Biochemistry Terms Starting With H

Heme is an iron-containing organic molecule built from a porphyrin ring with a central iron atom that binds oxygen, transfers electrons, or supports catalytic reactions inside proteins.

Hemoglobin in human red blood cells contains four heme groups, each capable of binding one oxygen molecule, enabling the transport of approximately 98 percent of oxygen throughout the body. Beyond oxygen transport, heme forms the functional core of cytochrome P450 enzymes in liver cells, where it facilitates the metabolism of drugs and toxins. The iron atom within heme switches between Fe2+ and Fe3+ oxidation states, making it indispensable for electron transfer reactions in cellular respiration.

Without this redox flexibility, mitochondria could not generate the electrochemical gradient that drives ATP synthesis.

Heme protein is a conjugated protein containing one or more heme groups as prosthetic cofactors, where each heme consists of an iron atom coordinated within a porphyrin ring that supports oxygen transport, electron transfer, or catalytic activity.

Hemoglobin in human red blood cells contains four heme groups, each binding one oxygen molecule to transport approximately 98 percent of oxygen throughout the bloodstream. Cytochrome c, a small heme protein of only 104 amino acids in humans, shuttles electrons between complexes III and IV of the mitochondrial electron transport chain, contributing directly to ATP synthesis. Peroxidases such as horseradish peroxidase (Armoracia rusticana) use their heme iron to oxidize a broad set of organic substrates, demonstrating that catalytic chemistry, not oxygen transport, defines many members of this protein family.

Henderson-Hasselbalch Equation is a mathematical expression that calculates the pH of a buffer solution from the pKa of a weak acid and the ratio of conjugate base concentration to weak acid concentration.

Human blood maintains a pH of approximately 7.4 through a carbonic acid-bicarbonate buffering system that obeys this equation, with a normal bicarbonate-to-carbonic acid ratio of about 20 to 1. The formula, pH equals pKa plus the logarithm of the conjugate base concentration divided by the weak acid concentration, lets biochemists predict how a buffer will shift when acids or bases are added. Clinicians apply this relationship when interpreting arterial blood gas measurements, where deviations from the normal ratio signal metabolic acidosis, metabolic alkalosis, or respiratory disorders.

Lawrence Henderson derived the original expression in 1908, and Karl Hasselbalch reformulated it in logarithmic form in 1917, giving the equation its modern shape.

High-energy phosphate bond is a chemical bond between phosphate groups in molecules such as ATP that releases more than 7 kilocalories per mole upon hydrolysis, providing the free energy that drives cellular work.

ATP contains two such bonds, located between its three phosphate groups, with each releasing approximately 7.3 kilocalories per mole under standard conditions. During intense exercise, human skeletal muscle cells hydrolyze ATP at rates approaching 0.5 kilograms per minute, far exceeding what mitochondria can regenerate instantly. Creatine phosphate in vertebrate muscle tissue stores a single high-energy phosphate bond that rapidly transfers its phosphate group back to ADP, replenishing ATP during the first 8 to 10 seconds of maximal effort before oxidative phosphorylation can accelerate.

Histidine is an amino acid with an imidazole side chain whose pKa near physiological pH lets it accept or donate protons, making it a key participant in enzyme catalysis and biological buffering.

The imidazole ring of histidine has a pKa of approximately 6.0, positioning it to shift between protonated and neutral forms near the physiological pH of 7.4. In serine proteases such as chymotrypsin, a histidine residue shuttles protons during peptide bond cleavage, accepting a proton from serine and donating it to the departing amine group in a sequence that accelerates catalysis by several orders of magnitude. Human infants cannot synthesize sufficient histidine to meet the demands of rapid growth, requiring dietary sources that supply at least 28 milligrams per kilogram of body weight each day.

Histidine also coordinates the iron atom in hemoglobin’s heme groups through what is called the proximal histidine, a direct bond that anchors iron in the correct geometry for reversible oxygen binding.

Hydrogen bond is a weak electrostatic attraction between a hydrogen atom covalently bonded to an electronegative atom and a lone pair of electrons on a second electronegative atom in the same or a neighboring molecule.

Each water molecule forms approximately 3.4 hydrogen bonds with neighboring molecules at room temperature, creating a dynamic network that raises water’s boiling point to 100 degrees Celsius, far above what its molecular weight alone would predict. In DNA, hydrogen bonds between complementary base pairs maintain the double helix, with adenine-thymine pairs sharing two bonds and guanine-cytosine pairs sharing three. This difference in bond number means guanine-cytosine-rich regions of DNA require more thermal energy to separate, a property that researchers exploit when designing polymerase chain reaction primers.

Proteins also depend on these bonds internally, where backbone carbonyl and amide groups form the repeating hydrogen bonds that define alpha helices and beta sheets.

Hydrogenation is a chemical reaction that adds hydrogen atoms across carbon-carbon double bonds in unsaturated molecules, reducing them to more saturated forms.

Industrial hydrogenation of vegetable oils typically uses a nickel catalyst at temperatures between 140 and 225 degrees Celsius to convert liquid oils into solid or semi-solid fats suitable for margarine and shortening. During partial hydrogenation, some double bonds rearrange from the natural cis configuration to the trans configuration, producing trans fats that raise LDL cholesterol levels in humans by approximately 2 percent for every 1 percent of total calories they replace. Certain rumen bacteria, particularly Butyrivibrio fibrisolvens, perform biohydrogenation naturally, converting dietary polyunsaturated fatty acids into saturated forms before those lipids are absorbed by the host animal.

This microbial activity means that beef and dairy products contain small amounts of naturally occurring trans fats, chemically distinct from the industrial trans fats linked to cardiovascular disease.

Hydrolysis is a chemical reaction in which a water molecule breaks a covalent bond within a larger molecule, splitting it into two smaller products by adding a hydroxyl group to one fragment and a hydrogen atom to the other.

During digestion in humans, specialized enzymes catalyze hydrolysis reactions that break proteins into amino acids, starches into simple sugars, and triglycerides into fatty acids and glycerol. Proteases secreted by the pancreas, including trypsin and chymotrypsin, cleave peptide bonds at specific amino acid sequences, releasing roughly 50 to 100 grams of dietary protein per day for absorption in the small intestine. Bacteria such as Bacillus subtilis secrete hydrolytic enzymes into surrounding soil, breaking down complex organic polymers externally before absorbing the resulting monomers.

This extracellular digestion strategy lets microorganisms exploit nutrient sources too large to enter the cell directly.

Hydrophobic interaction is the tendency of nonpolar molecules or molecular regions to cluster together in aqueous environments, driven by the gain in entropy of surrounding water molecules rather than by direct attraction between the nonpolar groups themselves.

This phenomenon drives protein folding, where approximately 60 percent of amino acid residues in a typical globular protein bury their nonpolar side chains in the interior, away from the surrounding water. Nonpolar residues such as leucine, valine, and phenylalanine cluster in these hydrophobic cores, and the stability of the folded structure depends more on this burial than on any other single force. Lipid bilayers also form through hydrophobic interactions, as fatty acid tails spontaneously associate to exclude water, creating the two-leaflet structure that defines all cellular membranes.

At 37 degrees Celsius, the entropic driving force for hydrophobic clustering is stronger than at lower temperatures, which is why some proteins actually become more stable as temperature rises slightly above room temperature.