Structural Proteins Can Form Very Strong, Complex Fibrous Structures Such as

Types and Functions of Proteins

Proteins perform many essential physiological functions, including catalyzing biochemical reactions.

Learning Objectives

Differentiate among the types and functions of proteins

Central Takeaways

Central Points

• Proteins are essential for the primary physiological processes of life and perform functions in every system of the human body.
• A poly peptide's shape determines its part.
• Proteins are composed of amino acrid subunits that grade polypeptide bondage.
• Enzymes catalyze biochemical reactions past speeding up chemic reactions, and can either pause down their substrate or build larger molecules from their substrate.
• The shape of an enzyme's agile site matches the shape of the substrate.
• Hormones are a type of protein used for cell signaling and advice.

Cardinal Terms

• amino acid: Any of 20 naturally occurring α-amino acids (having the amino, and carboxylic acid groups on the same carbon atom), and a variety of side chains, that combine, via peptide bonds, to form proteins.
• polypeptide: Any polymer of (same or different) amino acids joined via peptide bonds.
• catalyze: To accelerate a process.

Types and Functions of Proteins

Proteins perform essential functions throughout the systems of the human body. These long chains of amino acids are critically of import for:

• catalyzing chemic reactions
• synthesizing and repairing DNA
• transporting materials across the jail cell
• receiving and sending chemical signals
• responding to stimuli
• providing structural support

Proteins (a polymer) are macromolecules equanimous of amino acid subunits (the monomers ). These amino acids are covalently attached to one another to form long linear chains called polypeptides, which then fold into a specific three-dimensional shape. Sometimes these folded polypeptide chains are functional by themselves. Other times they combine with additional polypeptide chains to course the final protein structure. Sometimes non-polypeptide groups are also required in the final poly peptide. For instance, the blood protein hemogobin is fabricated upward of four polypeptide chains, each of which also contains a heme molecule, which is ring structure with an iron atom in its center.

Proteins accept dissimilar shapes and molecular weights, depending on the amino acid sequence. For instance, hemoglobin is a globular poly peptide, which means information technology folds into a meaty world-like structure, only collagen, institute in our skin, is a fibrous protein, which means information technology folds into a long extended cobweb-similar chain. You probably wait similar to your family members because you share similar proteins, simply yous look different from strangers because the proteins in your eyes, pilus, and the rest of your body are different.

Human Hemoglobin: Structure of human hemoglobin. The proteins' α and β subunits are in ruby-red and blue, and the iron-containing heme groups in greenish. From the protein data base of operations.

Because class determines part, whatever slight change to a protein's shape may cause the protein to become dysfunctional. Small changes in the amino acid sequence of a protein tin cause devastating genetic diseases such as Huntington's disease or sickle cell anemia.

Enzymes

Enzymes are proteins that catalyze biochemical reactions, which otherwise would not take identify. These enzymes are essential for chemical processes like digestion and cellular metabolism. Without enzymes, most physiological processes would proceed so slowly (or non at all) that life could non exist.

Because class determines function, each enzyme is specific to its substrates. The substrates are the reactants that undergo the chemical reaction catalyzed by the enzyme. The location where substrates bind to or interact with the enzyme is known as the active site, because that is the site where the chemistry occurs. When the substrate binds to its active site at the enzyme, the enzyme may aid in its breakdown, rearrangement, or synthesis. By placing the substrate into a specific shape and microenvironment in the active site, the enzyme encourages the chemic reaction to occur. There are two bones classes of enzymes:

Enzyme reaction: A catabolic enzyme reaction showing the substrate matching the verbal shape of the active site.

• Catabolic enzymes: enzymes that break down their substrate
• Anabolic enzymes: enzymes that build more circuitous molecules from their substrates

Enzymes are essential for digestion: the process of breaking larger food molecules downwards into subunits small enough to diffuse through a prison cell membrane and to be used by the cell. These enzymes include amylase, which catalyzes the digestion carbohydrates in the rima oris and modest intestine; pepsin, which catalyzes the digestion of proteins in the breadbasket; lipase, which catalyzes reactions need to emulsify fats in the small intestine; and trypsin, which catalyzes the further digestion of proteins in the small intestine.

Enzymes are likewise essential for biosynthesis: the procedure of making new, complex molecules from the smaller subunits that are provided to or generated by the cell. These biosynthetic enzymes include DNA Polymerase, which catalyzes the synthesis of new strands of the genetic material before cell segmentation; fatty acid synthetase, which the synthesis of new fatty acids for fatty or membrane lipid formation; and components of the ribosome, which catalyzes the formation of new polypeptides from amino acid monomers.

Hormones

Some proteins function as chemical-signaling molecules called hormones. These proteins are secreted by endocrine cells that act to control or regulate specific physiological processes, which include growth, development, metabolism, and reproduction. For case, insulin is a protein hormone that helps to regulate blood glucose levels. Other proteins act as receptors to detect the concentrations of chemicals and ship signals to respond. Some types of hormones, such every bit estrogen and testosterone, are lipid steroids, not proteins.

Other Poly peptide Functions

Proteins perform essential functions throughout the systems of the homo body. In the respiratory organization, hemoglobin (composed of iv protein subunits) transports oxygen for utilise in cellular metabolism. Boosted proteins in the blood plasma and lymph carry nutrients and metabolic waste material products throughout the trunk. The proteins actin and tubulin form cellular structures, while keratin forms the structural support for the dead cells that become fingernails and pilus. Antibodies, likewise called immunoglobins, help recognize and destroy foreign pathogens in the immune system. Actin and myosin allow muscles to contract, while albumin nourishes the early evolution of an embryo or a seedling.

Tubulin: The structural protein tubulin stained red in mouse cells.

Amino Acids

An amino acrid contains an amino group, a carboxyl grouping, and an R group, and it combines with other amino acids to form polypeptide chains.

Learning Objectives

Depict the construction of an amino acrid and the features that confer its specific properties

Primal Takeaways

Cardinal Points

• Each amino acrid contains a cardinal C atom, an amino group (NH2), a carboxyl group (COOH), and a specific R group.
• The R group determines the characteristics (size, polarity, and pH) for each type of amino acid.
• Peptide bonds form between the carboxyl grouping of i amino acid and the amino group of some other through aridity synthesis.
• A concatenation of amino acids is a polypeptide.

Key Terms

• amino acid: Whatsoever of xx naturally occurring α-amino acids (having the amino, and carboxylic acid groups on the aforementioned carbon cantlet), and a variety of side chains, that combine, via peptide bonds, to grade proteins.
• R group: The R grouping is a side concatenation specific to each amino acid that confers particular chemical properties to that amino acrid.
• polypeptide: Whatsoever polymer of (same or unlike) amino acids joined via peptide bonds.

Structure of an Amino Acrid

Amino acids are the monomers that make upward proteins. Each amino acid has the same fundamental structure, which consists of a key carbon cantlet, also known as the blastoff (α) carbon, bonded to an amino group (NH_ii), a carboxyl group (COOH), and to a hydrogen atom. In the aqueous surroundings of the cell, the both the amino group and the carboxyl group are ionized nether physiological conditions, and so take the structures -NH₃ ⁺ and -COO^–, respectively. Every amino acid also has another atom or group of atoms bonded to the central atom known as the R group. This R grouping, or side concatenation, gives each amino acid proteins specific characteristics, including size, polarity, and pH.

Amino acid structure: Amino acids have a central asymmetric carbon to which an amino group, a carboxyl group, a hydrogen cantlet, and a side chain (R group) are attached. This amino acid is unionized, only if information technology were placed in h2o at pH vii, its amino group would pick upwards some other hydrogen and a positive charge, and the hydroxyl in its carboxyl group would lose and a hydrogen and gain a negative charge.

Types of Amino Acids

The name "amino acid" is derived from the amino grouping and carboxyl-acid-grouping in their basic structure. There are 21 amino acids present in proteins, each with a specific R group or side chain. Ten of these are considered essential amino acids in humans because the human body cannot produce them and they must be obtained from the diet. All organisms have different essential amino acids based on their physiology.

Types of amino acids: At that place are 21 common amino acids unremarkably found in proteins, each with a different R group (variant group) that determines its chemic nature. The 21st amino acid, not shown here, is selenocysteine, with an R group of -CH₂-SeH.

Characteristics of Amino Acids

Which categories of amino acrid would you expect to find on the surface of a soluble poly peptide, and which would yous expect to find in the interior? What distribution of amino acids would you wait to observe in a protein embedded in a lipid bilayer?

The chemic composition of the side concatenation determines the characteristics of the amino acrid. Amino acids such equally valine, methionine, and alanine are nonpolar (hydrophobic), while amino acids such as serine, threonine, and cysteine are polar (hydrophilic). The side chains of lysine and arginine are positively charged so these amino acids are also known as basic (high pH) amino acids. Proline is an exception to the standard structure of an amino acid because its R group is linked to the amino group, forming a band-like structure.

Amino acids are represented by a unmarried upper case letter or a 3-letter abbreviation. For example, valine is known by the letter V or the 3-alphabetic character symbol val.

Peptide Bonds

The sequence and the number of amino acids ultimately make up one's mind the protein's shape, size, and role. Each amino acid is attached to another amino acid by a covalent bond, known as a peptide bail. When two amino acids are covalently attached past a peptide bond, the carboxyl group of i amino acid and the amino group of the incoming amino acid combine and release a molecule of water. Any reaction that combines ii monomers in a reaction that generates H₂O as 1 of the products is known as a dehydration reaction, so peptide bond formation is an case of a aridity reaction.

Peptide bond formation: Peptide bond formation is a dehydration synthesis reaction. The carboxyl group of one amino acid is linked to the amino group of the incoming amino acrid. In the process, a molecule of h2o is released.

Polypeptide Chains

The resulting chain of amino acids is called a polypeptide chain. Each polypeptide has a free amino grouping at i end. This end is called the N terminal, or the amino last, and the other end has a gratis carboxyl group, also known as the C or carboxyl terminal. When reading or reporting the amino acid sequence of a protein or polypeptide, the convention is to utilize the N-to-C direction. That is, the kickoff amino acid in the sequence is assumed to the be one at the N terminal and the terminal amino acid is assumed to be the one at the C terminal.

Although the terms polypeptide and protein are sometimes used interchangeably, a polypeptide is technically any polymer of amino acids, whereas the term protein is used for a polypeptide or polypeptides that take folded properly, combined with whatever additional components needed for proper performance, and is at present functional.

Protein Structure

Each successive level of poly peptide folding ultimately contributes to its shape and therefore its function.

Learning Objectives

Summarize the 4 levels of poly peptide structure

Primal Takeaways

Key Points

• Protein structure depends on its amino acrid sequence and local, depression-free energy chemical bonds between atoms in both the polypeptide courage and in amino acid side bondage.
• Protein structure plays a key role in its function; if a protein loses its shape at whatsoever structural level, it may no longer be functional.
• Primary structure is the amino acid sequence.
• Secondary structure is local interactions between stretches of a polypeptide chain and includes α-helix and β-pleated canvas structures.
• Tertiary structure is the overall the iii-dimension folding driven largely by interactions between R groups.
• Quarternary structures is the orientation and arrangement of subunits in a multi-subunit poly peptide.

Key Terms

• antiparallel: The nature of the opposite orientations of the two strands of DNA or two beta strands that comprise a protein's secondary structure
• disulfide bond: A bond, consisting of a covalent bond between two sulfur atoms, formed by the reaction of two thiol groups, specially between the thiol groups of two proteins
• β-pleated sheet: secondary construction of proteins where N-H groups in the backbone of ane fully-extended strand establish hydrogen bonds with C=O groups in the backbone of an adjacent fully-extended strand
• α-helix: secondary structure of proteins where every backbone N-H creates a hydrogen bail with the C=O group of the amino acid 4 residues earlier in the same helix.

The shape of a protein is critical to its part because it determines whether the protein can interact with other molecules. Protein structures are very complex, and researchers have only very recently been able to easily and quickly determine the structure of complete proteins downward to the diminutive level. (The techniques used date dorsum to the 1950s, but until recently they were very slow and laborious to use, so complete protein structures were very boring to exist solved.) Early structural biochemists conceptually divided poly peptide structures into four "levels" to make it easier to get a handle on the complication of the overall structures. To make up one's mind how the protein gets its concluding shape or conformation, nosotros need to understand these four levels of protein construction: principal, secondary, third, and 4th.

Primary Structure

A protein'due south primary structure is the unique sequence of amino acids in each polypeptide chain that makes up the protein. Actually, this is only a list of which amino acids announced in which society in a polypeptide chain, not really a construction. Merely, because the concluding poly peptide structure ultimately depends on this sequence, this was called the chief structure of the polypeptide concatenation. For case, the pancreatic hormone insulin has ii polypeptide chains, A and B.

Master construction: The A chain of insulin is 21 amino acids long and the B chain is 30 amino acids long, and each sequence is unique to the insulin protein.

The cistron, or sequence of DNA, ultimately determines the unique sequence of amino acids in each peptide concatenation. A modify in nucleotide sequence of the cistron's coding region may pb to a different amino acid existence added to the growing polypeptide chain, causing a change in protein structure and therefore function.

The oxygen-transport protein hemoglobin consists of iv polypeptide chains, two identical α chains and two identical β chains. In sickle cell anemia, a single amino exchange in the hemoglobin β concatenation causes a alter the structure of the unabridged protein. When the amino acrid glutamic acid is replaced by valine in the β chain, the polypeptide folds into an slightly-different shape that creates a dysfunctional hemoglobin poly peptide. So, merely one amino acid substitution can cause dramatic changes. These dysfunctional hemoglobin proteins, under low-oxygen conditions, beginning associating with one some other, forming long fibers made from millions of aggregated hemoglobins that distort the red blood cells into crescent or "sickle" shapes, which clog arteries. People affected by the disease ofttimes feel breathlessness, dizziness, headaches, and abdominal hurting.

Sickle cell illness: Sickle cells are crescent shaped, while normal cells are disc-shaped.

Secondary Structure

A protein'south secondary structure is any regular structures arise from interactions between neighboring or virtually-past amino acids as the polypeptide starts to fold into its functional three-dimensional form. Secondary structures arise equally H bonds class betwixt local groups of amino acids in a region of the polypeptide concatenation. Rarely does a single secondary structure extend throughout the polypeptide chain. It is unremarkably just in a section of the concatenation. The most common forms of secondary construction are the α-helix and β-pleated sheet structures and they play an important structural role in almost globular and fibrous proteins.

Secondary structure: The α-helix and β-pleated sheet form because of hydrogen bonding between carbonyl and amino groups in the peptide courage. Certain amino acids take a propensity to form an α-helix, while others have a propensity to form a β-pleated sail.

In the α-helix chain, the hydrogen bail forms between the oxygen atom in the polypeptide courage carbonyl group in one amino acrid and the hydrogen atom in the polypeptide backbone amino group of some other amino acid that is four amino acids further along the concatenation. This holds the stretch of amino acids in a right-handed roll. Every helical turn in an alpha helix has 3.6 amino acid residues. The R groups (the side chains) of the polypeptide beetle out from the α-helix chain and are not involved in the H bonds that maintain the α-helix structure.

In β-pleated sheets, stretches of amino acids are held in an almost fully-extended conformation that "pleats" or zig-zags due to the non-linear nature of unmarried C-C and C-N covalent bonds. β-pleated sheets never occur alone. They have to held in place by other β-pleated sheets. The stretches of amino acids in β-pleated sheets are held in their pleated sheet construction because hydrogen bonds course between the oxygen atom in a polypeptide backbone carbonyl group of one β-pleated sail and the hydrogen cantlet in a polypeptide courage amino group of some other β-pleated sheet. The β-pleated sheets which hold each other together align parallel or antiparallel to each other. The R groups of the amino acids in a β-pleated sheet signal out perpendicular to the hydrogen bonds holding the β-pleated sheets together, and are non involved in maintaining the β-pleated sail construction.

3rd Structure

The tertiary structure of a polypeptide chain is its overall three-dimensional shape, one time all the secondary structure elements have folded together among each other. Interactions between polar, nonpolar, acidic, and basic R grouping inside the polypeptide chain create the circuitous 3-dimensional 3rd structure of a protein. When poly peptide folding takes place in the aqueous environment of the trunk, the hydrophobic R groups of nonpolar amino acids mostly lie in the interior of the poly peptide, while the hydrophilic R groups lie mostly on the outside. Cysteine side chains course disulfide linkages in the presence of oxygen, the only covalent bond forming during protein folding. All of these interactions, weak and strong, determine the final three-dimensional shape of the protein. When a poly peptide loses its three-dimensional shape, it will no longer be functional.

Third structure: The tertiary construction of proteins is determined by hydrophobic interactions, ionic bonding, hydrogen bonding, and disulfide linkages.

Quaternary Construction

The quaternary structure of a protein is how its subunits are oriented and bundled with respect to one another. Every bit a result, 4th construction only applies to multi-subunit proteins; that is, proteins made from more than than one polypeptide concatenation. Proteins made from a single polypeptide will non accept a fourth construction.

In proteins with more than than one subunit, weak interactions betwixt the subunits help to stabilize the overall structure. Enzymes often play key roles in bonding subunits to grade the concluding, functioning protein.

For example, insulin is a ball-shaped, globular protein that contains both hydrogen bonds and disulfide bonds that concur its two polypeptide chains together. Silk is a fibrous protein that results from hydrogen bonding betwixt different β-pleated chains.

Four levels of protein structure: The four levels of protein structure tin exist observed in these illustrations.

Denaturation and Poly peptide Folding

Denaturation is a process in which proteins lose their shape and, therefore, their function because of changes in pH or temperature.

Learning Objectives

Discuss the process of protein denaturation

Key Takeaways

Key Points

• Proteins modify their shape when exposed to different pH or temperatures.
• The body strictly regulates pH and temperature to prevent proteins such as enzymes from denaturing.
• Some proteins tin can refold later denaturation while others cannot.
• Chaperone proteins help some proteins fold into the correct shape.

Key Terms

• chaperonin: proteins that provide favorable weather for the correct folding of other proteins, thus preventing aggregation
• denaturation: the modify of folding structure of a protein (and thus of concrete backdrop) caused by heating, changes in pH, or exposure to certain chemicals

Each protein has its own unique sequence of amino acids and the interactions between these amino acids create a specify shape. This shape determines the poly peptide'southward office, from digesting protein in the stomach to carrying oxygen in the claret.

Changing the Shape of a Protein

If the poly peptide is subject to changes in temperature, pH, or exposure to chemicals, the internal interactions between the protein'due south amino acids can be altered, which in turn may alter the shape of the protein. Although the amino acid sequence (also known equally the protein'due south primary structure) does not change, the protein's shape may change so much that it becomes dysfunctional, in which instance the protein is considered denatured. Pepsin, the enzyme that breaks downward protein in the stomach, only operates at a very low pH. At college pHs pepsin'south conformation, the mode its polypeptide chain is folded up in 3 dimensions, begins to alter. The stomach maintains a very low pH to ensure that pepsin continues to digest protein and does not denature.

Enzymes

Considering almost all biochemical reactions require enzymes, and considering about all enzymes only work optimally within relatively narrow temperature and pH ranges, many homeostatic mechanisms regulate appropriate temperatures and pH then that the enzymes can maintain the shape of their active site.

Reversing Denaturation

It is often possible to reverse denaturation because the primary structure of the polypeptide, the covalent bonds belongings the amino acids in their correct sequence, is intact. Once the denaturing amanuensis is removed, the original interactions betwixt amino acids return the protein to its original conformation and it tin resume its function.

However, denaturation can be irreversible in extreme situations, like frying an egg. The heat from a pan denatures the albumin protein in the liquid egg white and it becomes insoluble. The protein in meat also denatures and becomes firm when cooked.

Denaturing a protein is occasionally irreversible: (Pinnacle) The poly peptide albumin in raw and cooked egg white. (Lesser) A paperclip analogy visualizes the process: when cross-linked, paperclips ('amino acids') no longer move freely; their structure is rearranged and 'denatured'.

Chaperone proteins (or chaperonins ) are helper proteins that provide favorable weather condition for protein folding to take place. The chaperonins dodder around the forming protein and forestall other polypeptide chains from aggregating. Once the target poly peptide folds, the chaperonins disassociate.

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