Intro

Question 1

Enzymes and Equilibrium

Answer 1

• Reusable biological catalyst that speed up reactions
• Forward and reverse reactions create kinetic equilibrium with equal generation of substrate/product
• Enzymes and metabolites often only find each other by random events in cells or fluids
• Cell compartments help to concentrate them and accelerate the reactions

Question 2

Metabolic Pathways

Answer 2

• Linear
• Branches
• Cyclical

Question 3

Atomic/molecular rearrangement Reactions

Answer 3

• Changing a functional group for another (not replacing it)
• Changing the position of atoms within a molecule

Question 4

Substitution Reactions

Answer 4

• Replacement of a functional group with another

Question 5

Redox Reactions

Answer 5

• Dual oxidation and reduction reactions that usually involve co-enzymes.

Question 6

Cleavage Reactions

Answer 6

• Hydrolysis and splitting of molecules into 2

Question 7

Condensation Reactions

Answer 7

• 2 molecules joined with the loss of water

Question 8

Addition Reactions

Answer 8

• Two molecules are joined together but water is NOT eliminated, or addition across a double bond

Question 9

Transfer Reactions

Answer 9

• Transfer of a functional group
• Swapping of functional groups between substrates

Question 10

Enzyme Kinetics

Answer 10

Zone 1 = 1st order kinetics with regard to substrate
Zone 2 = Transitional zone
Zone 3 = Increasing substrate concentration has no effect

Question 11

Kinetic Parameters

Answer 11

• Vmax: Maximal rate at substrate saturation
• Km: Substrate concentration for half Vmax
• Kcat: Vmax/Enzyme concentration
• IU: amount of enzyme which converts 1 micromole of S to P per min
• Kat (SI Unit): amount of enzyme which converts 1mole of S to P per min
• Specific activity: enzyme activity per mg of protein (measures purity)

Question 12

Tissue Specificity

Answer 12

Km kinetic variation allows tissue specificity:

Glucokinase and hexokinase both catalyse the same reaction, converting glucose to glucose-6-phosphate (G6P)
- Hexokinase has a low Km (high affinity) and readily converts glucose to G6P
- Glucokinase has a high Km (low affinity) slowing the conversion, so liver can store glycogen

Question 13

Metabolic Control

Answer 13

• All these reactions need to be under control in the cell to maintain a steady-state
• Metabolic control of reactions occurs at three levels: substrate (substrate and coenzyme availability), enzyme (inhibitors, allosteric and covalent modification) and genetic

Question 14

Substrate Availability

Answer 14

• More substrate = faster rate of reaction
• Compartmentalisation using organelles affects availability of substrates, allows competing reactions to be carried out in the cell
• If two enzymes at a metabolic branch point are competing for the same substrate, the enzyme with the lower Km (high affinity) will preferentially bind to the substrate

Question 15

Coenzyme Availability

Answer 15

• Total cellular concentrations of co-enzymes/factors remains constant but their form (and ratio) differs, e.g.: ATP vs ADP and NAD+ vs NADH
  Example: pyruvate can be metabolised in several ways
• A key branch point exists with a choice between glycolysis (pyruvate dehydrogenase) and gluconeogenesis (pyruvate carboxylase)
• This branch choice is influenced by NAD+/NADH ratios

Question 16

Reversible Inhibitors

Answer 16

• They maintain the biochemical steady-state in the cell, by reducing the Km/Vmax of enzymes
• These are mostly concerned with metabolic control (i.e. negative feedback)
• Do not change the structure of the enzyme and bind to the active or regulatory domains
• They can be competitive, non-competitive, uncompetitive or mixed inhibitors

Question 17

Irreversible Inhibitors

Answer 17

• These tend to permanently destroy the enzyme’s catalytic activity
• Bind at the active site using a covalent bond and denature the enzyme permanently
• They include poisons and pharmaceutical drugs

Question 18

Competitive Inhibition

Answer 18

• The substrate and inhibitorcompetefor access to the enzyme’s active site
• Overcome by sufficiently high concentrations of substrate (Vmaxremains constant) (Km increased)

Question 19

Uncompetitive Inhibition

Answer 19

• Inhibitor binds only to the substrate-enzyme complex
• causesVmaxto decrease andKmto decrease

Question 20

Non-competitive Inhibition

Answer 20

• binding of the inhibitor to the enzyme reduces itsactivitybut does not affect the binding of substrate
• extent of inhibition depends only on the concentration of the inhibitor
• Vmax will decrease due to the inability for the reaction to proceed as efficiently
• Km will remain the same as the actual binding of the substrate, by definition, will still function properly.

Question 21

Mixed Inhibition

Answer 21

• Inhibitor can bind to the enzyme at the same time as the enzyme’s substrate
• binding of the inhibitor affects the binding of the substrate, vice versa
• can be reduced, but not overcome by increasing concentrations of substrate

Question 22

Allosteric Regulation

Answer 22

• Regulator molecules bind (reversibly) to regulatory sites on an enzyme and influence the binding of substrates to the active site
• Common on multimeric proteins

Question 23

Reversible Covalent Control

Answer 23

• chemically modifies the enzyme to increase/decrease activity
  Example: Phosphorylation kinases and phosphatases

Question 24

Isoenzymes + Isoforms

Answer 24

Isoenzymes - come from different genes, can catalyse same reaction
Isoforms - come from a single gene but result from PTMs, splice variants, etc

Question 25

Enzyme Cascade

Answer 25

• The first enzyme actually acts on another (enzyme) as its substrate, this newly activated enzyme may then activate another, and so on

Question 25

Substrate Cycling

Answer 25

• Often an anabolic state in the cell will prevent activity of catabolic enzymes
• Enzymes at important branch points can be inhibited by many different substrates allowing multiple feedback loops

Question 26

Gene Level Control

Answer 26

• Concentration of an enzyme in a cell is controlled by genetic expression and can be altered depending on the availability of substrate.