Biochemistry Terms Starting With T

Tertiary protein structure is the complete three-dimensional arrangement of all atoms in a single polypeptide chain, stabilized by hydrogen bonds, disulfide bridges, ionic interactions, and hydrophobic forces between amino acid side chains.

Lysozyme, found in human tears and saliva, demonstrates tertiary structure through its compact globular shape, with 129 amino acids folded into distinct alpha helices and beta sheets that create a cleft where bacterial cell wall fragments bind and are cleaved. Disulfide bonds between cysteine residues lock specific regions in place, contributing to the enzyme’s stability across a broad pH range. Myoglobin, the oxygen-storage protein in muscle cells, achieves its tertiary structure by burying hydrophobic amino acids in its core while exposing polar residues on its surface, keeping the protein soluble in the aqueous cytoplasm.

Hydrophobic burial is thermodynamically favorable because it releases ordered water molecules from around nonpolar side chains, increasing the entropy of the surrounding solvent and driving folding spontaneously.

Thermodynamics is the branch of physics and chemistry that studies energy transformations and the relationships between heat, work, and the energy states of systems.

Biochemical processes obey thermodynamic principles that determine whether reactions can occur spontaneously. Mitochondria in human muscle cells convert the chemical energy of glucose into ATP through oxidative phosphorylation, releasing approximately 686 kilocalories per mole of glucose while obeying the second law of thermodynamics. Cellular respiration illustrates how living organisms harness chemical energy while increasing the entropy of the universe, as heat dissipates into surrounding tissues and metabolic waste products disperse into the environment.

No biological process violates these laws; even the most efficient enzyme-catalyzed reaction can only redirect energy flow, never create energy from nothing or destroy it entirely.

Thioester bond is a chemical linkage formed between a carboxylic acid group and a sulfhydryl group, storing approximately 31 kilojoules per mole more free energy of hydrolysis than ordinary ester bonds.

In cellular metabolism, thioester bonds appear most prominently in acetyl-CoA, where the bond between the acetyl group and coenzyme A drives energy transfer during the citric acid cycle. Hydrolysis of this bond releases enough free energy to couple with otherwise unfavorable biosynthetic reactions, including the condensation steps of fatty acid synthesis. Escherichia coli relies on thioester linkages between acyl carrier proteins and growing fatty acid chains during each elongation cycle, with each round adding a two-carbon unit before the thioester is cleaved and reformed.

The sulfur atom in the linkage weakens the bond’s resonance stabilization compared to oxygen-based esters, which is precisely why thioester hydrolysis releases more usable energy.

Transamination is a reversible biochemical reaction in which an amino group transfers from one amino acid to a keto acid, producing a new amino acid and a new keto acid.

This reaction requires pyridoxal phosphate, the active form of vitamin B6, as a cofactor for enzymes called aminotransferases, which form a Schiff base intermediate with the amino group during catalysis. In human liver cells, alanine aminotransferase transfers an amino group from alanine to alpha-ketoglutarate, generating pyruvate and glutamate at rates exceeding 1,000 molecules per second per enzyme molecule. Glutamate produced by these reactions feeds into the urea cycle, linking amino acid catabolism directly to nitrogen excretion.

Through transamination, cells synthesize nonessential amino acids on demand without requiring dietary intake of all 20 proteinogenic amino acids.

Transition state is the highest-energy configuration that reacting molecules must pass through during a chemical reaction, representing the point at which bonds are simultaneously partially broken and partially formed.

During glycolysis in human cells, hexokinase stabilizes the transition state of glucose phosphorylation by approximately 13 kcal/mol, accelerating the reaction roughly 10 million times compared to the uncatalyzed version. Each transition state persists for only about 10^-13 seconds, making isolation impossible but structural inference achievable through kinetic isotope effects and computational modeling. Enzymes achieve catalytic power primarily by binding the transition state more tightly than either substrates or products, a principle first articulated by Linus Pauling in 1948.

This preferential binding lowers the activation energy barrier without altering the thermodynamics of the reaction.

Triglyceride is a lipid molecule consisting of one glycerol backbone esterified to three fatty acid chains through covalent ester linkages.

Adipose tissue in humans stores between 80,000 and 100,000 kilocalories of energy as triglycerides, far exceeding the roughly 2,000 kilocalories stored as glycogen. When blood glucose rises after a meal, the liver converts excess carbohydrates into triglycerides and packages them into very-low-density lipoproteins for transport to adipose tissue. During fasting or sustained exercise, hormone-sensitive lipase cleaves stored triglycerides into glycerol and three free fatty acids; skeletal muscle then oxidizes those fatty acids through beta-oxidation, generating approximately 106 ATP molecules per molecule of the 16-carbon fatty acid palmitate.

Glycerol released during lipolysis travels to the liver, where it enters gluconeogenesis as a substrate.

Turnover number is the maximum number of substrate molecules that a single enzyme molecule converts to product per second when the enzyme is fully saturated with substrate, expressed as the catalytic rate constant kcat.

Catalase, found in nearly all aerobic organisms, converts approximately 40 million hydrogen peroxide molecules per second per enzyme molecule, one of the highest turnover numbers measured for any enzyme. Carbonic anhydrase in human red blood cells reaches a turnover number of 600,000 per second, sustaining the rapid interconversion of carbon dioxide and bicarbonate needed to buffer blood pH during respiration. Comparing kcat values across enzymes reveals the catalytic cost of specificity: highly specific enzymes like DNA polymerase operate at only about 15 per second, sacrificing speed for fidelity.

Dividing kcat by the Michaelis constant Km yields the catalytic efficiency, which approaches the diffusion limit of roughly 10^8 to 10^9 per molar per second for the fastest known enzymes.