagon are peptide hormones involved in maintaining a steady level of blood glucose.

Why Genes Are Regulated (Gene Expression)

- Gene expression is the process whereby the information coded in a gene is used to produce a protein, and is based on the need for the given protein.

Proteins Are Continually Broken Down and Resynthesized

- Protein turnover is the continuous synthesis and breakdown of body proteins, which is necessary for normal growth and maintenance of body tissues.

How Proteins Facilitate and Regulate Body Processes

- Proteins are the molecules that do most of the body's work. -They are necessary for virtually every activity in the body, from promoting chemical reactions and transporting substance into and out of cells to protecting us from disease and allowing us to move

Do We Need Protein and Amino Acid Supplements?

Protein supplements are not needed to meet the protein needs of healthy individuals.

Nutrients that Accompany Protein Sources

- Recommendations for a healthy diet suggest that our diets be based on whole grains, vegetables, and fruits, and provide smaller amounts of lean meats and low-fat dairy products. - Animal sources of protein are high in iron, zinc, and calcium, but also add saturated fat and cholesterol to the diet. - Plant sources of protein are rich in fiber, phytochemicals, and monounsaturated and polyunsaturated fats.

Protein Recommendations

- The RDA for protein for healthy adults is 0.8 g/kg of body weight per day. - Healthy diets can include 10 to 35% of energy from protein.

Protein Digestion

- The chemical digestion of protein begins in the stomach and where hydrochloric acid denatures proteins, activates pepsin. -The acid also activates the protein-digesting enzyme pepsin, which breaks some of the peptide bonds in the polypeptide chains, leaving shorter polypeptides. - Most protein digestion occurs in the small intestine where polypeptides are broken into down by pancreatic protein-digesting enzymes (trypsin and chymotrypsin), and by protein-digesting enzymes in the microvilli of the small intestine. Protein must be broken down into small peptides and amino acids before it can be absorbed into the mucosal cells

Why Proteins Have Three-Dimensional Shapes

- The chemical properties, such as its charge and attraction to water, of the amino acids in the polypeptide chain cause it to fold, contributing to the three-dimensional structure of the protein. -Folds and bends in the chain occur when some of the amino acids attract each other and others repel each other

How Protein Structure Contributes to Function

- The shape of the protein determines its function with changes to its structure changing the functionality of the protein. - Denaturation is the alteration of a protein's three-dimensional structure.

Protein Structure

- To form proteins, amino acids are linked together by peptide bonds between the acid group of one amino acid and the nitrogen atom of the next amino acid. -Like carbohydrates and lipids, protein molecules contain the elements carbon, hydrogen, and oxygen, but proteins are distinguished from carbohydrates and triglycerides by the presence of the element nitrogen in their structure. -To form proteins, amino acids are linked together by peptide bonds, which are the linkages that form between the acid group of one amino acid and the nitrogen atom of the next amino acid (Figure 6.4). When two amino acids are linked with a peptide bond, the molecule formed is called a dipeptide; when three amino acids are linked, they form a tripeptide. Many amino acids bonded together constitute a polypeptide. -The chemical properties of the amino acids in the polypeptide chain cause it to fold, contributing to the three-dimensional structure of the protein.

Translating Recommendations into Healthy Diets (Protein Quality)

- To meet protein needs, it is important to consider both the amount and the quality of the protein. - Protein quality is the measure of how efficiently a protein in the diet can be by used to make body. - Animal proteins are a higher quality protein as they are complete proteins.

Why Some Nutrients Are at Risk of Deficiency in Vegetarian Diets

- Vegetarian diets can easily meet protein needs, but care must be taken to include enough iron and zinc in lacto-ovo vegetarian diets. - Well-planned vegan diets can provide adequate amounts of calcium, vitamin D, iron, zinc and omega-3 fatty acids, but must include supplements or fortified foods to meet the need for vitamin B12.

Marasmus

-A condition in which the individual isn't consuming enough energy which leads to severe body wasting. -Marasmus is due to a deficiency of energy, but protein and other nutrients are usually also insufficient to meet needs. -Essentially it is starvation because calorie intake is not sufficient

Kwashiorkor

-A deficiency of protein in the presence of adequate energy that is more commonly seen in children. -he word "kwashiorkor" comes from the Ga language of coastal Ghana. It means the disease that the first child gets when a second child is born.4 When the new baby is born, the older child is no longer breast-fed. Rather than receiving protein-rich breast milk, the young child is fed a watered-down version of the diet eaten by the rest of the family. This diet is low in protein and is often high in fiber and difficult to digest. -Because protein is needed for the synthesis of new tissue, kwashiorkor hampers growth in height and weight in children. Because proteins are important in immune function, there is an increased susceptibility to infection. There are changes in hair color because less of the skin pigment melanin is made -The bloated belly typical of this condition is a result of both fat accumulating in the liver, because there is not enough protein to transport it to other tissues, and fluid accumulating in the abdomen, because there is not enough protein in the blood to keep water from diffusing out of the blood vessels

Protein, Amino Acids, and Health

-A diet adequate in protein is essential to health. -Dietary protein is needed for growth and to replace body protein that is broken down and lost each day. If too little protein is consumed, the consequences can be dramatic and devastating. Too much protein, particularly if it is derived primarily from animal sources, may also have negative health effects

Protein Quality

-A measure of how efficiently a protein in the diet can be used to make body proteins - Protein quality is a measure of how good the protein in a food is at providing the essential amino acids needed by the body. Because animal amino acid patterns are similar to those of humans, the animal proteins in our diet generally provide a mixture of amino acids that better matches our needs than the amino acid mixtures provided by plant proteins. Animal proteins also tend to be digested more easily than plant proteins; only protein that is digested can contribute amino acids to meet requirements. -Because they are easily digested and supply essential amino acids in the proper proportions for human use, foods of animal origin are generally sources of high-quality protein, or complete dietary protein

How to Meet Nutrient Needs with a Vegetarian Diet

-A vegetarian diet is a pattern of food intake that includes plant-based foods and eliminates some or all foods of animal origin. -Despite the health and economic benefits of vegetarian diets, a poorly planned vegetarian diet can cause nutrient deficiencies. Protein deficiency is a risk when poorly planned vegan diets that contain little high-quality protein are consumed by small children or by adults with increased protein needs, such as pregnant women and those recovering from illness or injury -Vitamin and mineral deficiencies are a greater concern for vegetarians than is protein deficiency>B12 deficiency is a risk because B12 primarily found in animals>fortified foods with B12, vitamin D and calcium can help

How Amino Acids Provide Energy

-Although carbohydrate and fat are more efficient energy sources, amino acids from the diet and from body proteins are also used to provide energy - In order for the body to use amino acids as an energy source, the nitrogen-containing amino group must be removed (deamination) and converted into urea which goes to the kidneys for removal. - The remaining carbon skeleton can be broken down to produce ATP or used to make glucose or fatty acids. -The use of amino acids as an energy source increases both when the diet does not provide enough total energy to meet needs, as in starvation, and when protein is consumed in excess of needs.

High-Quality/Complete dietary protein vs. incomplete dietary protein

-Because animal proteins are easily digested and supply essential amino acids in the proper proportions for human use, foods of animal origin are generally sources of high-quality protein, or complete dietary protein -Compared to animal proteins, plant proteins are usually more difficult to digest and are lower in one or more of the essential amino acids. They are therefore generally referred to as incomplete dietary protein. Exceptions include quinoa and soy protein

Why Aspartame is Dangerous for People with Phenylketonuria (PKU)

-Because the phenylalanine released from aspartame digestion is absorbed into the blood, food products containing this alternative sweetener must be avoided by individuals with a genetic disorder called phenylketonuria (PKU). Individuals with PKU inherit a defective gene for an enzyme called phenylalanine hydroxylase that is needed to metabolize phenylalanine. In those with this faulty gene, the enzyme does not function properly, and they are unable to convert the essential amino acid phenylalanine to the semiessential amino acid tyrosine. Instead, phenylalanine is converted to compounds called phenylketones, which build up in the blood. In infants and children, high phenylketone levels can interfere with brain development, causing intellectual disability. -PKU is an inherited disease in which the body cannot metabolize the amino acid phenylalanine.

Denaturation

-Changes in protein structure can be caused by changes in the physical environment of the protein, such as an increase in temperature or a change in acidity. Such changes cause protein denaturation. In food, the heat of cooking denatures protein, thereby changing its shape and physical properties. -the alteration of a protein's three-dimensional structure.

Hormones

-Chemical messengers that are secreted into the blood and act on target cells in other parts of the body. -Hormones made from amino acids are classified as protein or peptide hormones. For instance, insulin and glucagon are peptide hormones involved in maintaining a steady level of blood glucose.

Protein deficiency

-Diets deficient in protein are most often deficient in energy as well, but a pure protein deficiency can occur when food choices are extremely limited and the staple food of a population is very low in protein. The term protein-energy malnutrition (PEM) is used to refer to the continuum of protein deficiency conditions ranging from pure protein deficiency, called kwashiorkor, to overall energy deficiency, called marasmus. Most protein-energy malnutrition is a combination of the two.

What Are Limiting Amino Acids?

-During the synthesis of a protein, a shortage of one needed amino acid can stop the process. Just as on an assembly line, if one part is missing, the line stops—a different part cannot be substituted -If the missing amino acid is a nonessential amino acid, it can be synthesized, most often by transamination, and protein synthesis can continue. If the missing amino acid is an essential amino acid, the body cannot make the amino acid, but it can break down some of its own proteins to obtain it. If an amino acid cannot be supplied, protein synthesis stops. The limiting amino acid is the essential amino acid that is available in the lowest concentration in relation to the body's need. -The essential amino acid present in shortest supply relative to need is called the limiting amino acid because lack of this amino acid limits the ability to synthesize the needed protein (Figure 6.10). ----- -Proteins from different foods provide different combinations of amino acids, so the limiting amino acids differ in different foods.

Why Undigested Protein Can Cause Food Allergies

-Food allergies are triggered when a protein from the diet is absorbed without being completely digested - Undigested protein fragments that are absorbed can trigger a food allergy. - The eight most common food allergens are the milk, eggs, peanuts, tree nuts, wheat, soy, fish, and shellfish. -A rapid severe allergic reaction that involves more than one part of the body is called anaphylaxis. A severe anaphylactic reaction can cause breathing difficulty or a dangerous drop in blood pressure and can be fatal.

How Protein Needs Change throughout the Life Cycle

-Growth, pregnancy, and lactation increase protein requirements. - Infants and children need more protein per kg of body weight than adults, even though they need less total protein. -The recommended dietary pattern is based on whole grains, legumes, fruits, and vegetables, with smaller amounts of lean meats and low-fat milk products -For an individual to grow, new body proteins must be synthesized. Because infants and children are small, they need less total protein than adults do, but because new protein must be synthesized for growth to occur, protein requirements per unit of body weight are greater than for adults (Figure 6.16). Protein needs are also increased during pregnancy and lactation.

Nitrogen Balance Tests

-Nitrogen balance occurs when nitrogen in=nitrogen out, healthy adults who consume adequate amounts of protein and are maintaining a constant body weight -Negative nitrogen balance (body breaks down more protein then it synthesizes): nitrogen in<nitrogen out. Total body protein decreases, occurs due to injury or illness as well as when diet is too low in protein and energy -Positive nitrogen balance (body synthesizing more protein than it breaks down): nitrogen in >nitrogen out. Total body protein increases. Occurs when the body is growing, during pregnancy, and when individuals are increasing mas by lifting weights

Amino Acid Structure

-Of the 20 amino acids commonly found in protein, 9 cannot be made by the adult human body. - The 9 amino acids the body is unable to make in sufficient amounts are essential or indispensable amino acids and must be consumed in the diet. - The 11 nonessential or dispensable amino acids can be made by the body through the process of transamination, in which an amino acid from one amino acid is transferred to a carbon-containing molecule to form a different amino acid. -Approximately 20 amino acids are commonly found in proteins. Each amino acid consists of a carbon atom bound to four chemical groups: a hydrogen atom; an amino group, which contains nitrogen; an acid group; and a fourth group or side chain that varies in length and structure

Amino Acid Functions in the Body

-Once dietary proteins have been digested and absorbed, their constituent amino acids become available to the body. The amino acids in body tissues and fluids are referred to collectively as the amino acid pool Amino acids available for use in the body is referred to as the amino acid pool. These amino acids come from the diet and from the breakdown of body proteins. -Amino acids in this pool can be metabolized to provide energy—4 kcals/g -When the diet is low in energy amino acids are also used to synthesis glucose and when protein and energy are consumed in excess, amino acids can be used to synthesize fatty acids

How Proteins Protect Us

-Proteins play an important role in protecting the body from injury and invasion by foreign substances. Skin, which is made up primarily of protein, is the first barrier against infection and injury. Foreign particles such as dirt or bacteria that are on the skin cannot enter the body and can be washed away. If the skin is broken and blood vessels are injured, blood-clotting proteins, including fibrin and thrombin, help prevent too much blood from being lost -If a foreign particle such as a virus or bacterium enters the body, the immune system fights it off by synthesizing proteins called antibodies. Each antibody has a unique structure that allows it to attach to a specific invader.

Functions of Body Protein

-Proteins provide structure and facilitate body activities. -Proteins help us move during muscle contraction -The protein keratin gives strength to hair -Proteins help transport substances in the body -Almost all chemical reactions in the body require the help of enzymes, which are proteins

How Proteins Provide Structure

-Proteins provide structure at the cellular level as an integral component of cell membranes. -Proteins provide structure to individual cells and to the body as a whole. In cells, proteins are an integral part of the cell membrane, the cytosol, and the organelles. Skin, hair, and muscle are body structures that are composed largely of protein. The most abundant protein in the body is collagen; it holds cells together and forms the protein framework of bones and teeth. It also forms tendons and ligaments, strengthens artery walls, and is a major constituent of scar tissue.

Transport Proteins

-Proteins transport substances into and out of individual cells and throughout the body. -At the cellular level, transport proteins present in cell membranes help move substances such as glucose and amino acids across the cell membrane into and out of cells. Transport proteins in the blood carry substances from one organ to another

Nitrogen balance

-Since protein is the only macronutrient that contains a significant amount of nitrogen, the amount of protein used by the body can be estimated by comparing nitrogen intake with nitrogen loss. Nitrogen intake is calculated from dietary protein intake. Nitrogen loss or output is measured by totaling the amounts of nitrogen excreted in urine and feces and that lost from skin, sweat, hair, and nails. -The majority of the nitrogen lost is excreted in the urine as urea

Conditionally essential amino acids

-Some amino acids are conditionally essential. These are essential only under certain conditions. For example, the conditionally essential amino acid tyrosine can be made in the body from the essential amino acid phenylalanine. If phenylalanine is in short supply, tyrosine cannot be made and becomes essential in the diet

Nonessential or dispensable amino acids

-The 11 nonessential or dispensable amino acids can be made by the human body and are not required in the diet

The Acceptable Macronutrient Distribution Range (AMDR)

-The Acceptable Macronutrient Distribution Range (AMDR) for protein is 10 to 35% of energy for adults.6,17 This range allows for different food preferences and eating patterns.

How Amino Acids Are Used to Synthesize Proteins

-The amino acids used to synthesize body proteins come from the amino acid pool -The instructions that dictate which amino acids are needed, and in what order they should be combined, are contained in stretches of DNA called genes. - In transcription, this information is copied into a molecule of mRNA, which carries it to the cytosol. - In translation, tRNA translates the mRNA code into a sequence of amino acids.

Protein Turnover

-The continuous synthesis and breakdown of body proteins, which is necessary for normal growth and maintenance of body tissues. -Body proteins are not static but rather are continually broken down and resynthesized. This process, referred to as protein turnover, is necessary for normal growth and maintenance of body tissues and for adaptation to changing situations.

Transcription

-The first step in protein synthesis involves copying, or transcribing, the DNA code from the gene into a molecule of messenger RNA (mRNA). This process is called transcription

Protein Synthesis

-The first step in protein synthesis involves copying, or transcribing, the DNA code from the gene into a molecule of messenger RNA (mRNA). This process is called transcription -Step 2,The mRNA then takes this information from the nucleus of the cell to ribosomes in the cytosol, where proteins are made. -Step 3, Here the information in mRNA is translated through another type of RNA, called transfer RNA (tRNA). Transfer RNA reads the code and delivers the needed amino acids to form a polypeptide chain. This process is called translation

Translation

-The mRNA then takes this information from the nucleus of the cell to ribosomes in the cytosol, where proteins are made. -Here the information in mRNA is translated through another type of RNA, called transfer RNA (tRNA). Transfer RNA reads the code and delivers the needed amino acids to form a polypeptide chain. This process is called translation

Contractile Proteins

-The proteins in muscles allow us to move. When you climb a flight of stairs, walk across the room, or run around the block, you are relying on the muscle proteins actin and myosin, which interact to cause muscles to contract.

High Protein Low Carb Diet (i.e. Atkins)

-To lose weight, you need to eat fewer calories than you expend. So, any diet that limits calorie intake will promote weight loss. High-protein, low-carbohydrate diets seem to defy the laws of thermodynamics because they don't limit calories but they do result in rapid weight loss. -The real reason high-protein diets promote weight loss is that protein in the diet promotes satiety and a lack of carbohydrate causes ketone production, which suppresses appetite.

Synthesis of Nonprotein Molecules

Amino acids are used to make nonprotein molecules that contain nitrogen, such as DNA, RNA, and neurotransmitters. -Amino acids are needed for the synthesis of a variety of nonprotein molecules that contain nitrogen

Heart Disease and Cancer Risk

High-(animal) protein diets can be high in saturated fat and cholesterol, thus leading to increased risk of heart disease and certain types of cancer.

Hydration and Kidney Function

High-protein diets increase the production of urea and other waste products that must be excreted in the urine and therefore can increase water losses>dehydration

Proteins That Regulate Fluid Balance

If protein levels in the blood fall too low, water leaks out of the blood vessels and accumulates in the tissues, causing swelling known as edema. -The distribution of fluid among body cells, the bloodstream, and the spaces between cells is important for homeostasis -Proteins help regulate this fluid balance in two ways. First, protein pumps located in cell membranes transport substances from one side of a membrane to the other. Second, large protein molecules present in the blood hold fluid in the blood by contributing to the concentration of dissolved particles in the bloodstream

Deamination

In order for the body to use amino acids as an energy source, the nitrogen-containing amino group must be removed (deamination) and converted into urea which goes to the kidneys for removal. -The remaining compounds are composed of carbon, hydrogen, and oxygen and can be broken down to produce ATP or used to make glucose or fatty acids.

Sources of Protein in Our Diet

In the United States, about two-thirds of the dietary protein comes from animal sources.

Meeting Recommendations for Protein Intake

Protein consumed in the diet must supply amino acids to replace losses that occur during protein turnover, to repair damaged tissues, and to synthesize new body proteins for growth.

Protein in the Digestive Tract

Protein enters the digestive tract from food, from digestive secretions, and from sloughed gastrointestinal cells. -No matter what the source, the protein must be broken down to amino acids before entering the bloodstream.

Enzymes

Protein molecules that speed up metabolic reactions but are NOT used up or destroyed in the process. - Without the help of enzymes, the metabolic reactions that break down molecules to provide energy and build molecules needed by the body would occur too slowly to support life.

How Protein Requirements Are Determined

Protein requirements can be determined using nitrogen balance, which compares the amount of nitrogen consumed in the diet with the amount excreted. - A negative nitrogen balance occurs when the body breaks down more protein than it synthesizes. - A positive nitrogen balance occurs when total body protein increases, such as in times of growth.

Protein-energy malnutrition (PEM)>>protein deficiency

Protein-energy malnutrition (PEM) is a concern, primarily in developing countries, and is characterized by wasting and an increased susceptibility to infection.

Transamination

The process in which an amino acid from one amino acid is transferred to a carbon-containing molecule to form a different amino acid.

Gene expression

The process whereby the information coded in a gene is used to produce a protein, and is based on the need for the given protein.

How Protein Quality is Measured

The protein digestibility-corrected amino acid score is a measure of protein quality that reflects a protein's digestibility as well as the proportions of amino acids it provides.

What Are the Benefits of Vegetarian Diets?

Vegetarian diets are associated with a lower risk for obesity, diabetes, cardiovascular disease, high blood pressure, and some types of cancer.

How to Plan Healthy Vegetarian Diets

Well-planned vegetarian diets can meet the nutritional needs of individuals throughout the life cycle.

Bone Health

When calcium intake is adequate, high-(animal) protein diets are associated with greater bone mass and fewer fractures. -Adequate protein is essential for healthy bones, but protein intakes above the current RDA have been shown to increase the amount of calcium lost in the urine. Some studies suggest that urinary calcium losses are greater when protein comes from animal rather than vegetable sources.7 These findings have contributed to a widely held belief that high-protein diets (especially diets that are high in animal protein) result in bone loss.

When Energy Intake is Low

When energy is deficient, body proteins, such as enzymes and muscle proteins, are broken down into amino acids that can then be used to generate ATP or synthesize glucose.