Energy, metabolism and enzymes

Denaturation unfolds a protein by breaking the weak bonds, such as hydrogen bonds, that hold its shape. The chain of amino acids stays intact, but a protein's job depends on its shape, so it stops working. Enzymes are proteins, so this matters on this page.

• Correct: It loses its folded shape, and with it the ability to do its job:
• It is split into its separate amino acids:
• It gains extra phosphate groups:

pH goes down as H+ concentration goes up. A fall of 0.4 pH units means the H+ concentration rose about two and a half times. Enzymes are sensitive to these changes.

• It fell:
• Correct: It rose:
• It stayed the same:

Reactants are the starting substances, and products are what the reaction makes. A solute is a substance dissolved in a solution, whether or not it reacts.

• Products:
• Solutes:
• Correct: Reactants:

An enzyme lowers the activation energy, the energy hill between reactants and products. It leaves the starting and ending energies, and so everything that depends on them, unchanged.

• Correct: The activation energy the reactants need to get over: Correct. The active site holds substrates in a strained, well-aligned position, which lowers the energy needed to reach the transition state.
• The total energy released when reactants become products: The energy released depends only on how much energy the reactants and products hold. The enzyme changes neither, so the energy released is the same with or without it.
• The balance point the reaction settles at when it reaches equilibrium: An enzyme speeds the forward and reverse reactions by the same factor. Equilibrium is reached sooner, but at the same balance point.
• Whether the overall reaction releases energy or needs an energy input: Whether a reaction is exergonic or endergonic is set by the energy of its reactants and products. An enzyme cannot make an endergonic reaction release energy; that reaction still needs energy, usually from ATP.

A competitive inhibitor and the substrate compete for the same active site. When substrate greatly outnumbers the inhibitor, substrate wins most encounters, so more active sites are doing the reaction and the rate rises.

• It stays low: the inhibitor has permanently changed the enzyme's shape: That describes a noncompetitive or permanent inhibitor. A competitive inhibitor binds the active site and comes off again; it does not change the enzyme's shape.
• It falls further: extra substrate crowds the enzyme and blocks the active site: The substrate is what the active site is built to bind, so more substrate means more reactions, not fewer. Crowding does not block the site.
• Correct: It rises: methanol molecules now outnumber the inhibitor at the active site: Correct. The inhibitor's effect depends on how often it, rather than the substrate, occupies the active site. Flooding the site with substrate outcompetes it.
• It stays low: the enzyme was already working at its maximum rate: The rate was low because the inhibitor occupied many active sites and substrate was scarce, not because every active site was busy with substrate. There was room to speed up.

Most human enzymes work best near 37 °C. Sustained heat well above that denatures them, so activity falls sharply, but the chemistry of the reaction itself, including the energy it releases, is unchanged.

• Enzyme molecules with a working active site: down. Heat far above 37 °C breaks the hydrogen bonds that hold the enzyme's folded shape. The protein unfolds (denatures), and its active site loses the shape the substrate fits.
• Rate of the catalyzed reaction: down. With fewer working active sites, fewer substrate molecules are converted each second. The speed-up from faster molecular motion is far outweighed by the loss of working enzyme.
• Energy released per reaction (products compared with reactants): no change. The energy released is set by the energy held in the reactants and products. Neither the enzyme nor its shape changes that; they change only how fast the reaction runs.

Joining small molecules into a large one is anabolism. Building a larger, more ordered molecule takes energy in, so the reaction is endergonic; your cells pay for it with ATP.

• Catabolic and exergonic: Catabolism breaks molecules down. Linking glucose into glycogen builds a larger molecule, and it takes energy rather than releasing it.
• Anabolic and exergonic: Building glycogen is anabolic, but it takes in energy, so it is endergonic, not exergonic.
• Catabolic and endergonic: The reaction does take energy in, but it builds a larger molecule, so it is anabolic, not catabolic.
• Correct: Anabolic and endergonic: Correct. It builds a larger molecule (anabolic) and needs an energy input (endergonic).

Once every active site is busy almost all the time, the enzyme is working at its maximum rate. More substrate has nowhere to bind. Only more enzyme would raise the rate.

• Correct: Every active site is already occupied nearly all the time: Correct. With every active site busy, the enzyme cannot convert substrate any faster.
• The extra substrate has started to denature the enzyme: Substrate binds the active site; it does not unfold the enzyme. Denaturation comes from heat, extreme pH and similar changes.
• The reaction has become endergonic at high substrate levels: Whether a reaction releases or needs energy depends on its reactants and products, not on how much substrate is present.
• The extra substrate has lowered the pH below the enzyme's optimum: Nothing in the question suggests the substrate is an acid. The rate levels off even when pH is held constant, because the active sites are full.

The energy released is the difference between the reactants (ATP and water) and the products (ADP and phosphate surrounded by water). The products are more stable, so they hold less energy, and the difference is released.

• From breaking the bond that holds the end phosphate onto ATP: Breaking a bond always takes energy. The release comes from the new, more stable arrangement of the products, not from the bond breaking.
• Correct: From the products (ADP and phosphate) holding less energy than ATP and water did: Correct. ATP's crowded negative phosphates make it a high-energy molecule; the products are more stable and lower in energy.
• From the ATPase adding its own energy as it splits ATP: An ATPase is an enzyme. It lowers the activation energy for the reaction but adds no energy of its own.
• From heat taken up from the surrounding fluid: ATP hydrolysis releases heat to its surroundings rather than taking it up.

NADH loses electrons, so it is oxidized (back to NAD+). Molecule X gains them, so it is reduced. Oxidation and reduction happen together in every redox reaction.

• NADH is reduced and molecule X is oxidized: This reverses the two. The molecule that gives up electrons is oxidized; the one that receives them is reduced.
• Both molecules are oxidized: electrons are moving between them: One molecule's loss is the other's gain, so one is oxidized and the other is reduced.
• Correct: NADH is oxidized and molecule X is reduced: Correct. Oxidation is loss of electrons (NADH) and reduction is gain (molecule X).
• Neither molecule changes: moving electrons is not a chemical reaction: A transfer of electrons is a redox reaction. Both molecules change: NADH becomes NAD+, and molecule X gains energy along with the electrons.