2nd Law of thermodynamics Universe tends to disorder so why don’t proteins, carbohydrates & other biomolecules breakdown? at temperatures typical of the cell, molecules don’t make it over the hump of activation energy but, a cell must be metabolically active heat would speed reactions, but… would denature proteins & kill cells
Living with oxygen is dangerous. We rely on oxygen to power our cells, but oxygen is a reactive molecule that can cause serious problems if not carefully controlled. One of the dangers of oxygen is that it is easily converted into other reactive compounds. Inside our cells, electrons are continually shuttled from site to site by carrier molecules, such as carriers derived from riboflavin and niacin. If oxygen runs into one of these carrier molecules, the electron may be accidentally transferred to it. This converts oxygen into dangerous compounds such as superoxide radicals and hydrogen peroxide, which can attack the delicate sulfur atoms and metal ions in proteins. To make things even worse, free iron ions in the cell occasionally convert hydrogen peroxide into hydroxyl radicals. These deadly molecules attack and mutate DNA. Fortunately, cells make a variety of antioxidant enzymes to fight the dangerous side-effects of life with oxygen. Two important players are superoxide dismutase, which converts superoxide radicals into hydrogen peroxide, and catalase, which converts hydrogen peroxide into water and oxygen gas. The importance of these enzymes is demonstrated by their prevalence, ranging from about 0.1% of the protein in an E. coli cell to upwards of a quarter of the protein in susceptible cell types. These many catalase molecules patrol the cell, counteracting the steady production of hydrogen peroxide and keeping it at a safe level. Catalases are some of the most efficient enzymes found in cells. Each catalase molecule can decompose millions of hydrogen peroxide molecules every second. The cow catalase shown here and our own catalases use an iron ion to assist in this speedy reaction. The enzyme is composed of four identical subunits, each with its own active site buried deep inside. The iron ion, shown in green, is gripped at the center of a disk-shaped heme group. Catalases, since they must fight against reactive molecules, are also unusually stable enzymes. Notice how the four chains interweave, locking the entire complex into the proper shape.
Why is it a good adaptation to organize the cell in organelles? Sequester enzymes with their substrates!
Why is it a good adaptation to organize the cell in organelles? Sequester enzymes with their substrates!
Enzymes work within narrow temperature ranges. Ectotherms, like snakes, do not use their metabolism extensively to regulate body temperature. Their body temperature is significantly influenced by environmental temperature. Desert reptiles can experience body temperature fluctuations of ~40°C (that’s a ~100°F span!). What mechanism has evolved to allow their metabolic pathways to continue to function across that wide temperature span?
Hemoglobin is aided by Fe Chlorophyll is aided by Mg
Ethanol is metabolized in the body by oxidation to acetaldehyde, which is in turn further oxidized to acetic acid by aldehyde oxidase enzymes. Normally, the second reaction is rapid so that acetaldehyde does not accumulate in the body. A drug, disulfiram (Antabuse) inhibits the aldehyde oxidase which causes the accumulation of acetaldehyde with subsequent unpleasant side-effects of nausea and vomiting. This drug is sometimes used to help people overcome the drinking habit. Methanol (wood alcohol) poisoning occurs because methanol is oxidized to formaldehyde and formic acid which attack the optic nerve causing blindness. Ethanol is given as an antidote for methanol poisoning because ethanol competitively inhibits the oxidation of methanol. Ethanol is oxidized in preference to methanol and consequently, the oxidation of methanol is slowed down so that the toxic by-products do not have a chance to accumulate.
Basis of most chemotherapytreatments is enzyme inhibition. Many health disorders can be controlled, in principle, by inhibiting selected enzymes. Two examples include methotrexate and FdUMP, common anticancer drugs which inhibit enzymes involved in the synthesis of thymidine and hence DNA. Since many enzymes contain sulfhydral (-SH), alcohol, or acid groups as part of their active sites, any chemical which can react with them acts as a noncompetitive inhibitor. Heavy metals such as silver (Ag+), mercury (Hg2+), lead ( Pb2+) have strong affinities for -SH groups. Cyanide combines with the copper prosthetic groups of the enzyme cytochrome C oxidase, thus inhibiting respiration which causes an organism to run out of ATP (energy) Oxalic and citric acid inhibit blood clotting by forming complexes with calcium ions necessary for the enzyme metal ion activator.
Another example of irreversible inhibition is provided by the nerve gas diisopropylfluorophosphate (DFP) designed for use in warfare. It combines with the amino acid serine (contains the –SH group) at the active site of the enzyme acetylcholinesterase. The enzyme deactivates the neurotransmitter acetylcholine. Neurotransmitters are needed to continue the passage of nerve impulses from one neurone to another across the synapse. Once the impulse has been transmitted, acetylcholinesterase functions to deactivate the acetycholine almost immediately by breaking it down. If the enzyme is inhibited, acetylcholine accumulates and nerve impulses cannot be stopped, causing prolonged muscle contration. Paralysis occurs and death may result since the respiratory muscles are affected. Some insecticides currently in use, including those known as organophosphates (e.g. parathion), have a similar effect on insects, and can also cause harm to nervous and muscular system of humans who are overexposed to them.
Metabolism & EnzymesAP Biology 2007-2008
From food webs to the life of a cell energy energy energyAP Biology
Flow of energy through life Life is built on chemical reactions transforming energy from one form to organic molecules → another ATP & organic molecules sun organic molecules → ATP & organic molecules solar energy →AP Biology ATP & organic molecules
Metabolism Chemical reactions of life forming bonds between molecules dehydration synthesis synthesis anabolic reactions breaking bonds between molecules hydrolysis digestion catabolic reactions That’s why they’re calledAP Biology anabolic steroids!
Chemical reactions & energy Some chemical reactions release energy exergonic digesting molecules= LESS organization= digesting polymers lower energy state hydrolysis = catabolism Some chemical reactions require input of energy building molecules= endergonic MORE organization= building polymers higher energy state dehydration synthesis = anabolismAP Biology
Endergonic vs. exergonic reactions exergonic endergonic - energy released - energy invested - digestion - synthesis +∆G -∆GAP Biology ∆G = change in free energy = ability to do work
Energy & life Organisms require energy to live where does that energy come from? coupling exergonic reactions (releasing energy) with endergonic reactions (needing energy) + + energydigestionsynthesis + + energy AP Biology
What drives reactions? If reactions are “downhill”, why don’t they just happen spontaneously? because covalent bonds are stable bonds Why don’t stable polymers starch spontaneously digest into their monomers?AP Biology
Activation energy Breaking down large molecules requires an initial input of energy activation energy large biomolecules are stable must absorb energy to break bonds cellulose energy CO2 + H2O + heatAP Biology
Too much activation energy for life Activation energy amount of energy needed to destabilize the bonds of a molecule moves the reaction over an “energy hill” glucose Not a match! That’s too much energy to expose living cells to!AP Biology
Reducing Activation energy Catalysts reducing the amount of energy to start a reaction uncatalyzed reaction Pheeew… that takes a lot less energy! catalyzed reaction NEW activation energy reactant productAP Biology
Catalysts So what’s a cell got to do to reduce activation energy? get help! … chemical help… ENZYMES Call in the ENZYMES! ∆GAP Biology
Enzymes Biological catalysts proteins (& RNA) facilitate chemical reactions increase rate of reaction without being consumed reduce activation energy don’t change free energy (∆G) released or required required for most biological reactions highly specific thousands of different enzymes in cells control reactions of lifeAP Biology
Enzymes vocabulary substrate reactant which binds to enzyme enzyme-substrate complex: temporary association product end result of reaction active site enzyme’s catalytic site; substrate fits into active site active site substrate productsenzymeAP Biology
Properties of enzymes Reaction specific each enzyme works with a specific substrate chemical fit between active site & substrate H bonds & ionic bonds Not consumed in reaction single enzyme molecule can catalyze thousands or more reactions per second enzymes unaffected by the reaction Affected by cellular conditions any condition that affects protein structure temperature, pH, salinityAP Biology
Naming conventions Enzymes named for reaction they catalyze sucrase breaks down sucrose proteases break down proteins lipases break down lipids DNA polymerase builds DNA adds nucleotides to DNA strand pepsin breaks down proteins (polypeptides)AP Biology
Lock and Key model Simplistic model of In biology… enzyme action Size doesn’t matter… substrate fits into 3-D Shape matters! structure of enzyme’ active site H bonds between substrate & enzyme like “key fits into lock”AP Biology
Induced fit model More accurate model of enzyme action 3-D structure of enzyme fits substrate substrate binding cause enzyme to change shape leading to a tighter fit “conformational change” bring chemical groups in position to catalyze reactionAP Biology
How does it work? Variety of mechanisms to lower activation energy & speed up reaction synthesis active site orients substrates in correct position for reaction enzyme brings substrate closer together digestion active site binds substrate & puts stress on bonds that must be broken, making it easier to separate moleculesAP Biology
Factors affecting enzyme function Substrate concentration as ↑ substrate = ↑ reaction rate more substrate = more frequently collide with enzyme reaction rate levels off all enzymes have active site engaged enzyme is saturated maximum rate of reaction ubstrate concentrationAP Biology
Temperature What’s happening here?! reaction rate 37° temperatureAP Biology
Factors affecting enzyme function Temperature Optimum T° greatest number of molecular collisions human enzymes = 35°- 40°C body temp = 37°C Heat: increase beyond optimum T° increased energy level of molecules disrupts bonds in enzyme & between enzyme & substrate H, ionic = weak bonds denaturation = lose 3D shape (3° structure) Cold: decrease T° molecules move slower decrease collisions between enzyme & substrateAP Biology
Enzymes and temperature Different enzymes function in different organisms in different environments hot spring human enzyme bacteria enzyme reaction rate 37°C 70°CAP Biology temperature (158°F)
Factors affecting enzyme function pH changes in pH adds or remove H+ disrupts bonds, disrupts 3D shape disrupts attractions between charged amino acids affect 2° & 3° structure denatures protein optimal pH? most human enzymes = pH 6-8 depends on localized conditions pepsin (stomach) = pH 2-3 trypsin (small intestines) = pH 8AP Biology 0 1 2 3 4 5 6 7 8 9 10 11
Salinity What’s happening here?! reaction rate salt concentrationAP Biology
Factors affecting enzyme function Salt concentration changes in salinity adds or removes cations (+) & anions (–) disrupts bonds, disrupts 3D shape disrupts attractions between charged amino acids affect 2° & 3° structure denatures protein enzymes intolerant of extreme salinity Dead Sea is called dead for a reason!AP Biology
Compounds which help enzymes Fe in Activators hemoglobin cofactors non-protein, small inorganic compounds & ions Mg, K, Ca, Zn, Fe, Cu bound within enzyme molecule coenzymes non-protein, organic molecules bind temporarily or permanently to enzyme near active site many vitamins Mg in chlorophyll NAD (niacin; B3) FAD (riboflavin; B2)AP Biology Coenzyme A
Competitive Inhibitor Inhibitor & substrate “compete” for active site penicillin blocks enzyme bacteria use to build cell walls disulfiram (Antabuse) treats chronic alcoholism blocks enzyme that breaks down alcohol severe hangover & vomiting 5-10 minutes after drinking Overcome by increasing substrate concentration saturate solution with substrate so it out-competes inhibitor for active site on enzymeAP Biology
Non-Competitive Inhibitor Inhibitor binds to site other than active site allosteric inhibitor binds to allosteric site causes enzyme to change shape conformational change active site is no longer functional binding site keeps enzyme inactive some anti-cancer drugs inhibit enzymes involved in DNA synthesis stop DNA production stop division of more cancer cells cyanide poisoning irreversible inhibitor of Cytochrome C, an enzyme in cellular respiration stops production of ATPAP Biology
Irreversible inhibition Inhibitor permanently binds to enzyme competitor permanently binds to active site allosteric permanently binds to allosteric site permanently changes shape of enzyme nerve gas, sarin, many insecticides (malathion, parathion…) cholinesterase inhibitors doesn’t breakdown the neurotransmitter, acetylcholineAP Biology
Allosteric regulation Conformational changes by regulatory molecules inhibitors keeps enzyme in inactive form activators keeps enzyme in active formAP Biology Conformational changes Allosteric regulation
Metabolic pathways A→ B→ C→ D→ E→ F→ G enzyme enzyme enzyme enzyme enzyme enzyme enzyme → → →→ → → → 3 1 2 4 5 6 Chemical reactions of life are organized in pathways divide chemical reaction into many small steps artifact of evolution ↑ efficiency intermediate branching pointsAP Biology ↑ control = regulation
Efficiency Organized groups of enzymes enzymes are embedded in membrane and arranged sequentially Link endergonic & exergonic reactions Whoa! All that going on in those little mitochondria!AP Biology
Feedback Inhibition Regulation & coordination of production product is used by next step in pathway final product is inhibitor of earlier step allosteric inhibitor of earlier enzyme feedback inhibition no unnecessary accumulation of product A→ B→ C→ D→ E→ F→ G enzyme enzyme enzyme enzyme enzyme enzyme → → → → → → X 3 1 2 4 5 6AP Biology allosteric inhibitor of enzyme 1
threonine Feedback inhibition Example synthesis of amino acid, isoleucine from amino acid, threonine isoleucine becomes the allosteric inhibitor of the first step in the pathway as product accumulates it collides with enzyme more often than substrate doesAP Biology isoleucine
Don’t be inhibited! Ask Questions!AP Biology 2007-2008
Ghosts of Lectures Past (storage)AP Biology 2007-2008
Cooperativity Substrate acts as an activator substrate causes conformational change in enzyme induced fit favors binding of substrate at 2nd site makes enzyme more active & effective hemoglobinHemoglobin 4 polypeptide chains can bind 4 O2; 1st O2 binds now easier for otherAP Biology bind 3 O2 to