3. Regulation of Biochemical Pathways

Which of the following statements regarding biochemical pathways are true?

• All cell types are capable of carrying out the same biochemical pathways.

• All cell types will have the enzymes required to carry out biochemical pathways that produce energy necessary for living.

• Specialised cells carry out unique biochemical pathways required for cellular respiration.

• Biochemical pathways are reactions that start with an initial reactant which is then converted directly into a final product.

Pepsin, trypsin and chymotrypsin are three endogenous digestive enzymes produced in the gastrointestinal tract to digest proteins. Proteins are partially digested within the stomach by pepsin and, as they enter the small intestine, they are acted on by other enzymes such as trypsin and chymotrypsin. The digestion of proteins in the GI tract

• are sped up by the enzymes as the activation energy is lowered, allowing peptide chains to form from smaller amino acids.

• would never occur without the presence of pepsin, trypsin and chymotrypsin.

• results in the direct formation of amino acids from proteins.

• is facilitated by the lowering in activation energy needed to activate the proteins.

Cellular respiration and photosynthesis are both processes that require a variety of different enzymes, however they can differ in that

• cellular respiration requires coenzymes to facilitate the steps, whereas photosynthesis relies on the actions of Rubisco.

• cellular respiration is a catabolic pathway that results in the formation of energy-rich glucose, whereas photosynthesis is an anabolic pathway that requires chlorophyll to first absorb energy from the Sun.

• they utilise different coenzymes that assist the enzymes involved by acting as electron donors and acceptors.

• photosynthesis results in the assembly of low-energy inorganic carbon dioxide molecules into energy-rich glucose, whereas cellular respiration breaks down glucose into ADP molecules for use in important metabolic reactions.

Coenzymes assist the catalytic actions of enzymes. NAD is a coenzyme that functions to

• accept hydrogens and their associated electrons in its unloaded form

• accept oxygen and their associated electrons in its unloaded form

• donate hydrogens and their associated electrons in its unloaded form

• donate only protons in its unloaded form

A coenzyme helper that can accept a phosphate group to catalyse an energy-requiring reaction

• ATP

• NADP+

• ADP

• FAD

Above the optimum temperature, enzymes tend to

• move more slowly, and thus colliding less frequently with its substrate, resulting in a slower rate of reaction

• have less energy to interact with its substrate, resulting in a slower rate of reaction

• denature irreversibly at the active site, such that it is unable to bind to its substrate

• collide more frequently with the substrate, resulting in a faster rate of reaction until denaturation occurs.

When all the active sites of an enzyme are occupied by substrate molecules, the rate of reaction

• increases steadily, producing more products.

• decreases quickly, as less substrates can be reacted with enzymes.

• stays the same, until more substrates are added to the reaction.

• stays the same, until another limiting factor is changed

In the biochemical pathway of glycolysis, ATP acts as

• a cofactor

• a non-competitive inhibitor

• an allosteric inhibitor

• an allosteric activator.

Which of the following is true in a prokaryote when tryptophan levels are high

• The trp repressor protein unbinds to the operator region

• The ribosome pauses at adjacent trp codons

• Two trp molecules bind to the repressor protein causing a conformational change

• An anti-terminator loop is formed and the ribosome continues downstream

Enzymes are associated with the biochemical pathways of cellular respiration and photosynthesis. Consider a cell that carries out both cellular respiration and photosynthesis. Which one of the following is a correct statement about the enzymes associated with these pathways?

• Both pathways are catalysed by the same enzymes

• An increase in pH above the optimal pH for cellular respiration may cause the enzymes to denature

• The rates of these biochemical pathway reactions are not affected by changes in the concentrations of the enzymes.

• . A decrease in temperature below the optimal temperature for photosynthesis will cause the enzymes to denature.