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Learning Outcomes Define the basic building blocks of a protein and describe what makes each protein unique. Describe the process by which cells make proteins. Explain the significance of a protein’s shape. Define and understand genetics and epigenetics and how they relate to protein synthesis. Explain protein digestion, absorption, and circulation.

Learning Outcomes 6 Describe the major functions of proteins in the body. 7 Explain how the body recycles and reuses amino acids. 8 Calculate the amount of protein you need on a daily basis. 9 Describe vegetarianism and variations of vegetarian diets. 10 Know the consequences of protein deficiency and excess.

Nitrogen-containing macronutrient What Are Proteins? Nitrogen-containing macronutrient Amino acid subunits Peptide bond Most abundant organic substance in body Classification Number of amino acids

What Are Proteins? Amino acid structure Common components Central carbon bonded to hydrogen Amino group Nitrogen R-group Carboxylic acid group Shape of protein imparts its function

FIGURE 5.1 Components of an Amino Acid The structure of the R-group makes one amino acid different from another. These 3 components are the same for all amino acids. Amino acids have four parts: an amino group, a central carbon, a carboxylic acid group, and an R-group. 6

What Are Proteins? Amino acids Essential Nine Nonessential Eleven Transamination Conditionally essential Six PKU Animal-derived foods such as meat, milk, fish, and eggs are considered complete protein sources because they contain all of the essential amino acids.

TABLE 5.1 Essential, Nonessential, and Conditionally Essential Amino Acids

Proteins of animal origin Proteins found in plant-derived foods What Are Proteins? Proteins of animal origin Proteins found in plant-derived foods Complete protein source Balanced amount of essential amino acids Incomplete protein source Limiting amino acid Protein complementation Different incomplete proteins

How Do Cells Make Proteins? Protein synthesis Step 1: Cell signaling Cellular needs conveyed to cell nucleus Step 2: Transcription mRNA is constructed using DNA as a template Exits nucleus Chromosome Gene

How Do Cells Make Proteins? Protein synthesis Step 3: Translation mRNA binds to a ribosome tRNA Carry amino acids to the ribosome Peptide is released from ribosome

FIGURE 5.2 Protein Synthesis 1. Cell signaling Cell signaling communicates the need to synthesize a protein to the nucleus. 2. Transcription Transcription of a gene in the nucleus results in the synthesis of a strand of mRNA. 3. Translation The mRNA strand leaves the nucleus and binds to ribosomes in the cytoplasm. tRNA translates the information carried by mRNA by delivering amino acids in the correct sequence to the ribosome, resulting in the production of a polypeptide strand. The steps involved in protein synthesis are three-fold: cell signaling, transcription, and translation. 12

Why Is a Protein’s Shape Critical to Its Function? Primary structure Number and sequence of amino acids in a single chain Determined by DNA code Critical to function Sickle cell anemia

FIGURE 5.3 Primary Structure of a Protein The number and sequence of amino acids in a single polypeptide chain is referred to as a protein’s primary structure. 14

Why Is a Protein’s Shape Critical to Its Function? Secondary structure α-helix β-folded sheet Tertiary structure Interactions between R-groups

FIGURE 5.4 Secondary Structure of a Protein Weak bonds between carboxylic acid and amino groups cause the protein to fold into a secondary structure. Weak chemical bonds fold proteins into three-dimensional spiral- and fan-like shapes, resulting in secondary structures 16

Why Is a Protein’s Shape Critical to Its Function? Quaternary structure Two or more peptide chains join together Not all proteins have this structure Prosthetic groups Often contain minerals Denaturation Denaturing agents

FIGURE 5.5 Quaternary Structure and Prosthetic Groups of Hemoglobin Hemoglobin is made from four polypeptide chains and four iron-containing prosthetic groups called heme. 18

What Is Meant by Genetics and Epigenetics? Genotype DNA Phenotype Physical or biochemical characteristics Mutations Alteration in a gene Inherited if present in DNA of egg or sperm

What Is Meant by Genetics and Epigenetics? Inheritable changes in gene expression not involving DNA sequence May be passed on to next generation Nutritional status May impact long-term epigenetic modifications

How Are Proteins Digested, Absorbed, and Circulated? Digestion Begins in the stomach Gastrin Hydrochloric acid Disrupts chemical bonds Converts proenzyme pepsinogen to pepsin Pepsinogen Proenzyme for pepsin

How Are Proteins Digested, Absorbed, and Circulated? Digestion Small intestine In lumen and within cells Secretin Stimulates pancreas to release bicarbonate Cholecystokinin (CCK) Stimulates release of proenzymes from pancreas

FIGURE 5.6 Overview of Protein Digestion 1. Stomach cells release the hormone gastrin, which enters the blood, causing the release of hydrochloric acid (HCl) and pepsinogen from other cells in the stomach. 2. Hydrochloric acid denatures proteins and converts pepsinogen to pepsin, which begins to digest proteins. 3. Partially digested proteins enter the small intestine and cause the release of the hormones secretin and cholecystokinin (CCK). 4. Secretin stimulates the pancreas to release bicarbonate and proenzymes into the intestine. Bicarbonate neutralizes chyme. CCK also stimulates the pancreas to release proenzymes into the small intestine 5. Pancreatic proenzymes are converted to active enzymes in the small intestine. These enzymes digest polypeptides into tripeptides, dipeptides, and free amino acids. 6. Intestinal enzymes complete protein digestion. Protein digestion occurs in both the stomach and small intestine via pepsin, pancreatic proteases, and intestinal proteases. 23

How Are Proteins Digested, Absorbed, and Circulated? Absorption Amino acids are absorbed in the duodenum Amino acids enter the bloodstream Circulation Travel to the liver for processing

How Are Proteins Digested, Absorbed, and Circulated? Food allergy Major allergens Food labels Immune response Common signs and symptoms Anaphylaxis Food intolerance or sensitivity Lactose intolerance

Why Do You Need Proteins and Amino Acids? Structure Structural materials of the body Growth and development Enzymes Biological catalysts Movement Contraction and relaxation of muscles Voluntary and involuntary movements Actin and myosin

Why Do You Need Proteins and Amino Acids? Transport Escorting substances into and around the body Secondary malnutrition Communication Hormones Cell-signaling proteins Protection Antibodies

Why Do You Need Proteins and Amino Acids? Fluid balance Albumin Edema Regulation of pH Source of glucose and energy Proteolysis Gluconeogenesis Lipogenesis Other purposes Because protein is needed for fluid balance, severe protein deficiency can cause edema. Pressure has been applied to the tops of these feet, demonstrating what is referred to as pitting edema.

FIGURE 5.7 Regulation of Fluid Balance by Albumin 1. Blood is pumped out of the heart and into blood vessels. 2. The narrow diameter of the blood vessels surrounding organs and tissues causes blood pressure to increase. This forces fluid and nutrients (but not albumin) out of the capillary vessels. 3. The increased concentration of albumin causes water to be drawn back into the blood. When this does not happen, edema can occur. Edema is common in protein deficiency when albumin synthesis is limited. Albumin is a protein in the blood that helps regulate fluid balance within and around tissues. © 2014 Cengage Learning® 29

TABLE 5.2 Major Functions of Proteins in the Body

How Does the Body Recycle and Reuse Amino Acids? Protein turnover Adaptation to periods of growth and development Nitrogen excretion Ammonia Liver converts to urea Nitrogen balance and protein status Negative nitrogen balance Positive nitrogen balance

How Much Protein Do You Need? Reasons to consume dietary protein For adequate amounts of essential amino acids For nitrogen DRIs for amino acids Milligrams per kilogram per day (mg/kg/day) Set relative to body size No ULs

FIGURE 5.8 RDAs for the Essential Amino Acids in Adults Source: Institute of Medicine. Dietary Reference Intakes for energy, carbohydrate, fiber, fatty acids, cholesterol, protein, and amino acids. Washington, DC: National Academies Press; 2005. 33

How Much Protein Do You Need? DRIs for protein Expression Grams per day (g/day) Grams per kilogram body weight per day (g/kg/day) Times for greater protein intake No UL Athlete protein needs AMDRs

Can Vegetarian Diets Be Healthy? Forms of vegetarianism Lacto-ovo-vegetarian Lactovegetarian Vegan Micronutrient deficiencies Special recommendations for vegetarians Enjoy assortment of foods Consume foods in moderation

What Are the Consequences of Protein Deficiency and Excess? Protein-energy malnutrition (PEM) Micronutrient deficiencies Risk for infection and illness Marasmus Severe, chronic, overall malnutrition Kwashiorkor Severe edema in extremities Ascites Children with kwashiorkor often have distended abdomens (ascites), edema in their hands and feet, cracked and peeling skin, and an apathetic nature. These children are at especially increased risk for infections.

What Are the Consequences of Protein Deficiency and Excess? Adults Rarely experience kwashiorkor Extreme muscle loss Protein excess High intakes of protein accompany high intakes of fat, saturated fat, and cholesterol Red meat consumption Cancer risk