Biochemistry Terms Starting With M

Macromolecule is a very large molecule composed of thousands of covalently bonded atoms, typically formed by linking smaller subunits called monomers into long chains or branched structures.

Living organisms rely on four major classes of macromolecules: proteins, nucleic acids, carbohydrates, and lipids. A single hemoglobin protein in human red blood cells contains over 10,000 atoms arranged in a precise three-dimensional structure. Proteins found in bacterial ribosomes can exceed 2.5 million daltons in molecular weight, illustrating the size range these molecules achieve.

Cellulose, the most abundant organic macromolecule on Earth, consists of glucose monomers linked by beta-1,4-glycosidic bonds into chains that can reach molecular weights above 1 million daltons in plant cell walls.

Metabolic flux is the rate at which molecules move through a specific biochemical reaction or pathway within a cell, measured as the quantity of substrate converted per unit time.

In the bacterium Escherichia coli, glycolytic flux reaches approximately 10 millimoles per gram of dry cell weight per hour during exponential growth on glucose. Scientists measure this flow by tracking isotope-labeled substrates as they move through sequential enzyme-catalyzed reactions, a technique called metabolic flux analysis. Cancer cells typically exhibit elevated glycolytic flux compared to normal cells, consuming glucose up to 200 times faster even when oxygen is available, a phenomenon described by Otto Warburg in the 1920s.

Regulatory enzymes at branch points in a pathway, called flux control points, determine how much substrate is directed into each branch.

Metabolic pathway is a series of connected, enzyme-catalyzed chemical reactions within a cell that converts a starting molecule into one or more products through sequential steps.

Glycolysis converts one glucose molecule into two pyruvate molecules through exactly 10 enzyme-catalyzed reactions, yielding a net gain of 2 ATP and 2 NADH per glucose. The human body runs over 2,000 distinct metabolic pathways simultaneously to maintain cellular function, with each pathway featuring specific enzymes whose products feed directly into the next reaction as substrates. Pathway activity is regulated at committed steps, where allosteric enzymes respond to product concentrations and energy signals to speed up or slow down the entire sequence.

In the soil bacterium Streptomyces coelicolor, specialized secondary metabolic pathways produce antibiotics such as actinorhodin through more than 20 sequential enzymatic reactions.

Metabolism is the complete set of chemical reactions occurring within living cells that converts nutrients into energy and molecular building blocks needed for growth, maintenance, and reproduction.

Human metabolism at rest burns approximately 1,200 to 2,000 kilocalories daily through processes including breathing, blood circulation, and cellular maintenance. The ruby-throated hummingbird (Archilochus colubris) sustains one of the fastest metabolic rates among vertebrates, with its heart beating up to 1,200 times per minute and its body consuming more than its own weight in nectar each day to fuel flight. Metabolic reactions are organized into pathways where enzymes catalyze each step, extracting energy from glucose, fats, and proteins while synthesizing molecules needed for cell structure.

Two broad categories divide these reactions: catabolism breaks molecules down to release energy, and anabolism uses that energy to build larger molecules from smaller precursors.

Metabolite is any small molecule that is produced, consumed, or chemically modified during metabolic reactions within a living cell.

Metabolites include building blocks, energy carriers, and signaling molecules that coordinate cellular activity. The human body produces over 100,000 distinct metabolites, ranging from simple sugars like glucose to steroid hormones derived from cholesterol. In baker’s yeast (Saccharomyces cerevisiae), the metabolite ethanol accumulates as a byproduct of anaerobic fermentation, reaching concentrations up to 18% by volume in tolerant strains.

Photosynthesizing plants export the metabolite sucrose through phloem tissue to deliver chemical energy from leaves to roots and other non-photosynthetic organs.

Michaelis constant is the substrate concentration at which an enzyme-catalyzed reaction proceeds at exactly half its maximum velocity, and it reflects the affinity of an enzyme for its substrate.

For hexokinase in yeast, the Michaelis constant for glucose is approximately 0.1 millimolar, indicating high substrate affinity that supports efficient glucose phosphorylation even at low intracellular glucose concentrations. A lower Km value means the enzyme reaches half-maximal activity with less substrate present, signaling tighter binding between enzyme and substrate. Different isozymes of the same enzyme often carry distinct Km values matched to their cellular environments: glucokinase in human liver cells has a Km for glucose near 10 millimolar, roughly 100 times higher than hexokinase, which lets the liver act as a glucose buffer only when blood sugar is elevated.

Km is determined experimentally by measuring reaction velocity across a range of substrate concentrations and fitting the data to the Michaelis-Menten equation.