2 Enzymes Flashcards
Give the seven key features of enzymes
- Lower activation energy
- Increase rate of reaction
- Do not alter equilibrium constant
- Are not changed or consumed in the reaction (which means that they will appear in both the reactants and products)
- Are pH and temperature-sensitive, with optimal activity at specific pH ranges and temperature
- Do not affect the overall ΔG (Gibb’s free energy: quantitative measure of the favorability of a given reaction at constant temperature and pressure) of the reaction
- Are specific for a particular reaction or class of reactions
Give the six classes of enzymes
mnemonic: LIL HOT
Oxidoreductases: RedOx reactions, electron transfer, NAD+/NADP+ cofactors sometimes, oxidase, dehydrogenase, reductase
Transferases: Move functional groups, kinases
Hydrolases: Use addition of water to cleave molecules, phosphatase, peptidase, nuclease, lipases.
Lyases/synthases: Cleave or synthesize molecules without water or RedOx reactions
Isomerases: Rearrange bonds within molecules (e.g. between isomers and stereoisomers)
Ligases: catalyze addition or synthesis reactions between large similar molecules using ATP. E.g. nucleic acid synthesis and repair
Which of the following do enzymes impact?
- Overall free energy of a reaction
- Equilibrium of the reaction
- Rate of a reaction
Rate of a reaction (kinetics)
They can affect how quickly a reaction gets to equilibrium, but not effect equilibrium itself
Enzymes ensure that many important reactions can occur in a reasonable amount of time in biological systems
Enzymes do this by lowering activation energy (make it easier for the substrate to reach transition state)
What are the two theories for how enzymes and substrates interact and which is the more supported?
- Lock and key theory: No alteration of tertiary or quaternary structure necessary
Induced fit model: (better one) the substrate and enzyme undergo conformational change to fit, requiring energy (endergonic). Letting go of the substrate is easier and releases energy (exergonic) and the enzymes spontaneously returns to its native shape.
What is the difference between apoenzymes and holoenzymes?
Apoenzymes: have cofactor
Holoenzymes: cofactor not present
What are prosthetic groups in enzymes?
Cofactors coenzymes that are necessary for enzyme function
What is the difference between cofactors and coenzymes?
Cofactors: inorganic molecules or metal ions
Coenzymes: small organic groups, mostly vitamins (e.g. NAD+, FAD, and coenzyme A)
Why must b complex vitamins and ascorbic acid (vitamin C) be replenished often by diet?
They are water soluble and excreted easily
A Michaelis-Menten plot of enzyme kinetics shows that as you add substrate to a solution with enzyme the rate of the reaction will increase until it reaches Vmax, at which point the solution is at saturation and it cannot speed up any more.
Halfway to Vmax is called 1/2Vmax and the substrate concentration is called Km (the concentration of substrate which permits the enzyme to achieve half Vmax.)
What is the Michaelis-Menten equation?
E + S ⇋ ES → E + P
⇋ formation of ES forms at rate k1 or k-1.
→ The formation of E + P is unidirectional and occurs at rate kcat
Because enzyme concentration is kept constant on the mcat we can relate the velocity of the enzyme to substrate concentration using the MM equation
v = vmax[S] / km + [S]
km (michalis constant) is equal to [S] (substrate concentration) when….?
When the reaction rate is equal to half of Vmax
In this way km can be used to compare enzymes. It denotes the affinity of enzymes for their substrates
Changing concentration of enzyme or substrate never changes km
Vmax represents maximum enzyme velocity and is measured in moles of enzyme per second. Also, Vmax, can be mathematically related to Kcat, which has units of s^-1. Give the equation for this
Vmax = [E]Kcat
What is the catalytic efficiency?
The ratio of Kcat/Km
A large Kcat (high turnover) or a small Km (high substrate affinity) will result in a higher catalytic efficiency, which indicates a more efficient enzyme
What is a Lineweaver-Burk plot?
How can you find Km and Vmax with it?
A double reciprocal graph of the Michaelis-Menten equation. Yields a straight line (rather than a hyperbolic one of the Michaelis-Menten plot).
The intercept of the line with the x-axis gives the value of -1/Km.
The intercept of the line with y-axis gives the value of 1/Vmax
When is the curve on a Michaelis-Menten plot sigmoidal (rather than hyperbolic)?
When enzymes have cooperativity among substrate binding sites. Cooperative enzymes have multiple subunits (quaternary structure) and multiple active sites (hemoglobin has a sigmoidal Michaelis-Menten plot!).
Subunits and enzymes may exist in a low-affinity tense state (T) or a high-affinity relaxed state (R). Binding of the substrate encourages transition of other subunits from T to R, increasing the likelihood of substrate binding to these other subunits. Loss of substrate does the opposite
Relate cooperative enzyme binding with Hill’s coefficient
If Hill’s coefficient > 1: positively cooperative binding is occurring, such that after one ligand is bound the affinity of the enzyme for further ligand(s) increases
If HIll’s coefficient < 1, negatively cooperative binding is occurring, such that after one ligand is bound the affinity of the enzyme for further ligand(s) decreases
If Hill’s coefficient = 1, the enzyme does not exhibit cooperative binding.
