This third section on macronutrients covers proteins, from chapter 6. At the end of this section, you should be able to distinguish between essential and non-essential amino acids and explain why adequate amounts of each of the essential amino acids are required for protein synthesis. Describe how amino acids form proteins, identify food sources of protein distinguished between high-quality and low-quality proteins, and describe the concept of complementary proteins. Describe how protein is digested, absorbed, and metabolized in the body.
List primary functions of protein in the body, apply current recommendations for protein intake to determine protein needs for healthy adults, and describe what is meant by positive protein balance, negative protein balance, and protein equilibrium. Describe how protein-calorie malnutrition eventually can lead to disease in the body, and develop healthy plant-based eating patterns that meet the body's nutritional needs. As an overview, the body is made up of thousands of proteins.
Although protein is of key importance in the human body, North Americans generally consume more than they need to maintain health. In the body, protein has a diverse variety of functions, including as a part of important structures in the body. It's involved in regulating and maintaining body functions, and it makes up a key part of the blood.
Protein can also supply energy with an energy density of four calories per gram, and protein provides an essential form of nitrogen in the form of amino acids. Proteins are made of amino acids and there are 20 different amino acids that make up all all proteins. Nine of these are essential amino acids and 11 are non-essential amino acids.
All amino acids are composed of carbon, hydrogen, oxygen, and nitrogen. It's important to note that all macronutrients are composed of carbon, hydrogen, and oxygen, but amino acids are unique in providing nitrogen. Each amino acid has an R group, which is a side chain of atoms that is specific to each amino acid and determines the differences between individual amino acids.
acids. Notice there's a picture of a generic amino acid at the far left. In the green rectangular box, notice there is a central carbon, and this carbon forms four bonds. The first, starting on the right and going clockwise, is the carboxylic acid group, COOH. Second, at the bottom, is a hydrogen atom.
Third, on the left, is an amino group. And finally, at the 12 o'clock position, is the R group. And this R group can vary in length.
length greatly depending on the amino acid from a simple hydrogen atom as shown on the center of the slide for glycine, a CH3 methyl group as shown on the right side of the slide for alanine, to much longer structures with multiple carbons and branches or rings. Each of these side chains is unique in structure and this dictates the configuration of the protein made from the amino acids. Which of the following groups accounts for the differences among amino acids?
A. The amine group. B. R group, C, acid group, or D, keto group? The correct answer is B.
The R group is the only difference between any two amino acids. Branched chain amino acids, BCAAs, are essential amino acids with a branched structure in the R group. Examples in the human diet include leucine, isoleucine, and vanadolidine.
valine. Again, these are essential amino acids. The carbon skeleton of these amino acids can be used by muscles for fuel. Whey protein from milk is a rich source of branched chain amino acids.
This slide includes some important terms. Essential amino acids cannot be synthesized by humans in sufficient amounts or at all, and they must be included in the diet. The quality of protein in foods is dictated by the amount of essential amino acids in that food.
The limiting amino acid is the essential amino acid in lowest concentration in food or in the diet relative to the body's needs. And finally, a conditionally essential amino acid is an amino acid made from essential amino acids in the body if insufficient amounts are eaten. Generally, they're not essential because they can be made in the body.
However, under certain conditions, they can become essential. essential. This may happen during times of rapid growth, disease, or metabolic stress.
Phenylketonuria, PKU, is a genetic disease in which the normally non-essential amino acid, tyrosine, becomes conditionally essential. The condition is PKU, which is characterized by a limited ability to metabolize another amino acid, phenylalanine, which is essential to tyrosine. The enzyme that is used in converting phenylalanine to tyrosine is insufficient. Therefore, tyrosine becomes essential and must be obtained from the diet. Also, since phenylalanine isn't getting converted, the levels of it build up in the blood, potentially to toxic levels that could disrupt brain function, leading to mental retardation.
Within the first few days of life, all newborns are tested for PKU, which is treated with a special diet that limits phenylalanine. As previously mentioned, there are 20 amino acids, and there are normally 9 essential amino acids and 11 non-essential amino acids. Valine, leucine, and isoleucine are the branched chain amino acids. The basic building block of a protein is called a fatty acid, monosaccharide, amino acid, or gene. The correct answer is C, amino acid.
Alright, so proteins are made from amino acids. The amino acids are linked together by chemical bonds called peptide bonds. These bonds form between the amino acid group of one amino acid and the acid carboxyl group of another.
Dipeptides have two amino acids. amino acids, tripeptides have 3 amino acids, oligo peptides 4-9 amino acids, and polypeptides have 10 or more amino acids. Most proteins are polypeptides with 50-2000.
Notice on the left side of the slide, amino acid 1 is the amino acid that is present in the protein. The amino acid that is present in the protein is joins to amino acid 2 to form a dipeptide. The acid group in amino acid 1 gives up the OH, and the amino group in amino acid 2 gives up a hydrogen. The H and the OH form H2O, and water leaves as the dipeptide is formed.
This is an example of a condensation reaction. It's also called dehydration synthesis. Proteins are synthesized using directions from DNA in the nucleus. Think of these as coded instructions, and copies of the code of transcribed to form mRNA, and then transferred to the cytoplasm.
Then amino acids are added one at a time with the aid of transfer RNA, tRNA, in an energy-requiring process, forming a peptide and eventually a functional protein. This process is illustrated on the figure on this slide. In step 1, which takes place in the nucleus of the cell, DNA contains the information necessary to produce proteins. Notice the DNA strand is unwound in the center portion.
In step 2, transcription takes place. This is the copying of the segment of DNA that results in mRNA. of the information in DNA needed to make a protein. In step 3, the transcript, mRNA strand, leaves the nucleus and goes to a ribosome.
In step 4, amino acids, the building blocks of proteins, are carried to the ribosome by tRNAs, transfer RNAs, containing the code that matches the one on the mRNA. Then in step 5 is the process of translation, where the information contained in mRNA is used to determine the number, types, and arrangement of amino acids in the protein. The instructions for making the protein proteins are located in the A.
cell membrane, B. nucleus, C. cytoplasm, or D.
lysosome? The correct answer to this question is B. The instructions for making proteins are located in the nucleus on the strand of DNA. As previously mentioned, the R group of an amino acid differentiates one amino acid from another. Because of the differences in R groups, the order of amino acids in a protein determines its ultimate shape.
The protein's final shape determines its function in the body. The picture on this slide represents a protein in its finished form with four peptides, the four different colors connected. may cause disease, and an example is sickle cell anemia, which is an illness that results from malformation of red blood cells because of an incorrect structure in part of its hemoglobin protein chains.
Glutamic acid is replaced with valine. Red blood cells become hard and sticky and may block blood flow and reduce oxygen getting to tissues, and this may result in cancer. Another important term is denaturation.
Denaturation of proteins is alteration of the protein's three-dimensional structure. As mentioned, the structure of protein determines its function, and denaturation results in loss of biological activity. This may be due to heat, enzymes, acid, or alkaline solutions, or agitation or some combination of these.
Two examples we'll discuss are cooking and digestion. As we'll discuss, the acidic environment of the stomach results in denaturation of proteins, which is one of the first steps in their digestion.