Biochemistry - Major Biomolecule Classes

Carbohydrates: The Energy Molecule What Are Carbohydrates? Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen atoms. The key to recognizing them is their characteristic ratio: for most carbohydrates, there are twice as many hydrogen atoms as oxygen atoms. This is expressed by the general formula $CnH{2n}On$, where n is any positive integer. You can think of carbohydrates as "hydrated carbons"—hence the name. The simplest carbohydrates are monosaccharides, or single sugars. The most important monosaccharide is glucose ($C6H{12}O6$), which serves as the primary fuel for cells throughout your body and is the building block for larger carbohydrate structures. Cyclic Structures: How Monosaccharides Form Rings Here's something that often confuses students: monosaccharides don't exist as the linear chain structures you might see drawn in textbooks. In aqueous solution, they spontaneously form ring structures through a reaction between the carbonyl group and a hydroxyl group within the same molecule. Monosaccharides can form two types of rings: Pyranoses are six-membered rings (glucose typically forms this structure) Furanoses are five-membered rings (fructose commonly exists this way) This ring formation is important because it affects how the molecule behaves and how it bonds with other molecules. The ring form is the biologically active form in most situations. <extrainfo> In glucose solutions, both the ring and open-chain forms exist in equilibrium, but the ring form is heavily favored (>99% of molecules are in ring form at any moment). </extrainfo> Disaccharides: Joining Two Sugars Together When two monosaccharides link together, they form a disaccharide. This occurs through a dehydration reaction: the –OH group from one sugar and an –H from another combine to form water ($H2O$), while a new bond forms between the two sugar molecules. This bond between sugars is called a glycosidic bond. Two important disaccharides you should know are: Sucrose (table sugar) = glucose + fructose Lactose (milk sugar) = glucose + galactose Understanding lactose is particularly relevant because of lactose intolerance. The enzyme lactase catalyzes the hydrolysis (breaking) of lactose into its component monosaccharides. If a person lacks sufficient lactase, they cannot break down lactose, leading to digestive discomfort. This is a common condition, especially in populations with less dietary history of milk consumption. Polysaccharides: Large Energy and Structural Molecules When hundreds or thousands of monosaccharides chain together through glycosidic bonds, they form polysaccharides. These molecules have two major roles in living organisms: Structural Support: Cellulose is a polysaccharide that provides structural rigidity to plant cell walls. It forms long, straight chains that pack tightly together, creating a strong, inflexible material. Notably, most animals (including humans) lack the enzymes needed to break down cellulose, which is why we cannot digest plant fiber as an energy source—though it remains important for digestive health. Energy Storage: Glycogen is the animal equivalent of a storage polysaccharide. It's a highly branched polymer of glucose that's stored primarily in the liver and muscles. When your body needs quick energy, enzymes break down glycogen to release glucose into the bloodstream. The key distinction is that cellulose is a linear, structural molecule, while glycogen is branched, which allows it to be mobilized quickly for energy. Lipids: The Hydrophobic Molecules Understanding Lipids as a Diverse Class Unlike carbohydrates, which have a clear structural definition, lipids are defined primarily by their physical properties: they are biological molecules that are largely water-insoluble (hydrophobic) or non-polar. This means lipids don't mix well with water. Lipids include several structurally distinct classes: fatty acids, triglycerides, phospholipids, sphingolipids, glycolipids, waxes, and sterols. Despite this diversity, most of them share a common structural foundation: a glycerol backbone with fatty acids attached to it. Fatty Acids: Saturated vs. Unsaturated A fatty acid is a long hydrocarbon chain with a carboxyl group (–COOH) at one end. The distinction between saturated and unsaturated is crucial and has significant health implications: Saturated fatty acids contain no double bonds between the carbon atoms in their chain. This allows them to pack tightly together, making them typically solid at room temperature (like butter). Unsaturated fatty acids contain one or more double bonds (C=C) in their hydrocarbon chain. These double bonds introduce "kinks" in the structure, preventing tight packing and making them typically liquid at room temperature (like olive oil). Why does this matter? The structure of fatty acids affects how they interact with other molecules and how our bodies metabolize them. Saturated fats have been associated with elevated cholesterol levels, while unsaturated fats are generally considered healthier. The Amphiphilic Nature of Lipids Here's the key insight that explains why lipids are so important in biology: most lipids are amphiphilic (sometimes called "amphipathic"). This means they have both a hydrophobic tail (the fatty acid chains—water-repelling) and a hydrophilic head (the phosphate group or other polar region—water-attracting). This dual nature is absolutely critical to understanding cell membrane structure. Lipids spontaneously arrange themselves into bilayers, with their hydrophobic tails pointing inward (away from water) and their hydrophilic heads facing outward (toward water). This is not something that requires energy—it happens naturally because it minimizes the contact between water and the hydrophobic regions. Lipid Digestion and Absorption In the digestive system, dietary lipids are broken down by enzymes into simpler components: primarily fatty acids and glycerol. These smaller molecules can be absorbed through the intestinal wall and transported to cells throughout the body, where they can be used for energy, structural components, or signaling molecules. Proteins: The Molecular Workhorses Amino Acids: The Building Blocks All proteins are polymers constructed from just 20 standard amino acids. Despite this limited variety, the combinatorial possibilities are enormous—imagine building an infinite variety of words using only 20 letters. Every amino acid has the same basic structure: Each amino acid contains: An alpha carbon (the central carbon atom) An amino group (–NH₂, which can accept a proton to become –NH₃⁺) A carboxyl group (–COOH, which can lose a proton to become –COO⁻) A hydrogen atom A distinctive side chain or R group (the part that varies between amino acids) The R group determines each amino acid's unique chemical properties—some are hydrophobic, some are hydrophilic, some are charged, and some are polar but uncharged. Peptide Bonds: Linking Amino Acids Together When amino acids join together, the carboxyl group of one amino acid reacts with the amino group of another in a dehydration reaction—water is released, and a new bond forms. This bond, called a peptide bond, links the carbonyl carbon of one amino acid to the nitrogen of the next. The resulting chain has directionality: we refer to the free amino end as the N-terminus and the free carboxyl end as the C-terminus. Protein sequences are always written from N-terminus to C-terminus. An important distinction: chains with fewer than 30 amino acid residues are called peptides, while longer chains are called proteins. This is a somewhat arbitrary boundary, but it reflects functional differences—shorter chains typically have simpler functions. The Four Levels of Protein Structure Understanding protein structure requires thinking at four different scales, each providing different information: Primary Structure is simply the linear sequence of amino acids, from N-terminus to C-terminus. This is like the alphabet sequence of a word—it contains all the information needed to build the protein, but doesn't tell you the 3D shape. Primary structure is determined by the genetic code. Secondary Structure refers to regular, repeating structural patterns that form through hydrogen bonding between atoms in the backbone (not the R groups). The two most common secondary structures are: α-helices: a coiled structure where the backbone spirals β-sheets: extended, pleated structures where multiple segments lie side-by-side These structures are stabilized by hydrogen bonds between the C=O group of one amino acid and the N-H group of another four residues down the chain (in an α-helix) or in adjacent strands (in a β-sheet). Tertiary Structure is the overall three-dimensional folding of the entire single polypeptide chain. This folding is determined by interactions between the R groups: hydrophobic interactions (nonpolar R groups cluster away from water), hydrogen bonding, ionic interactions (between charged R groups), and disulfide bonds (covalent bonds between cysteine residues). This level of structure gives the protein its functional shape. Quaternary Structure describes the assembly of multiple separate polypeptide chains (called subunits) into a multi-subunit complex. Not all proteins have quaternary structure—it only applies to proteins made of multiple polypeptide chains. Hemoglobin, for example, consists of four subunits working together to bind and transport oxygen. Functional Categories of Proteins Proteins perform nearly every function in living cells. Three major functional categories include: Structural Proteins provide shape and mechanical support to cells and tissues. Examples include actin and myosin, which are protein filaments responsible for muscle contraction. Enzymes are proteins that catalyze (speed up) biochemical reactions. They can increase reaction rates by factors up to $10^{11}$ by lowering the activation energy required for reactions to proceed. Nearly every metabolic pathway in your cells depends on enzymes. Antibodies (also called immunoglobulins) are specialized proteins produced by immune cells that bind specifically to foreign molecules (antigens) and mark them for destruction. An antibody molecule consists of four polypeptide chains: two identical heavy chains and two identical light chains, held together by disulfide bonds (covalent links between cysteine residues). Enzyme Kinetics and Regulation Because enzymes are so critical to life, cells have multiple mechanisms to control enzyme activity. Enzyme action can be modulated by: Inhibitors: molecules that decrease enzyme activity (either reversibly or irreversibly) Activators: molecules that increase enzyme activity Covalent modifications: phosphorylation being the most common, where a phosphate group is temporarily added or removed to turn the enzyme on or off This regulation allows cells to respond quickly to changing conditions without having to synthesize new enzymes. Protein Digestion and Amino Acid Nutrition When you eat protein, your digestive system breaks it down into amino acids and dipeptides (two amino acids linked together) in the small intestine. These are then absorbed and transported throughout the body. Here's a critical nutritional concept: your body cannot synthesize all amino acids it needs. The essential amino acids that must be obtained from diet are: Isoleucine Leucine Lysine Methionine Phenylalanine Threonine Tryptophan Valine In contrast, non-essential amino acids can be synthesized by your cells from metabolic intermediates (molecules produced during normal metabolism). This doesn't mean they're unimportant—it means your body can make them if dietary intake is insufficient. <extrainfo> There's also a category called "conditionally essential" amino acids that are normally non-essential but become essential during periods of illness, stress, or intense exercise. </extrainfo> Transamination and Ammonia Disposal One of the most important reactions involving amino acids is transamination, where amino groups are transferred between amino acids and other molecules. This is catalyzed by enzymes called transaminases. Transamination serves two purposes: it allows the synthesis of new amino acids and it removes amino groups from amino acids so their carbon skeletons can be used for energy. When amino groups are removed from amino acids, they're converted to ammonia (NH₃). Here's the problem: ammonia is toxic, even at relatively low concentrations. Rather than excreting ammonia directly, your body converts it to urea in the liver through a series of reactions called the urea cycle. Urea is much less toxic than ammonia and can be safely excreted in urine. <extrainfo> The urea cycle is a complex pathway with six steps and involves several enzymes. While the details are important for biochemistry courses, the key concept for general biology is understanding that the body converts toxic ammonia into non-toxic urea for excretion. </extrainfo> Nucleic Acids: Information Storage and Transfer Basic Structure of Nucleotides Nucleic acids are polymers of nucleotides, and each nucleotide is a molecule containing three distinct components: A nitrogenous base (a ring structure containing nitrogen atoms) A five-carbon sugar (either ribose in RNA or deoxyribose in DNA) A phosphate group These three components link together through covalent bonds to form the nucleotide. When nucleotides polymerize, phosphate groups from one nucleotide bond to the sugar of the next, creating a sugar-phosphate backbone with bases projecting from the side. DNA vs. RNA: Two Types of Genetic Molecules There are two major types of nucleic acids in cells, distinguished by their sugar and composition: Deoxyribonucleic acid (DNA) contains deoxyribose sugar (which lacks one oxygen atom compared to regular ribose) and stores genetic information. DNA is the permanent storage of hereditary information, organized into genes that code for proteins and RNA molecules. Ribonucleic acid (RNA) contains ribose sugar and plays multiple roles in cells. RNA is involved in reading the information from DNA (messenger RNA) and in translating that information into proteins (transfer RNA and ribosomal RNA). RNA is also increasingly recognized as having regulatory functions. Nitrogenous Bases and Base Pairing Rules DNA and RNA contain four different nitrogenous bases each. Two important bases appear in both: Adenine (A): a purine (double-ring structure) Guanine (G): a purine (double-ring structure) DNA also contains: Thymine (T): a pyrimidine (single-ring structure) Cytosine (C): a pyrimidine (single-ring structure) RNA contains cytosine as well, but has uracil (U) instead of thymine. The bases pair in a very specific way, held together by hydrogen bonds: Adenine pairs with thymine (in DNA) or adenine pairs with uracil (in RNA) via two hydrogen bonds Cytosine pairs with guanine via three hydrogen bonds The stronger bonding in the C-G pair (three H-bonds vs. two) means that DNA with higher C-G content is more stable and has a higher melting temperature. Nucleotides Beyond Genetic Material: ATP While nucleotides are best known as the building blocks of genetic material, they serve another critical function as cellular energy currency. Adenosine triphosphate (ATP) is a nucleotide containing adenine, ribose sugar, and three phosphate groups. ATP stores chemical energy in the high-energy bonds between its phosphate groups. When cells need energy for reactions, ATP is broken down (usually to ADP, with two phosphate groups, plus a free phosphate group), releasing the energy stored in those bonds. This energy is then used to drive otherwise unfavorable reactions. ATP is so central to cellular metabolism that it's been called the "energy currency of the cell."