Chapter 6 Summary
Proteins are made from amino acids that are linked together with peptide bonds. Molecules with 2 amino acids are called dipeptides, those with 3 are tripeptides, and those with over 20 are polypeptides. Amino acids contain a central carbon, a carboxylic acid group, an amino group, and a side chain, or R-group. The R-group defines each amino acid’s chemical nature. The body needs 20 amino acids, 9 of which are essential. However, in certain conditions a nonessential amino acid becomes essential, such as in phenylketonuria (PKU), in which tyrosine becomes conditionally essential. Foods that contain relatively high amounts of all the essential amino acids in the appropriate proportions are said to be complete protein sources. Foods lacking or having low amounts of at least 1 of the essential amino acids are said to contain incomplete proteins. In general, animal-derived foods contain complete proteins, whereas plant-derived foods contain incomplete proteins. Combining incomplete proteins can result in consumption of all the essential amino acids; this is called protein complementation.
The series of steps involving cell signaling, transcription, and translation produce hundreds of thousands of different polypeptides in the body. These steps involve the genetic material DNA as well as mRNA, ribosomes, tRNA, and amino acids. Each gene in a DNA strand contains information about how a protein should be constructed. This information is transcribed into strands of mRNA in the nucleus and then is translated into a polypeptide chain via tRNAs and amino acids in the cytosol.
The sequence of amino acids making up a protein is called its primary structure. The peptide chain then folds into secondary and tertiary structures depending on the chemical nature of its unique amino acid sequence. Finally, peptide chains can join and combine with other substances such as minerals to form a quaternary structure. Disruption of a protein’s shape can occur via mutations in the DNA or denaturing agents, such as heat and heavy metals.
Most cells in the body contain genetic material, or DNA, in their nuclei. Except for egg and sperm cells, human cells contain 46 strands of DNA—each strand representing a chromosome. At conception, the parents’ chromosomes are passed on to the embryo. These chromosomes constitute the offspring’s genetic makeup or genome. An alteration in a cell’s genome, called a mutation, can influence the cell’s ability to produce a functional protein. Some mutations are harmless, perhaps influencing physical characteristics. Other mutations can be dangerous, such as those causing cancer. The study of how nutrition interacts with a person’s genome is called nutrigenomics.
Protein digestion begins in the stomach where the gastric juices denature the protein via hydrochloric acid (HCl). Next, the enzyme pepsin begins the process of selectively hydrolyzing peptide bonds holding the amino acids together. Additional proteases produced in the pancreatic and intestinal cells complete the task of protein digestion. Amino acids and very small peptides are then transported into the cells lining the small intestine by both passive and active transport systems and circulated in the blood to the liver. Sometimes larger polypeptides are absorbed, triggering a food allergy.
Amino acids are used to synthesize proteins needed for structure, catalysis, movement, transport, communication, and protection. All enzymes are proteins, and muscle is made primarily of the proteins actin and myosin. Many hormones and transport molecules are also made of proteins, as are antibodies important for immunity. Proteins such as albumin regulate fluid balance between intravascular and interstitial spaces, whereas others help maintain appropriate pH. Amino acids, themselves, also have diverse roles such as regulation of metabolic reactions and formation of nitrogen-containing, nonprotein substances such as neurotransmitters, DNA, and RNA. Amino acids also play a key role in energy (ATP) production being converted to glucose and broken down for energy (ATP) during periods of need. During times of energy abundance, amino acids are transformed into fat and stored in adipose tissue.
Protein turnover represents the balance between protein synthesis and degradation. When protein (or nitrogen) intake exceeds loss, the body is in positive nitrogen balance. This typically occurs during periods of growth. When nitrogen loss is greater than intake, the body is in negative nitrogen balance. This occurs during illness and starvation. When amino acids are degraded, they undergo deamination. The resultant ammonia is converted to urea in the liver and excreted in the urine.
Dietary Reference Intakes (DRIs) have been established for both the essential amino acids and total protein. In general, requirements are greater for males than females and are highest during infancy, pregnancy, and lactation. It is recommended that healthy adults consume approximately 0.8 g of protein for each kilogram of body weight. This value is the same whether a person is physically active or not. There are no Tolerable Upper Intake Levels (ULs) for the amino acids or protein. The Institute of Medicine’s Acceptable Macronutrient Distribution Ranges (AMDRs) recommend that adults consume 10 to 35% of energy intake from protein. The USDA Dietary Guidelines for Americans and MyPyramid food guidance system suggest that we meet this goal by choosing a variety of protein sources such as low-fat dairy foods, lean meats, and legumes.
There are varying degrees of being vegetarian, from vegans who eat no animal products to lacto-ovo-vegetarians who consume dairy and egg products. Consuming adequate amounts of protein is typically not difficult even for the strictest of vegetarians. However, consuming a vegetarian diet does increase a person’s risk for selected micronutrient deficiencies such as calcium, zinc, iron, and vitamin B12. Consuming a wide variety of nutrient-dense foods can help vegetarians get enough of these nutrients.
Although most people consume adequate amounts of protein, protein deficiency remains a significant health concern worldwide. Protein deficiency is almost always accompanied by some degree of energy deficiency. Thus this state of malnutrition is typically referred to as protein energy malnutrition, or PEM. In children, PEM takes two forms: kwashiorkor and marasmus. Most adults with PEM have symptoms associated with marasmus. In almost all cases, treatment of children and adults with PEM is multifaceted, involving a variety of factors, including provision of adequate nutrition.
For most people, excess protein intake does not result in health complications. However, be careful to limit intake of fat, saturated fat, and cholesterol, which tend to be found in high-protein foods.
Although the availability of high-quality protein foods makes it easy to consume sufficient amounts of this nutrient, at times protein intake may still be inadequate. However, it is relatively simple to assess dietary protein intake using a three-day food record in addition to public-access dietary assessment programs. If protein intake is limited, changes such as consuming low-fat dairy products and lean meats can add important protein to the diet without adding fat.