Proteins are large macromolecules composed of long chains of amino acid residues. They play many roles in our bodies. Here, we will look at their Structure, Functions, and Amino Acid Sequence. These answers to the question, «What is a protein?» will provide a better understanding of this complex biological structure. However, they are not all that easy to understand. Let’s start with a brief definition of protein.nFunctionsnProteins play an incredibly important role in all kinds of biological processes. Not only do they catalyze biochemical reactions, but they also act as storage and transport molecules. An example is hemoglobin, which transports oxygen together with iron. Iron is stored in proteins called ferritin. These proteins help to keep our bodies hydrated and prevent the passage of germs. But these are just a few of the functions of proteins.nThe most common kinds of proteins are found in human tissues, and there are many more than one hundred different types of these macromolecules. The table below summarizes some of the more common types and functions of proteins in the body. Collagen is the most common structural protein found in the body, making up 30 percent of bone tissue and a large portion of soft tissues. Collagen is composed mainly of glycine and proline amino acids and forms ropes that overlap other proteins.nBlood contains several proteins that act as messengers and transport mediums. Some of these proteins, like immunoglobin, are produced by the lymphocytes of vertebrates. They recognise foreign substances and help the immune system heal itself. Other proteins, such as fibrinogen and thrombin, stop blood loss when tissues are damaged. While some toxins in plants and snakes also appear to have defensive functions, they are actually enzymes that perform other functions.nStructurenA protein’s structure is composed of multiple strands of amino acids arranged in a spiral structure. Hydrogen bonding between the strands is critical in determining the secondary structure of proteins. Alpha helices are the most stable of these structures. They are made of three amino acid residues, with the R group of the first three protruding outward. This type of structure is also called the ss-sheet, and is formed when the side-chain substituents of an amino acid group fit beside the N-H group.nThere are four levels to the structure of a protein. The primary structure refers to the sequence of amino acids found in the protein. These amino acids form the backbone of the protein. The secondary structure refers to the regular folding patterns of this backbone, including alpha helices and beta sheets. Alpha helices are the most common type of protein structure. The beta sheet is another type of structure, and has several distinct regions.nOne method of determining the structure of a protein is circular dichroism spectroscopy. Using circular dichroism spectroscopy, scientists can approximate the proportion of a protein that is made up of different secondary structures. If a protein has more than one type of structure, its spectra are often correlated. Therefore, a protein’s structure can be predicted using this method. If this method can be applied to a protein, it will give a better picture than a single structure.nAmino acid sequencenAmino acid sequences of proteins can be identified by a number of methods. A simple method is to add a reducing agent to a protein solution and trace the concentration of amino acids over time. The amino acids at the terminal end of the protein are known as terminal amino acids. Identifying the amino acid sequence of a protein is an important step in the discovery of the structure of a protein. This method has many advantages, including being an excellent tool for studying protein structure.nAmino acid sequences encode the message about the shape and activity of a protein. But these messages are highly degenerate, so that there are many different sequences encoding the same activity or structure. Comparing amino acid sequences with similar messages will reveal key features of the code and lead to a better understanding of how proteins function. Here are some important facts about amino acid sequencesnThe amino acid sequences of more than sixty plant hemoglobins have been deposited in databases. Elucidating the structure of plant Hbs is important for the discovery of its function. However, the experimental methods are costly and require large amounts of protein material. Fortunately, in silico methods are helping to reduce the gap between structure and sequence. These methods predict the tertiary structure of proteins from their amino acid sequences, which can be an important step toward the discovery of their functions.nStructure of a proteinnThe secondary structure of a protein is called an alpha helix. This structure forms when the amino acid chain extends in a spiral pattern. The O and N atoms of the helix main chain are represented by red and blue balls, respectively. This 3.6-residue-per-turn repeat produces a 7-pointed star structure. In addition to these three basic types of protein structures, there are also several more that we might not be aware of.nBiological proteins are made of twenty amino acids. Each one of them has a different side chain and chemistries. A majority of amino acids have nonpolar side chains. Others have polar, but uncharged, side chains. Regardless of the structure of the protein, the amino acid side chains interact with each other in a variety of ways. The polar side chains form hydrogen bonds, while the hydrophobic side chains interact with each other via weak van der Waals interactions.nSecondary protein structures are also called alpha helices. Alpha helices and beta pleated sheets contribute to the structure of a protein. The tertiary structure, on the other hand, consists of any number of three-dimensional shapes. Alpha helices, for example, contribute to a portion of the tertiary structure, while beta helices contribute to the rest of the tertiary structure.nFunctions of a proteinnProteins are complex molecules that undergo several modifications. The types of modifications that occur are dependent on several factors, including cell type, the cell cycle, extracellular and intracellular signals, and local concentrations of specific modifying molecules. The modifications allow a protein to possess a particular function within the right cell at the right time. As the number of functionally relevant protein domains increases, a protein’s structure may change.nProteins regulate many processes in the body. Enzymes accelerate chemical reactions and are essential for digestion and replication of DNA. Hormones are made of proteins and send signals to control the activity of cells. The hormone insulin regulates the metabolism by promoting glucose absorption in cells. Transport proteins move ions and molecules across cell membranes. A red blood cell contains hemoglobin, a type of protein that carries oxygen throughout the body.nAmino acids form the basis of proteins. These molecules are large compared to other molecules. They consist of long chains of amino acids that are joined together by a peptide bond. There are twenty different types of amino acids. Amino acids that share the same function often have the same amino acid composition and amino acid sequence. Amino acid sequences do not explain all the functions of a protein, but they do give a good insight into the relationship between structure and function.nSources of amino acidsnAmino acids are essential building blocks of proteins and are necessary for the body’s functions. In addition to their role in the construction of proteins, amino acids also have medical uses. Various types of amino acids can have positive and negative effects. L-arginine, for example, is found in many complete protein foods and is also sometimes taken orally or applied as a topical cream. Regardless of the source, L-arginine is generally considered safe as a dietary supplement, and it has been used for a number of medical conditions.nAll animal sources of protein provide the essential amino acids in varying amounts. Besides meat, fish, poultry, and eggs, plant sources of protein are also available. Meat, poultry, eggs, and fish are complete sources of protein, while soy, nuts, seeds, legumes, and whole soy are incomplete sources of protein. In addition to meat, plant-based proteins are plentiful and are considered an excellent source of protein. Some vegetarians find this to be an ideal source of protein because of its ease of use.nMeat and other meat-based products are a great source of histidine and leucine, while milk and dairy products are good sources of isoleucine and valine. Other good sources of BCAAs include nuts, grains, and poultry. Among grains, almonds and chia seeds are excellent sources of whey protein. However, research on how these proteins are absorbed in the body is lacking.nIf cells can produce proteins, why does the human body need them? That is the question that has long puzzled scientists and researchers. In this video, Assistant Professor of Biology, Nathan Ahlgren, explains what proteins are and how they are made. He also describes the diverse functions that proteins play in the human body. This informative video is a must-read for anyone who wants to understand why proteins are essential for our health.nEssential amino acidsnA question that often arises is whether essential amino acids are needed by our bodies. The fact is that amino acids play a pivotal role in making proteins. Some are essential while others are considered conditionally essential, meaning that our bodies are able to make them. These include cysteine, arginine, glutamine, lysine, proline, and tyrosine. If you do not get enough of these amino acids from your diet, you may need to supplement them with an amino acid supplement.nNot all organisms are able to synthesize all of the amino acids required for the production of proteins, so we must get them from food. In the US, we eat more protein than we need, but we’re missing out on essential amino acids. The body makes eight of the twenty amino acids, but the rest needs to be obtained from our diet. There are two classes of amino acids: essential and non-essential. The former are required for the building of proteins in the body, while the latter are not needed for growth and reproduction.nAmino acids have several biologically significant functions. They play a pivotal role in digestion, repair of body tissue, and the formation of new proteins. Amino acids also function as chemical messengers in inter-cell communication. Among other functions, they are important for hormone and enzyme production, and they may also help prevent muscle loss. They may improve mood, sleep, and athletic performance. Fortunately, most people can meet their amino acid needs with a good diet. Protein-rich foods are essential for building new cells and tissues.nIdeally, you should consume a variety of foods rich in protein, including animal and plant-based foods. Increasing your intake of proteins from a variety of sources is the best way to ensure you are getting adequate amounts of all the essential amino acids. While the modern diet provides enough protein for most people, a deficiency is rare in healthy individuals. If you’re not sure you’re getting enough, consult a doctor.nFunctions of proteinsnProteins are the workhorses of a cell, determining not only a cell’s shape and structure but also its functions. They play an integral role in cell signalling, defence mechanisms, facilitated diffusion, active transport, and intracellular response. They are also composed of R groups and differ from one another in their function and structure. In addition to being components of cells, proteins also play a role in intercellular communication.nA protein’s structure is based on large regions called «functional domains» that have specific functions. An enzyme’s active site binds its substrate, for example, and myoglobin has a central crevice containing alpha-helices that bind oxygen molecules. These domains work together to form the overall purpose of the protein. Membrane proteins, on the other hand, weave an alpha-helical structure throughout the membrane. They then project their carbohydrate hydrophilic side chains into extracellular surface domains.nAfter folding, proteins can be modified by enzymes called transferases. These enzymes add small modifier groups to proteins, changing their conformation. These modifier groups act as molecular switches. Some of these modifications are reversible, and different enzymes catalyze the opposite reaction. Enzymes called phosphatases add or remove phosphate groups from proteins. These enzymes are essential for the cell’s metabolism.nMany amino acids are involved in cellular function. Some of these molecules provide structural support and mobility, while others function as a cell’s defense against germs. All amino acids are organic molecules containing an acid and an amino group. The amino acids in a protein are linked together by peptide bonds. Among the most important macromolecules, proteins help perform numerous functions in the body. The three-dimensional structure of proteins in a cell allows them to carry genetic information and act as a catalyst for gene expression.nSources of proteinnProteins are the main actors in cells, carrying out the functions specified by their genes. Although most other biological molecules are relatively inert, proteins are essential to life. Proteins make up nearly half of an Escherichia coli cell’s dry weight, while DNA and RNA make up only about 3%. The proteins expressed in a cell are called a proteome. Here are some common types of protein.nAnimal tissues are rich in protein. The human body contains approximately 30 percent protein. Liver and muscles are high in protein, while red blood cells contain about 30 percent. Animal organs contain higher protein content than the rest of the body, especially those that are low in water. Most animals cannot synthesize some essential amino acids, such as lysine and threonine. Protein molecules are produced by stepwise alignment of amino acids, with the first step derived from aspartate. Only after the synthesis is complete is the protein released into the body’s fluids.nMany products today contain purified proteins. These are the main ingredients of protein supplements, energy bars, and exercise foods. Whole food protein sources, including legumes and nuts, are best because they contain other micronutrients that are complementary to proteins. Whole foods are better for us than purified proteins. They contain essential amino acids, but the added cost of the purified proteins is passed onto the consumer. Ultimately, the consumer will decide whether to buy proteins from a reputable source or not.nPlant foods are also important for protein. Legumes, nuts, and seeds are a great source of protein. Beans, legumes, and tofu are all rich sources of protein. These foods are often consumed as snacks in place of a higher-calorie snack. Similarly, you can add beans and peanut butter to salads and other meals as substitute for carbohydrates. Protein bars can be high in sugar and can be a calorie-laden alternative to regular snack food.nNeed for protein in a healthy dietnThe average adult needs about 0.75 grams of protein per kilo of body weight each day. This translates to approximately 75 to 90 grams of protein for a 75-kilogram man. During active and strenuous physical activities, our needs for protein increase, and we need more protein than a sedentary person does. The daily requirement for protein is dependent on your physical activity level and general health, so it is best to consult a registered dietitian to determine the right amount of protein for your body.nTo meet your protein requirements, make sure you eat a varied diet full of a variety of proteins. While meat, fish, poultry, eggs, and dairy products are the most common food sources, you can also find high-protein alternatives in soy, beans, tofu, nuts, seeds, and whole grains. You can even add protein-rich foods like peanut butter, roasted chickpeas, and nuts to your favorite dishes to get that extra boost of protein. But beware of protein bars — many are full of sugar and do not provide enough amino acids.nWhile there is a general need for a certain amount of protein in our diet, we should keep in mind that too much protein can lead to fat storage in the body. In addition to preventing disease and helping us build muscle, protein is an essential part of a healthy diet. Just keep in mind that you don’t need more than one gram of protein per day — the FDA recommends that adults consume 50 grams of protein per day, which is part of a 2,000-calorie diet.nWhen it comes to choosing the right proteins, you can get enough from meat, eggs, and poultry. However, you should be careful not to overeat these foods, as they contain excess sugar and may contain unhealthy ingredients. Try to add legumes, nuts, seeds, and soy products to your diet to get the recommended amount of protein. These foods are high-protein, low-calorie, and full of fiber. You should also limit your intake of saturated fat and sodium.nNeed for protein in times of illnessnWhen someone is ill, protein intake is particularly important. During times of illness, a person’s body needs more than enough protein to support normal growth and functioning. To meet this need, they should consume approximately 25 to 40 grams of protein at each meal and 10 to 20 grams of protein at snacks. A three-ounce serving of protein is equivalent to the size of a deck of cards. During times of illness, protein intake should be maximized by including more legumes and nuts, which contain high levels of protein.nAdults need approximately 1.2 grams of protein per kilogram of body weight each day. However, this amount can vary, depending on the severity of the disease and the person’s age. For example, a 150-pound woman needs 102 grams of protein a day, while a 180-pound man needs 123 grams of protein a day. Older adults and severely malnourished people may need more protein than this.nA recent study suggests that older adults who consume higher amounts of protein are more likely to be healthier. According to Paul Jacques, co-author of the study and director of the Jean Mayer USDA Human Nutrition Research Center on Aging, the study followed 2,000 older adults for six years. Researchers found that people who consumed the least amount of protein were twice as likely to have difficulty walking and climbing stairs, even after adjusting for other health factors.nBesides being needed for a healthy body, protein also plays an important role in our immune system. We need protein to build our immune system, make antibodies, keep our cells healthy, and perform our everyday activities. If we don’t get enough protein, we are at risk of losing vital muscle tissues. And protein is crucial for fighting infections. When we’re sick, we need more of this vital nutrient to fight off illnesses and improve our quality of life.