this key concept video is about the structure and function of proteins by the end of it we will have looked at amino acids the essential and non-essential amino acids the formation of dye and polypeptides the levels of structure of proteins conjugated and nonconjugated proteins fibrous and globular proteins and denaturation of proteins proteins contain the elements carbon hydrogen o oxygen nitrogen and a small amount of sulfur the roles of proteins are diverse and include enzymatic activity for example amas the digestive enzyme of starch and DNA polymerase for DNA replication structural support for example collagen in bones and microtubules within cells transport for example hemoglobin in red blood cells and carrier proteins in plasma membrane grains defense against infection for example antibodies and antimicrobial proteins cell signaling for example hormones and receptor proteins and regulation of gene expression for example transcription factors the monomers of proteins are amino acids and their basic structure is shown here in this diagram they have a central carbon atom known as the alpha carbon as it holds the whole structure together it is bound to a hydrogen atom here then to the left you can see one of the functioning groups the amine group this group consists of a nitrogen atom bonded to two hydrogen atoms and it gives amino acids their basic or alkaline properties to the right of the diagram we can see the carboxy group or coh group this consists of a carbon atom double bonded to an oxygen atom and single bonded to another o group this carboxy group gives amino acids their acidic properties below the alpha carbon we can see the variable R Group which is a name given to the side group this gives the different amino acids their different chemical properties there are 20 different naturally occurring amino acids in the human body each with a different R Group which determines the amino acids shape size charge and reactivity our groups can be hydrophobic repelling water or hydrophilic being attracted to water hydrophilic R groups are polar or charged and can be acidic or basic amino acids are classified as essential or nonessential based on whether the body can synthesize them or not essential amino acids are those that cannot be synthesized by the body and must be obtained from the diet there are nine essential amino acids none essential amino acids are those that the body can synthesize on its own by using other amino acids or through other metabolic processes at certain times non-essential amino acids may become conditionally essential and dietary intake may be necessary to meet the body's needs for example during pregnancy or infancy in pregnancy arginine is a conditionally essential amino acid as it plays a critical role in Fe growth and development and the regulation of blood flow to the uterus and placenta a diet lacking in any of the essential amino acids can lead to protein deficiency vegan diets can provide all the essential amino acids needed however they do require planning as unlike animal-based Foods many plant-based protein sources lack one or more of the essential amino acids in sufficient quantities amino acids can join together to form over a 100,000 different proteins these amino acid chains vary greatly in length from peptides which are only a few amino acids long to proteins made up of thousands of amino acids when amino acids are linked together they form the primary structure of a protein and this order of amino acids along the chain is determined by the genes that code for the proteins the primary structure ultimately determines the protein shape and function as we will soon see but let's start with the basics seeing how just two amino acids are linked together two amino acids can join together to form a dipeptide which is shown in this diagram the amino acids are held together by peptide linkages which are calent bonds these bonds form between the carboxy group of one amino acid and the amino group or Amin group of the next as shown on this diagram here the hydrogen from the amine group and the hydroxy from the carboxy group are lost as water so this is a condensation reaction more amino acids can join together to form polypeptides these are polymers as they are formed from repeating units the amino acids dipeptides can be broken apart by a hydrolysis reaction where water is added to split the bond apart the water splits and replaces the hydroxy group lost from the carboxy group of one amino acid and the hydrogen atom lost from the amine group of the other amino acid now let's look at the different levels of structures of proteins as we said earlier the primary structure of a protein refers to the specific sequence of amino acids that make up the polypeptide chain it is the most fundamental level of protein structure and plays a crucial role in determining the protein's overall structure and hence its role the sequence quence of amino acids is determined by the genetic code stored in an organism's DNA the secondary structure of a polypeptide chain refers to the regular local folding patterns that arise within a specific segment of the polypeptide chain there are two common types of secondary structure the most common being the alpha Helix the alpha Helix shown here has a right-handed spiral confirmation res resembling a coiled spring the structure is stabilized by hydrogen bonds formed between the amide or NH group of one amino acid and the carbonal or c bond o group of the amino acid located four positions down the chain the alpha Helix is a compact and stable structure the other common type of secondary structure is the beta plated sheet the beta plated sheet is made up of beta strands which are segments of the polypeptide chain aligned side by side forming a sheetlike structure the beta strands are typically 3 to 10 amino acids in length and the strands are connected by hydrogen bonds again between the amide group of one amino acid and the carbonal group of an amino acid in an adjacent strand the tertiary structure of the protein is the three-dimensional Arrangement that results from the folding and arrangement of its secondary structure elements and other regions of the polypeptide chain the tertiary structure is essential for a protein's biological function and is determined by various interactions and forces between amino acid residues within the polypeptide chain the bonds and interactions important in the structure include hydrogen bonds which involve electronegative atoms such as oxygen or nitrogen and hydrogen atoms ionic bonds which form between oppositely charged amino acid are groups disulfide bonds which are calent bonds between the sulfur containing amino acid cysteine and methionine these bonds can cross-link different parts of the protein chain and add further stability to the tertiary structure hydrophobic interactions are when the nonpolar hydrophobic R groups of amino acids are repelled by the surrounding aquous solution they form a hydrophobic core which stabilizes the overall protein structure and hydro philic interactions are when the polar or charged AR groups are attracted to the water molecules in the aquous environment the quaternary structure refers to the arrangement and interaction of two or more polypeptide chains to form a functional biologically active protein complex the polypeptide chains also known as subunits interact with each other through hydrogen bonds ionic bonds hydrophobic and hydrophilic interactions and disulfide bonds these interactions help stabilize the overall quary structure and maintain the overall shape of the protein complex proteins are conjugated or nonconjugated nonconjugated proteins also known as simple proteins consist solely of amino acids linked together by peptide linkages examples of these include enzymes hormones such as insulin and structural proteins such as collagen conjugated proteins also known as complex proteins have prosthetic groups which play essential roles in the proteins function the prosthetic groups can be organic or inorganic molecules an example of a conjugated protein is hemoglobin found in red blood cells with the heem groups that bind oxygen proteins can also be categorized by shape in general proteins are separated into fibrous and globular proteins fibrous proteins have elongated thread-like shapes and serve primarily structural or mechanical functions they are characterized by the repetitive amino acid sequences and extended secondary structures which are often rich in beta pleted sheets they are typically insoluble in water strong and not very sensitive to changes in the environment such as temperature and pH fluctuations examples include proteins of connective tissue such as collagen and contract our protein such as actin and myosin globular proteins have three-dimensional Compact and roughly spherical shapes they are essential in various biological processes and perform a wide range of metabolic functions in living organisms globular proteins have a more diverse and irregular amino acid sequence and resulting secondary structure than fibrous proteins they are typically soble in water and sensitive to fluctuations in the environment such as temperature and pH examples include enzymes such as amalay transport proteins such as hemoglobin and signaling proteins such as insulin collagen is the most abundant fibrous protein in the human body and plays a critical role in providing tent cell strength to connective tissues it forms a triple helix structure composed of three polypeptide chains known as Alpha chains each polypeptide chain has repeated amino acid sequences with every third amino acid being glycine the small R group of glycine enables the helical chains to wind around each other and they are held together by hydrogen bonds these triple heles lie parallel but staggered to each other with calent bonds between them which means that collagen provides resistant to stretching and gives tissues their structural Integrity actinomyosin are fibrous proteins found in muscle cells which are essential for muscle contraction actin helps form thin filaments it has binding sites for myosin myosin is a large elongated protein which is the main protein of the thick filaments in muscle its globular heads interact with actin filaments to create cross Bridges and use energy from ATP to undergo confirmational changes that pull the actin filaments closer together resulting in muscle contraction insulin is a hormone a type of signaling protein with a well-defined tertiary structure it is secreted by beta cells in the pancreas to help control blood glucose concentration it is a small protein composed of 51 amino acids in humans the amino acids are organized into two peptide chains the a chain of 21 amino acids and the B chain of 30 amino acids the two chains are connected by disulfide bonds that help stabilize the protein structure it specific structure means it combined to specific receptors on the surface of target cells signaling them to take up glucose hemoglobin is a transport protein it is composed of four protein subunits arranged in a quaternary structure in adults are two alpha and two beta subunits forming the hemoglobin molecule each of the four globin subunits contains a prosthetic hem group that contains an ion atom at its Center the ion atoms bind to oxygen molecules when oxygen binds to one ion atom it causes a confirmational change in the protein making it easier for the other subunits to bind additional oxygen molecules as a result hemoglobin becomes progressively better at binding oxygen as more oxygen molecules abound this is essential for efficient oxygen uptake in the lungs and oxygen release in tissues the bonds that help stabilize the structure of proteins are susceptible to changes in the environment in particular I temperature and pH in extreme cases proteins including enzymes can become denatured in other words their structures break down most of the properties of proteins rely on the 3D Shape if we take enzymes as a typical example of proteins we can see the effect of different temperatures on the activity of enzymes using the graph here as temperature increases the rate of reaction increases this is because the molecules gain kinetic energy move around more and there are more successful collisions between the substrates and enzymes the greater the temperature the greater the kinetic energy this is shown by line Y however at the same time the increase in temperature increases the kinetic energy of the atoms making up the protein molecule and these start to vibrate this can lead to the breaking of the hydrogen and ionic bonds the greater the energy the more bonds are broken and at very high temperatures calent bonds such as dulfi bonds can also break further destabilizing the protein structure as the 3D structure breaks down the protein or enzyme becomes denatured this is shown by line X the combination of line Y and line X results in the rate of reaction curve shown by line Zed you can see that the rate of reaction increases up to an Optimum temperature here then Above This temperature the ch in the shape of the 3D structure outweighs the benefit of the increased kinetic energy of the enzyme and substrate molecules and denaturation takes over as the main influencer of the rate of reaction proteins have specific pH ranges at which they are most stable and functional here you can see the range for pepsin is between 1 and 3.5 whereas for amalay it is between 5 and 8.5 proteins have different charges on their amino acid side chain due to the presence of acidic or basic groups pH affects protein structure by influencing the ionization state of these charge groups when the pH of the environment deviates from the protein's optimal pH it can disrupt the electrostatic interactions that maintain the protein structure at low PH an increased concentration of hydrogen ions disrupts the hydrogen bonds ionic bonds and hydrophobic interactions that help St ize the 3D structure of the protein at high pH hydroxide ions can similarly disrupt these bonds and interactions as a result the protein May unfold lose its secondary and tertiary structures and become less functional this denatured state is often irreversible now that we have looked at the structure and function of proteins the key points to take away are that proteins contain the elements carbon hydrogen oxygen nitrogen and a small amount of sulfur the roles of proteins include enzymes structural support transport defense signaling and Gene regulation amino acids which have amine carboxy and R groups are the monomers of polypeptides R groups can be hydrophobic or hydrophilic hydrophilic R groups can be acidic or basic essential amino acids have to be consumed in the diet non essential amino acids can be synthesized by the body amino acids can be joined together by peptide linkages via condensation reactions the primary structure is the specific sequence of amino acids that make up the polypeptide chain the secondary structure is the regular folding of the polypeptide chain most commonly Alpha heles and beta pled sheets the tertiary structure is the 3D arrangement of the polypeptide chain the quaternary structure is the arrangement and interaction of two or more polypeptide chains non conjugated proteins consists solely of amino acids conjugated proteins have prosthetic groups fibrous proteins have elongated shapes and serve primarily structural or mechanical functions globular proteins have 3D roughly spherical shapes and serve primarily metabolic functions proteins can be denatured by extreme pH or high temperatures