Zellbiologie

Free energy value for products is lower than value for reactants --> reaction is thermodynamically favourable but needs a push!

ΔG 0 = the amount of energy consumed or
liberated in a reaction. A reaction is thermo-
dynamically favourable or spontaneous if ΔG 0 is
negative

Transition state
• old bonds break and new ones form
• substance is neither substrate nor product
• unstable short lived species – equal probability of going forward or backward

substrates bind by weak non covalent interactions (hydrogen bonding, hydrophobic or ionic interactions)

where reaction takes place

The Lock and Key Model (Emil Fisher Model)

The Induced Fit Model (Hand in Glove Analogy, Daniel Koshland model)

Lock-Key Model: Substrate fits into a perfectly shaped space in the enzyme, there is a high similarity between the shape of the enzyme and the substrate and it is highly stereospecific --> implies a preformed, rigid and inflexible site

Induced Fit Model: The substrate fits into a general shape In the enzyme, causing the enzyme to change shape. There is a close but not perfect fit of E and S but change in protein configuration (proteins = flexible) leads to a nearly perfect fit.

K m = Michaelis constant
K m is substrate concentration at
which the enzyme operates at one
half of its max. velocity

If K_m is low (small number) -> substrate held tightly
--> high affinity
reaches V max at a lower [S]; small means less than 10 ^-3 M

If K_m is high (high number) -> substrate held weakly
--> low affinity
reaches V max at a higher [S]; high means 10 ^-1 bis 10 ^-3 M

y=m*x+b

double- reciprocal plot

• Km increases
• Vmax stays the same
• effects of competition can be overcome by increasing [S]
• inhibitor and substrate effectively compete for the active site
• a sufficiently high [S] will displace the inhibitor from the active site!

• inhibitor binds to a site other than the active site
• inhibitor can bind either before or after the substrate

• Km stays the same binding of S has no effect on inhibition; inhibitor can bind to ES complex
• V max decreases 1/V max increases; activity decreases
• effects of competition cannot be overcome by increasing [S]

• inhibitor binds to a site other than the active site but only when substrate is bound

• Km decreases
• V max decreases 1/V max increases; overall rate decreases
• effects of competition cannot be overcome by increasing [S]

Protein Phosphorylation: Phosphroylation can result either in an increase or decrease of protein
activity. The effects depends on the phosphorylation site and the structure of the protein.

• are amphiphilic
• are arranged as bilayers
• are mobile (--> fluidity)
• are asymmetrically arranged

• is tightly regulated
• is dependent of the temperature
• is dependent of the fatty acid composition
• is modulated by cholesterol

Cholestrol
- decrease in the mobility of the first
few CH2 groups -> lipid membrane less
deformable
- chol. enhances the permeability
barrier properties of bilayer and
thereby increases the stability of
membranes

• must accomodate in the hydrophobic environment
• have transmembrane domains (TMD)
• TMDs are rich in hydrophobic amino acids -> interactions with lipids
• membrane spanning regions contain mostly a– helices (less frequent: b-sheets/b barrels)
• cannot be released without destruction of the membrane

Lipid anchor covalent linked to AA side chains
Lipid anchor:

1. amid or ester bond to fatty acid acyl group from myristate or palmitate
2. covalently linked to an isoprenoid chain (through S of cysteins)
3. linked to a glycosylphosphatidylinositol (GPI) molecule

• - Simple Diffusion: Along a concentration gradient --> only apolar molecules
• - Facilitated diffusion: Along a concentration gradient --> passive or carrier mediated transport
• - Primary Active: Against electrochemical gradient --> active, mediated by e.g. ABC tresproters or Na + /K + -ATPase
• - Secondary Active: Against electrochemical gradient, ions move along concentration gradient
• - Ion Channel: Along electrochemical gradient --> ligand or ion gated (channel proteins: consist of multiple α-helical transmembrane segments or β barrels, are voltage or ligand gated and have a high substrate specificity)
• - Ion Transport via Ionophores: Along electrochemical gradient