All 20 of these amino acids share a similar structure Figure 2. All consist of a central carbon atom to which the following are bonded:.
For this reason, they make excellent buffers, helping the body regulate acid—base balance. What distinguishes the 20 amino acids from one another is their variable group, which is referred to as a side chain or an R-group. This group can vary in size and can be polar or nonpolar, giving each amino acid its unique characteristics.
For example, the side chains of two amino acids—cysteine and methionine—contain sulfur. Sulfur does not readily participate in hydrogen bonds, whereas all other amino acids do. This variation influences the way that proteins containing cysteine and methionine are assembled. Amino acids join via dehydration synthesis to form protein polymers Figure 2.
The unique bond holding amino acids together is called a peptide bond. A peptide bond is a covalent bond between two amino acids that is formed by dehydration synthesis.
A peptide, in fact, is a very short chain of amino acids. Strands containing fewer than about amino acids are generally referred to as polypeptides rather than proteins. The body is able to synthesize most of the amino acids from components of other molecules, however, nine cannot be synthesized and have to be consumed in the diet.
These are known as the essential amino acids. Free amino acids available for protein construction are said to reside in the amino acid pool within cells. Structures within cells use these amino acids when assembling proteins. If a particular essential amino acid is not available in sufficient quantities in the amino acid pool, however, synthesis of proteins containing it can slow or even cease.
The sequence is called the primary structure of the protein. Although some polypeptides exist as linear chains, most are twisted or folded into more complex secondary structures that form when bonding occurs between amino acids with different properties at different regions of the polypeptide. The most common secondary structure is a spiral called an alpha-helix.
If you were to take a length of string and simply twist it into a spiral, it would not hold the shape. Similarly, a strand of amino acids could not maintain a stable spiral shape without the help of hydrogen bonds, which create bridges between different regions of the same strand see Figure 2.
Less commonly, a polypeptide chain can form a beta-pleated sheet, in which hydrogen bonds form bridges between different regions of a single polypeptide that has folded back upon itself, or between two or more adjacent polypeptide chains.
In this configuration, amino acids that had been very distant in the primary chain can be brought quite close via hydrogen bonds or, in proteins containing cysteine, via disulfide bonds. A disulfide bond is a covalent bond between sulfur atoms in a polypeptide.
Often, two or more separate polypeptides bond to form an even larger protein with a quaternary structure see Figure 2. The polypeptide subunits forming a quaternary structure can be identical or different. For instance, hemoglobin, the protein found in red blood cells is composed of four tertiary polypeptides, two of which are called alpha chains and two of which are called beta chains.
When they are exposed to extreme heat, acids, bases, and certain other substances, proteins will denature. Denaturation is a change in the structure of a molecule through physical or chemical means.
Denatured proteins lose their functional shape and are no longer able to carry out their jobs. An everyday example of protein denaturation is the curdling of milk when acidic lemon juice is added. The contribution of the shape of a protein to its function can hardly be exaggerated. For example, the long, slender shape of protein strands that make up muscle tissue is essential to their ability to contract shorten and relax lengthen.
As another example, bones contain long threads of a protein called collagen that acts as scaffolding upon which bone minerals are deposited. These elongated proteins, called fibrous proteins, are strong and durable and typically hydrophobic.
In contrast, globular proteins are globes or spheres that tend to be highly reactive and are hydrophilic. The hemoglobin proteins packed into red blood cells are an example see Figure 2. Enzymes, introduced earlier as protein catalysts, are examples of this. The next section takes a closer look at the action of enzymes. If you were trying to type a paper, and every time you hit a key on your laptop there was a delay of six or seven minutes before you got a response, you would probably get a new laptop.
In a similar way, without enzymes to catalyze chemical reactions, the human body would be nonfunctional. It functions only because enzymes function. Enzymatic reactions—chemical reactions catalyzed by enzymes—begin when substrates bind to the enzyme. A substrate is a reactant in an enzymatic reaction.
This occurs on regions of the enzyme known as active sites Figure 2. Any given enzyme catalyzes just one type of chemical reaction. This characteristic, called specificity, is due to the fact that a substrate with a particular shape and electrical charge can bind only to an active site corresponding to that substrate.
Binding of a substrate produces an enzyme—substrate complex. It is likely that enzymes speed up chemical reactions in part because the enzyme—substrate complex undergoes a set of temporary and reversible changes that cause the substrates to be oriented toward each other in an optimal position to facilitate their interaction.
This promotes increased reaction speed. The enzyme then releases the product s , and resumes its original shape. The enzyme is then free to engage in the process again, and will do so as long as substrate remains.
Advertisements for protein bars, powders, and shakes all say that protein is important in building, repairing, and maintaining muscle tissue, but the truth is that proteins contribute to all body tissues, from the skin to the brain cells. Also, certain proteins act as hormones and chemical messengers that help regulate body functions.
For example, growth hormone is important for skeletal growth, among other roles. As was noted earlier, the basic and acidic components enable proteins to function as buffers in maintaining acid—base balance, but they also help regulate fluid—electrolyte balance. Are also important as energy carriers. Proteins Consist of long chains of amino acids. Are necessary for growth and repair of tissues. Are crucial to life and perform a wide range of functions. Related questions What are some examples of organic molecules vs inorganic molecules?
How does water dissolve organic molecules? Explain why organic molecules are based on carbon. How can you identify organic molecules? Living organisms contain inorganic compounds mainly water and salts and organic molecules.
Organic molecules contain carbon; inorganic compounds do not. Carbon oxides and carbonates are exceptions; they contain carbon but are considered inorganic because they do not contain hydrogen. The atoms of an organic molecule are typically organized around chains of carbon atoms. They are small, simple compounds that play important roles in the cell, although they do not form cell structures. Most of the carbon found in organic molecules originates from inorganic carbon sources such as carbon dioxide captured via carbon fixation by microorganisms.
Organic molecules in organisms are generally larger and more complex than inorganic molecules. Their carbon skeletons are held together by covalent bonds.
They form the cells of an organism and perform the chemical reactions that facilitate life. All of these molecules, called biomolecules because they are part of living matter, contain carbon, which is the building block of life. Carbon is a very unique element in that it has four valence electrons in its outer orbitals and can form four single covalent bonds with up to four other atoms at the same time.
The carbon skeleton of organic molecules can be straight, branched, or ring shaped cyclic. Organic molecules are built on chains of carbon atoms of varying lengths; most are typically very long, which allows for a huge number and variety of compounds. No other element has the ability to form so many different molecules of so many different sizes and shapes. Molecules with the same atomic makeup but different structural arrangement of atoms are called isomers.
The concept of isomerism is very important in chemistry because the structure of a molecule is always directly related to its function.
Slight changes in the structural arrangements of atoms in a molecule may lead to very different properties. Chemists represent molecules by their structural formula, which is a graphic representation of the molecular structure, showing how the atoms are arranged. Compounds that have identical molecular formulas but differ in the bonding sequence of the atoms are called structural isomers.
Isomers that differ in the spatial arrangements of atoms are called stereoisomers; one unique type is enantiomers. The properties of enantiomers were originally discovered by Louis Pasteur in while using a microscope to analyze crystallized fermentation products of wine. Enantiomers are molecules that have the characteristic of chirality, in which their structures are nonsuperimposable mirror images of each other. Many organisms are only able to use one enantiomeric form of certain types of molecules as nutrients and as building blocks to make structures within a cell.
Some enantiomeric forms of amino acids have distinctly different tastes and smells when consumed as food. For example, L-aspartame, commonly called aspartame, tastes sweet, whereas D-aspartame is tasteless. Drug enantiomers can have very different pharmacologic affects. For example, the compound methorphan exists as two enantiomers, one of which acts as an antitussive dextro methorphan, a cough suppressant , whereas the other acts as an analgesic levo methorphan, a drug similar in effect to codeine.
Enantiomers are also called optical isomers because they can rotate the plane of polarized light. Some of the crystals Pasteur observed from wine fermentation rotated light clockwise whereas others rotated the light counterclockwise. The d and l labels are derived from the Latin words dexter on the right and laevus on the left , respectively.
These two different optical isomers often have very different biological properties and activities. Certain species of molds, yeast, and bacteria, such as Rhizopus , Yarrowia , and Lactobacillus spp. Another important reason to be aware of optical isomers is the therapeutic use of these types of chemicals for drug treatment, because some microorganisms can only be affected by one specific optical isomer. Figure 4. Enantiomers are stereoisomers that exhibit chirality. Their chemical structures are nonsuperimposable mirror images of each other.
Enantiomers are also called optical isomers because they can rotate the plane of polarized light. Some of the crystals Pasteur observed from wine fermentation rotated light clockwise whereas others rotated the light counterclockwise. The d and l labels are derived from the Latin words dexter on the right and laevus on the left , respectively.
These two different optical isomers often have very different biological properties and activities. Certain species of molds, yeast, and bacteria, such as Rhizopus , Yarrowia , and Lactobacillus spp. Another important reason to be aware of optical isomers is the therapeutic use of these types of chemicals for drug treatment, because some microorganisms can only be affected by one specific optical isomer.
In addition to containing carbon atoms, biomolecules also contain functional groups —groups of atoms within molecules that are categorized by their specific chemical composition and the chemical reactions they perform, regardless of the molecule in which the group is found. Some of the most common functional groups are listed in Table 1.
R might symbolize just a single hydrogen atom or it may represent a group of many atoms. Notice that some functional groups are relatively simple, consisting of just one or two atoms, while some comprise two of these simpler functional groups. It is present in several classes of organic compounds as part of larger functional groups such as ketones, aldehydes, carboxylic acids, and amides.
In ketones, the carbonyl is present as an internal group, whereas in aldehydes it is a terminal group. Carbon chains form the skeletons of most organic molecules. Functional groups combine with the chain to form biomolecules. Because these biomolecules are typically large, we call them macromolecules. Many biologically relevant macromolecules are formed by linking together a great number of identical, or very similar, smaller organic molecules. The smaller molecules act as building blocks and are called monomers , and the macromolecules that result from their linkage are called polymers.
Cells and cell structures include four main groups of carbon-containing macromolecules: polysaccharides , proteins , lipids , and nucleic acids. The first three groups of molecules will be studied throughout this chapter. The biochemistry of nucleic acids will be discussed in Biochemistry of the Genome. Of the many possible ways that monomers may be combined to yield polymers, one common approach encountered in the formation of biological macromolecules is dehydration synthesis.
In this chemical reaction, monomer molecules bind end to end in a process that results in the formation of water molecules as a byproduct:. Figure 5 shows dehydration synthesis of glucose binding together to form maltose and a water molecule.
Table 2 summarizes macromolecules and some of their functions. Figure 5. In this dehydration synthesis reaction, two molecules of glucose are linked together to form maltose. In the process, a water molecule is formed.
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