Biochemistry SI Study Notes: Enzymes, Inhibition, Phosphorylation & Hemoglobin

Comprehensive vocabulary flashcards covering enzyme catalysis strategies, environmental effects, reversible and irreversible inhibition patterns, Lineweaver-Burk plots, phosphorylation, pharmacological targets, and hemoglobin heme structure.

Enzyme

A catalyst that speeds up a chemical reaction without being consumed.

Enzyme Regeneration

The requirement that an enzyme must return to its original functional form by the end of a catalytic cycle, even if temporarily altered during the reaction.

General Acid-Base Catalysis

A catalytic strategy involving proton ($\text{H}^+$) transfer.

General Acid-Base Catalysis Recognition Clue

Protons moving in reaction diagrams, $K_a$ notation, or acid-base chemistry.

Covalent Catalysis

A catalytic mechanism in which the enzyme forms a temporary covalent bond with the substrate.

Covalent Catalysis Example (Acetoacetate)

The conversion of acetoacetate to acetone, which proceeds through a temporary enzyme-substrate covalent intermediate to allow energetically favorable steps.

Metalloenzyme

An enzyme where the metal ion is closely associated with or embedded in the enzyme and directly participates in the reaction, commonly featuring transition metals.

Metal-Activated Enzyme

An enzyme where the metal ion is free-floating and functions to stabilize charges, commonly utilizing Group 1 and Group 2 metals such as $\text{Mg}^{2+}$ and $\text{Ca}^{2+}$.

Electrostatic Catalysis

A catalytic mechanism that uses positive and negative charge interactions (ion-ion or ion-dipole) to stabilize molecules and charges during a reaction.

Electrostatic Catalysis (ATP Example)

Uses ion-ion and ion-dipole interactions, where $\text{Mg}^{2+}$ helps stabilize negatively charged oxygens.

Proximity and Orientation Catalysis

A catalytic strategy that forces substrates close together and correctly aligned in the active site.

Transition-State Stabilization

A strategy where the active site is shaped to bind the high-energy transition state more tightly than the starting substrate, lowering the energetic barrier.

Temperature Effect on Enzyme Activity

Initially increases activity due to higher kinetic energy and more productive collisions; past an optimal temperature, excessive heat causes denaturation and a sharp crash in activity.

pH Effect on Enzyme Activity

An enzyme-dependent effect rather than a simple linear rule, as seen in pepsin functioning optimal at low pH.

Lysosomal Enzyme Safety Mechanism

Lysosomal digestive enzymes function optimal at low pH; if they escape into the more neutral cytosol, their activity decreases.

Reversible Inhibition

Inhibition mediated primarily by noncovalent interactions; includes competitive, uncompetitive, mixed, and noncompetitive types.

Reversible Covalent Modification

A modification where a functional group is covalently attached to a protein and can later be removed, with phosphorylation as the primary example.

Irreversible Inhibition (Suicide Inhibition)

Inhibition in which the target enzyme is permanently disabled under normal biochemical conditions and remains ineffective until degraded.

Beta-lactam Antibiotics

An example of suicide (irreversible) inhibition targeting bacterial enzymes involved in cell-wall synthesis.

Lineweaver-Burk Plot Equation

A double-reciprocal plot of the Michaelis-Menten relationship yielding a linear equation in the form $y = mx + b$.

Lineweaver-Burk Y-Intercept

Represents $\frac{1}{V_{max}}$.

Lineweaver-Burk X-Intercept

Represents $-\frac{1}{K_m}$.

Lineweaver-Burk Slope

Represents $\frac{K_m}{V_{max}}$.

Vmax ($V_{max}$)

The maximum reaction velocity that the enzyme system can reach.

Km ($K_m$)

The substrate concentration required to reach $\frac{1}{2}V_{max}$, used as a rough proxy for enzyme-substrate affinity.

Low Km ($K_m$) Significance

Indicates high apparent affinity of the enzyme for its substrate.

High Km ($K_m$) Significance

Indicates lower apparent affinity of the enzyme for its substrate.

Competitive Inhibition Mechanism

The inhibitor and substrate compete directly for binding at the same active site.

Competitive Inhibition Vmax ($V_{max}$)

Unchanged, because adding enough substrate can outcompete the inhibitor.

Competitive Inhibition Km ($K_m$)

Increases.

Competitive Inhibition Lineweaver-Burk Feature

Maintains the same Y-intercept ($\frac{1}{V_{max}}$) while the slope increases.

Uncompetitive Inhibition Mechanism

The inhibitor binds somewhere other than the active site and takes enzyme molecules out of action, regardless of substrate concentration.

Uncompetitive Inhibition Vmax ($V_{max}$)

Decreases.

Uncompetitive Inhibition Km ($K_m$)

Decreases.

Uncompetitive Inhibition Lineweaver-Burk Feature

The Y-intercept moves up and the plotted lines are parallel.

Mixed Inhibition Mechanism

Combines competitive- and uncompetitive-like behavior; decreases $V_{max}$ and changes $K_m$.

Mixed Inhibition Vmax ($V_{max}$)

Decreases.

Mixed Inhibition Km ($K_m$)

Changes (direction depends on relative inhibitor binding).

Noncompetitive Inhibition (SI Framework)

A special case of mixed inhibition in which $K_m$ remains unchanged while $V_{max}$ decreases.

Noncompetitive Inhibition Vmax ($V_{max}$)

Decreases.

Noncompetitive Inhibition Km ($K_m$)

Unchanged.

Noncompetitive Inhibition Lineweaver-Burk Feature

Maintains the same X-intercept ($-\frac{1}{K_m}$).

Phosphorylation

The covalent attachment of a phosphate group to a protein, recognized as the most common reversible covalent modification.

Kinase

An enzyme that adds a phosphate group to a target molecule or protein.

Phosphatase

An enzyme that removes a phosphate group from a target molecule or protein.

Phosphorylation Amino Acid Side Chains

Serine, threonine, and tyrosine (hydroxyl-containing side chains), as well as histidine.

Receptor Tyrosine Kinases

Cellular receptors that make extensive use of phosphorylation to regulate function and signaling.

Warfarin Target and Action

Inhibits VKORC1 to interfere with vitamin K recycling.

Statins Target and Action

Inhibit HMG-CoA reductase to reduce endogenous cholesterol synthesis.

NSAIDs Target

Inhibit COX (cyclooxygenase).

ACE Inhibitors Target

Inhibit ACE (angiotensin-converting enzyme).

Proton-Pump Inhibitors Mechanism

Act via suicide or irreversible inhibition of proton pumps.

Clopidogrel Mechanism

Irreversible inhibition affecting platelet activation.

Beta Blockers Mechanism

Act via competitive antagonism at beta-adrenergic receptors.

Heme Group Composition

Consists of a porphyrin ring bound to a central iron ion.

Iron Oxidation State for Oxygen Binding

$\text{Fe}^{2+}$ (ferrous state), required for normal oxygen binding.

Iron Oxidation State Incapable of Normal O2 Binding

$\text{Fe}^{3+}$ (ferric state).

Coordinate Covalent Bond

A chemical bond in which one atom donates both electrons in the shared electron pair.

Deoxy-Heme Coordinate Bonds

5 total coordinate bonds: 4 from porphyrin nitrogens and 1 from the proximal histidine.

Oxy-Heme Coordinate Bonds

6 total coordinate bonds: 4 from porphyrin nitrogens, 1 from the proximal histidine, and 1 from bound $\text{O}_2$.

Proximal Histidine Sequence Name

His 93.

Distal Histidine Sequence Name

His 64.

Proximal Histidine Helix Designation

F8.

Distal Histidine Helix Designation

E7.

Proximal Histidine Relative Location

Situated below / very close to the heme iron.

Distal Histidine Relative Location

Situated above / farther from the heme iron.

Proximal Histidine Direct Coordination to Iron

YES (directly forms a coordinate covalent bond with $\text{Fe}$).

Distal Histidine Direct Coordination to Iron

NO (does not directly coordinate-bond to $\text{Fe}$).

Proximal Histidine Primary Interaction

Forms a coordinate covalent bond directly with $\text{Fe}$.

Distal Histidine Primary Interaction

Forms a hydrogen bond with bound $\text{O}_2$.

Proximal Histidine Major Roles

Anchors heme/$\text{Fe}$ to the globin protein; transmits $\text{Fe}$ movement to induce the T to R conformational state shift.

Distal Histidine Major Roles

Stabilizes bound $\text{O}_2$; helps protect heme iron; limits problematic carbon monoxide ($\text{CO}$) binding.

Proximal Histidine Memory Cue

P = Proximal = Physically attached to $\text{Fe}$.

Distal Histidine Memory Cue

D = Distal = Doesn't directly bond $\text{Fe}$.

Iron Movement Upon O2 Binding

When $\text{O}_2$ binds, $\text{Fe}^{2+}$ moves directly into the plane of the porphyrin ring.

Hemoglobin Conformational Transition Trigger

Movement of $\text{Fe}^{2+}$ into the porphyrin plane pulls the attached proximal histidine, transmitting movement to the protein to change T state to R state.

Competitive Inhibition Anchor Rule

Competitive changes $K_m$, not $V_{max}$.

Uncompetitive Inhibition Anchor Rule

Uncompetitive = parallel lines.

Noncompetitive Inhibition Visual Clue

Same X-intercept on a Lineweaver-Burk plot.

Competitive Inhibition Visual Clue

Same Y-intercept on a Lineweaver-Burk plot.

Uncompetitive Inhibition Visual Clue

Parallel lines on a Lineweaver-Burk plot.

Mixed Inhibition Visual Clue

Both parameters ($V_{max}$ and $K_m$) are affected on a Lineweaver-Burk plot.

Porphyrin Nitrogens Role in Heme

Four nitrogens in the porphyrin ring that each form a coordinate covalent bond with the central iron atom.

Sixth Coordinate Position of Heme Iron

Occupied by oxygen ($\text{O}_2$) when oxygen is bound.

Oxygen-Binding Sequence Step 1

$\text{Fe}^{2+}$ sits in heme.

Oxygen-Binding Sequence Step 2

Proximal His93/F8 is attached below the iron.

Oxygen-Binding Sequence Step 3

$\text{O}_2$ enters from the distal side.

Oxygen-Binding Sequence Step 4

$\text{O}_2$ binds to $\text{Fe}^{2+}$.

Oxygen-Binding Sequence Step 5

Distal His64/E7 stabilizes bound $\text{O}_2$ via hydrogen bonding.

Oxygen-Binding Sequence Step 6

$\text{Fe}^{2+}$ moves into the porphyrin plane, pulling the proximal histidine along with it.

Oxygen-Binding Sequence Step 7

Globin conformation changes, driving the transition from the T state to the R state.

VKORC1

The target enzyme of Warfarin involved in vitamin K recycling.

HMG-CoA Reductase

The target enzyme of Statins involved in endogenous cholesterol synthesis.

COX (Cyclooxygenase)

The enzyme target inhibited by nonsteroidal anti-inflammatory drugs (NSAIDs).

ACE (Angiotensin-Converting Enzyme)

The target enzyme inhibited by ACE inhibitors.

Proton Pumps Target

Inhibited irreversibly/via suicide inhibition by proton-pump inhibitors.

Platelet Activation Target

Target process affected by Clopidogrel via irreversible inhibition.

Beta-Adrenergic Receptors

Receptor targets bound competitively by beta blockers.

His 93

The specific residue number for the proximal histidine.

His 64

The specific residue number for the distal histidine.