Source: Alberts et al., Ch. 4 / PCB 3023 Review Sheet
Difficulty: Intermediate to Advanced | Prerequisites: Parts 1 and 2 of these notes (protein structure, binding sites, allosteric regulation).
Enzymes are the catalysts that make cellular chemistry possible. Without them, most biological reactions would be far too slow to sustain life. This section covers what enzymes are, how they speed up reactions, how their activity is measured and described mathematically, and how cells regulate enzyme activity to respond to changing conditions. These concepts are tested heavily, particularly the Michaelis-Menten equation, K_M, competitive versus noncompetitive inhibition, and phosphorylation. If you are behind, make sure you understand binding sites and allosteric regulation (Part 2) before tackling this material.
Enzymes are proteins that catalyse specific reactions by lowering activation energy, without being consumed. Their activity is described by K_M and V_max. Cells control enzymes through inhibitors (competitive and noncompetitive), feedback loops, phosphorylation, and GTP-binding switches. Understanding the [S] vs. velocity graph is essential for the exam.
Enzyme
A protein that acts as a biological catalyst, speeding up a specific chemical reaction without being consumed or permanently altered. Think of it as a molecular machine that grabs its raw materials, does its work, and releases the product, ready to go again.
Active site
The specific region of an enzyme where the substrate binds and the catalytic reaction occurs. Formed by a precise arrangement of amino acid side chains. In simple terms, this is the enzyme's workshop.
Substrate
The molecule (or molecules) an enzyme acts upon. The substrate binds to the active site and is converted into product. Think of it as the raw material the enzyme processes.
Catalyst
A substance that increases the rate of a chemical reaction by lowering the activation energy, without itself being permanently changed. All enzymes are catalysts; they emerge from the reaction unchanged and ready to catalyse another round.
V_max (maximum velocity)
The maximum rate of an enzymatic reaction, achieved when all enzyme molecules are saturated with substrate. In simple terms, it is the fastest the enzyme can work when it has more substrate than it can handle.
K_M (Michaelis constant)
The substrate concentration at which the reaction rate is half of V_max. Mathematically, K_M = (k₂ + k₃) / k₁. A small K_M means the enzyme reaches half-maximal speed at low substrate concentrations (high affinity for its substrate). A large K_M means the enzyme needs a lot of substrate to work efficiently (lower apparent affinity).
Competitive inhibitor
A molecule that competes with the substrate for binding to the enzyme's active site. It resembles the substrate in shape. Increases the apparent K_M (enzyme needs more substrate to overcome the inhibitor) but does not change V_max (at very high substrate concentrations, the substrate outcompetes the inhibitor).
Noncompetitive inhibitor
A molecule that binds to the enzyme at a site other than the active site, reducing catalytic activity regardless of substrate concentration. Does not change K_M but decreases V_max (even saturating substrate cannot restore full speed because some enzyme molecules are locked in an inactive conformation).
Feedback inhibition (negative feedback)
A regulatory mechanism in which the end product of a metabolic pathway inhibits an enzyme early in the pathway, preventing overproduction. Think of it as a thermostat: the output turns down the input.
Positive feedback
A regulatory mechanism in which the product of a reaction stimulates further production. Less common than negative feedback but important in processes that need to commit fully once started (e.g., blood clotting). Think of it as a snowball rolling downhill, getting bigger.
Phosphorylation
The addition of a phosphate group (PO₄³⁻) to a protein, typically on the hydroxyl group of serine, threonine, or tyrosine. Catalysed by kinases. Can activate or inactivate a protein by changing its conformation. In simple terms, it is a molecular on/off switch.
Kinase
An enzyme that transfers a phosphate group from ATP to a target protein (phosphorylation). The counterpart is a phosphatase, which removes the phosphate.
Phosphatase
An enzyme that removes a phosphate group from a protein, reversing the effect of a kinase.
GTP-binding protein (GTPase)
A protein regulated by the nucleotide GTP. Active when GTP is bound; inactive when it hydrolyses GTP to GDP. The protein acts as a molecular switch: GTP-bound = on, GDP-bound = off. Intrinsic GTPase activity turns the switch off over time.
Every enzyme shares these properties:
It is a protein (with rare exceptions such as ribozymes, which are RNA-based).
It has an active site where substrate binds and catalysis occurs.
It lowers the activation energy of the reaction, increasing the reaction rate.
It is specific to its substrate (or a small set of related substrates).
It is not consumed or permanently altered by the reaction; it is released unchanged.
It does not change the equilibrium of the reaction, only the speed at which equilibrium is reached.
Enzymes use a combination of strategies to speed up reactions. The three main mechanisms:
Orienting substrates: The active site holds the substrate(s) in precisely the right orientation for the reaction, reducing the randomness of molecular collisions. Without the enzyme, the substrates would need to collide in just the right way by chance.
Straining or distorting bonds: The active site can physically stress the substrate's bonds, making them easier to break. The enzyme stabilises the transition state (the high-energy intermediate), lowering the energy barrier.
Providing a favourable microenvironment: The active site may offer acidic, basic, or hydrophobic conditions not present in the surrounding solution. Specific amino acid side chains can donate or accept protons, form temporary covalent bonds with the substrate, or position metal ions that participate in catalysis.
The reaction scheme:
E + S ⇌ ES → E + P
Where:
E = free enzyme
S = substrate
ES = enzyme-substrate complex
P = product
k₁ = rate constant for E + S → ES (substrate binding)
k₂ = rate constant for ES → E + S (substrate release without reaction)
k₃ = rate constant for ES → E + P (catalysis and product release)
The Michaelis constant: K_M = (k₂ + k₃) / k₁
K_M is the substrate concentration at which the reaction velocity is half of V_max. It is read directly from a [S] vs. velocity graph: find the point on the y-axis where v = V_max / 2, then drop down to the x-axis to read K_M.
Small K_M: The enzyme reaches half-maximal speed at low [S]. The enzyme has high apparent affinity for its substrate; it grabs substrate efficiently even when substrate is scarce.
Large K_M: The enzyme needs a high substrate concentration to work at half-maximal speed. Lower apparent affinity; the enzyme is less efficient at capturing substrate.
K_M is a property of each specific enzyme-substrate pair.
Competitive inhibition:
The inhibitor binds the active site, competing directly with the substrate.
Increasing substrate concentration can overcome the inhibition.
Effect on kinetic parameters: K_M increases (apparent, because more substrate is needed), V_max is unchanged (at saturating [S], all active sites are occupied by substrate).
On a [S] vs. velocity graph: the curve shifts to the right but reaches the same V_max.
Noncompetitive inhibition:
The inhibitor binds at a site other than the active site (an allosteric site).
It works whether or not substrate is bound; increasing [S] does not overcome it.
Effect on kinetic parameters: K_M is unchanged, V_max decreases (some fraction of enzyme molecules are permanently inactive).
On a [S] vs. velocity graph: the curve reaches a lower plateau (lower V_max) but the K_M point does not shift.
Feedback inhibition (negative feedback): The final product of a metabolic pathway binds to and inhibits an enzyme early in that same pathway. This prevents the cell from overproducing the end product. The inhibition is usually allosteric. Example: the amino acid isoleucine inhibits the first enzyme in its own biosynthetic pathway (threonine deaminase).
Positive feedback: The product of a reaction activates the same enzyme or pathway, amplifying the response. Less common, but used when a biological process needs to go to completion rapidly once triggered. Example: blood clotting cascades, where each activated protease activates the next, amplifying the signal.
Regulation of enzyme activity (without changing enzyme amount): Allosteric regulation, competitive and noncompetitive inhibition, covalent modification (phosphorylation), and binding of regulatory molecules. These all change how fast existing enzyme molecules work.
Regulation of enzyme amount: Changing the rate of gene expression (transcription/translation) or the rate of enzyme degradation changes how many enzyme molecules are present. More enzyme = higher total capacity.
Phosphorylation is the most common form of covalent modification used to regulate protein activity. A kinase enzyme transfers the terminal phosphate group from ATP to the hydroxyl (–OH) side chain of serine, threonine, or tyrosine on the target protein. This adds a bulky, negatively charged group, which can:
Change the protein's conformation (switching it on or off)
Create or destroy a binding site for other proteins
Alter the protein's localisation within the cell
The reverse reaction is catalysed by a phosphatase, which removes the phosphate group by hydrolysis.
The kinase/phosphatase system is like a toggle switch: kinases flip it one way, phosphatases flip it back. Both are tightly regulated.
GTP-binding proteins (also called GTPases) function as molecular switches:
GTP-bound form = active. The protein is "on" and can interact with downstream partners.
GDP-bound form = inactive. The protein is "off."
The protein has intrinsic GTPase activity: it slowly hydrolyses its bound GTP to GDP, turning itself off. This built-in timer ensures the signal does not persist indefinitely.
Guanine nucleotide exchange factors (GEFs) activate the protein by promoting the exchange of GDP for GTP.
GTPase-activating proteins (GAPs) inactivate it by stimulating GTP hydrolysis.
Ras (a proto-oncogene product involved in cell growth signalling) is the textbook example of a GTP-binding protein.
The Michaelis-Menten reaction scheme:
E + S ⇌ (k₁ forward, k₂ reverse) ES → (k₃) E + P
E = enzyme (free)
S = substrate
ES = enzyme-substrate complex
P = product
k₁ = rate constant for substrate binding
k₂ = rate constant for substrate release (without catalysis)
k₃ = rate constant for catalysis and product release (also called k_cat in some texts)
Michaelis constant:
K_M = (k₂ + k₃) / k₁
Reading the [S] vs. velocity graph: V_max is the plateau of the curve. K_M is the [S] value at which v = V_max / 2.
"Enzymes provide energy for reactions." They do not. Enzymes lower the activation energy barrier, making it easier for the reaction to proceed, but they do not add energy to the system or change the thermodynamics.
"Competitive inhibitors reduce V_max." They do not. At sufficiently high substrate concentrations, the substrate outcompetes the inhibitor, and V_max is achieved normally. Competitive inhibitors increase the apparent K_M.
"Noncompetitive inhibitors bind the active site." They bind elsewhere (an allosteric site). Because they do not compete with substrate, increasing [S] does not overcome their effect.
"K_M measures how tightly an enzyme binds its substrate." This is a simplification. K_M is an apparent affinity measure, not a true dissociation constant, because it includes the catalytic rate constant (k₃) as well. It is more precisely the substrate concentration at half-maximal velocity.
"Phosphorylation always activates a protein." Phosphorylation can either activate or inactivate a protein, depending on the specific protein and the site of phosphorylation.
⚠️ Be able to read a [S] vs. velocity graph and identify V_max, K_M, and the effect of competitive versus noncompetitive inhibition. This is a near-certainty on the exam.
⚠️ Know every term in the Michaelis-Menten equation: E, S, ES, P, k₁, k₂, k₃. You may be asked what each represents.
⚠️ Competitive inhibition: K_M increases, V_max unchanged. Noncompetitive inhibition: K_M unchanged, V_max decreases. Memorise this; it appears in multiple formats.
⚠️ Know the difference between feedback inhibition (end product inhibits an early enzyme) and positive feedback (product stimulates the pathway).
⚠️ Phosphorylation: kinase adds phosphate from ATP to serine, threonine, or tyrosine; phosphatase removes it. Can switch protein on or off. This is tested in both Chapter 4 and later signalling chapters.
⚠️ GTP-binding proteins: GTP-bound = on, GDP-bound = off, intrinsic GTPase turns it off. Know this switch mechanism.
⚠️ Be prepared to work problems of types 4-10 through 4-15 from the textbook.
True or False: Enzymes change the equilibrium of a reaction. __________
K_M is the substrate concentration at which the reaction rate is __________ of V_max.
A competitive inhibitor increases / decreases / does not change K_M. __________
True or False: Phosphorylation of a protein is always catalysed by a phosphatase. __________
A GTP-binding protein is active when it has __________ bound and inactive when it has __________ bound.
Answers: 1. False (only the rate, not the equilibrium). 2. Half. 3. Increases. 4. False (catalysed by a kinase; phosphatase removes the phosphate). 5. GTP; GDP.
Q: What are enzymes? List the essential aspects common to all enzymes.
A: Enzymes are proteins that catalyse specific chemical reactions. All enzymes have an active site, lower the activation energy, are substrate-specific, are not consumed by the reaction, and do not alter the reaction's equilibrium.
Q: Summarise three ways by which enzymes catalyse chemical reactions.
A: (1) Orienting substrates in the correct position for reaction. (2) Straining or distorting substrate bonds to stabilise the transition state. (3) Providing a favourable chemical microenvironment (donating/accepting protons, positioning metal ions, creating hydrophobic or charged surroundings).
Q: Explain both the mathematical and practical meaning of K_M.
A: Mathematically, K_M = (k₂ + k₃) / k₁. Practically, it is the substrate concentration at which the enzyme operates at half its maximum velocity. A small K_M means the enzyme has high apparent affinity for its substrate (efficient at low [S]); a large K_M means it requires more substrate to work at half speed.
Q: Contrast competitive and noncompetitive inhibition. How does each affect K_M and V_max?
A: A competitive inhibitor binds the active site and competes with substrate. It increases the apparent K_M but does not change V_max. A noncompetitive inhibitor binds at an allosteric site. It does not change K_M but decreases V_max.
Q: Contrast feedback inhibition with positive feedback.
A: In feedback inhibition, the end product of a pathway inhibits an early enzyme, preventing overproduction (self-limiting). In positive feedback, the product of a reaction stimulates the same enzyme or pathway, amplifying the response (self-reinforcing). Negative feedback is far more common.
Q: What two main ways can an enzyme's activity be altered?
A: (1) Regulating the activity of existing enzyme molecules (allosteric regulation, inhibitors, covalent modification such as phosphorylation). (2) Regulating the amount of enzyme present (changing transcription, translation, or degradation rates).
Q: How does phosphorylation influence protein function? Which enzymes and amino acids are involved?
A: Phosphorylation adds a phosphate group from ATP to the hydroxyl side chain of serine, threonine, or tyrosine, catalysed by a kinase. The added charge and bulk change the protein's conformation, switching its activity on or off. A phosphatase reverses this by hydrolysing off the phosphate group.
Q: How is the activity of GTP-binding proteins regulated by GTP?
A: GTP-binding proteins are active when GTP is bound and inactive when GDP is bound. The protein's intrinsic GTPase activity slowly hydrolyses GTP to GDP, turning itself off. GEFs (guanine nucleotide exchange factors) activate it by swapping GDP for GTP; GAPs (GTPase-activating proteins) accelerate inactivation.
Enzyme kinetics appears again whenever metabolic pathways are discussed (glycolysis, citric acid cycle, oxidative phosphorylation). Phosphorylation and GTP-binding protein switches are central to cell signalling (Chapters 15 and 16 in Alberts). Feedback inhibition is a recurring theme in metabolic regulation and homeostasis.
The concept of competitive inhibition is directly relevant to pharmacology: many drugs work by competing with a substrate for an enzyme's active site.
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