Biochemistry Terms Starting With I

Induced-fit model is a description of enzyme-substrate binding in which the enzyme undergoes a conformational change upon substrate approach, reshaping its active site to achieve optimal geometric and chemical complementarity with the substrate.

Unlike the rigid lock-and-key model proposed by Emil Fischer in 1894, the induced-fit model, introduced by Daniel Koshland in 1958, accounts for the flexibility observed in enzymes such as hexokinase, which closes around glucose by shifting active-site residues by approximately 8 to 12 angstroms. This closure excludes water from the active site, preventing the unproductive hydrolysis of ATP that would otherwise compete with glucose phosphorylation. Conformational change also repositions catalytic residues into precise orientations that lower the activation energy of the reaction, explaining why many enzymes are far more efficient than rigid binding alone would predict.

Structural studies using X-ray crystallography have captured hexokinase in both open and closed conformations, providing direct evidence for the movement Koshland described.

Inhibition is the process by which a molecule decreases or prevents the catalytic activity of an enzyme by binding to it and interfering with its normal function.

Inhibitors regulate metabolic pathways by modulating enzyme activity in response to cellular conditions. The human body produces endogenous acetylcholinesterase inhibitors that control nerve signal transmission, while the antibiotic penicillin inhibits bacterial transpeptidase enzymes that cross-link peptidoglycan strands in cell walls. Competitive inhibitors bind at the active site and can be displaced by increasing substrate concentration, whereas noncompetitive inhibitors bind at separate allosteric sites and reduce catalytic rate regardless of substrate levels.

These mechanistic differences determine how tightly a drug or metabolite controls a given pathway.

Inorganic phosphate is a free phosphate ion that exists independently of organic molecules, released during ATP hydrolysis and recycled during ATP synthesis.

When human muscle cells hydrolyze one mole of ATP during contraction, approximately 7.3 kilocalories of energy are released along with one molecule of inorganic phosphate and ADP. This phosphate group does not remain idle in the cytoplasm; oxidative phosphorylation in mitochondria recaptures it to regenerate ATP. Yeast cells such as Saccharomyces cerevisiae maintain intracellular inorganic phosphate concentrations between 1 and 10 millimolar, with levels fluctuating based on nutrient availability and energy demand.

Beyond energy metabolism, inorganic phosphate regulates enzyme activity and buffers cytoplasmic pH by accepting or donating protons near physiological pH.

Ionic Bond ionic bond is an electrostatic attraction between oppositely charged ions that forms when one atom transfers one or more electrons to another atom.

Within proteins, ionic bonds frequently stabilize three-dimensional structures by linking positively charged lysine or arginine residues with negatively charged aspartate or glutamate residues. Thermus aquaticus, a bacterium that thrives in hot springs at temperatures exceeding 70°C, relies heavily on ionic bonds between amino acids to maintain protein stability under extreme heat. These bonds are typically weaker than covalent bonds but stronger than van der Waals interactions, with energies ranging from 5 to 20 kilocalories per mole in biological systems.

At physiological pH, the precise balance of charged residues at an enzyme’s active site often determines substrate specificity and catalytic rate.

Isoelectric point is the specific pH value at which a molecule such as a protein or amino acid carries no net electrical charge because its positive and negative charges are equal in number.

Human hemoglobin has an isoelectric point of approximately 6.8, meaning it carries no net charge at that pH. At pH values below the isoelectric point, proteins gain protons and become positively charged, while at pH values above it, they lose protons and become negatively charged. This property lets scientists separate proteins using isoelectric focusing, where molecules migrate through a pH gradient until they reach their isoelectric point and stop moving.

Proteins at their isoelectric point also show minimal solubility, because reduced electrostatic repulsion between molecules promotes aggregation.

Isoenzymes are different molecular forms of the same enzyme that catalyze the same biochemical reaction but differ in amino acid sequence, structure, or regulatory properties.

Isoenzymes often show tissue-specific expression patterns that reflect the metabolic needs of different cell types. Human lactate dehydrogenase exists as 5 distinct isoenzymes formed from combinations of two subunit types, with LDH-1 predominating in heart muscle and LDH-5 predominating in skeletal muscle and liver. This distribution makes isoenzyme analysis clinically valuable for diagnosing tissue damage, since elevated serum levels of specific isoenzymes indicate injury to particular organs.

Kinetic differences between isoenzymes are equally significant: LDH-1 favors the conversion of lactate to pyruvate, while LDH-5 favors the reverse reaction, matching the distinct metabolic priorities of cardiac and hepatic tissue.

Isomerase is an enzyme that catalyzes the rearrangement of atoms within a single molecule to convert it into a structural or geometric isomer without adding or removing any atoms.

These enzymes facilitate structural transformations without adding or removing atoms from the substrate molecule. Phosphoglucose isomerase in human cells converts glucose-6-phosphate to fructose-6-phosphate during glycolysis, achieving an equilibrium ratio of approximately 70:30 in favor of glucose-6-phosphate. Triose phosphate isomerase operates with near-perfect catalytic efficiency, approaching the diffusion-controlled limit at roughly 10^8 per molar per second, making it one of the fastest enzymes known.

Without this enzyme, only half of each glucose molecule would enter the most productive portion of glycolysis, sharply reducing ATP yield per glucose.