英国留学

Macromolecules
Amino Acids
An amino acid is a molecule containing both amine and carboxyl functional groups. They are the building blocks of proteins. They have the following general structure.

……

The carboxyl group (-COOH) and the amino group (-NH2) can be ionized and unionized. Hence the amino acid can adopt a Non-Ionized, Ionized from and a Zwitterion form (Zwitter coming from the German word for "hybrid".)

……

Phi and Psi angles
There are 20 different amino acids which make up proteins. Their side chains can be separated into 3 groups:
1. Non-Polar (E.g Glycine, Alanine etc.)
2. Polar. (E.g Serine, Cysteine)
3. Charged. (E.g Aspartic Acid, Lysine)
These side chains have different reactivities and disulphide bonds between Cystine amino acids provide common bonds in proteins. Amino acids also joint together by Peptide bonds.
There are two degrees of freedom around each residue of the peptide chain. These are termed Phi (φ) and Psi (Ψ). Phi is the angle about NH to Cα. Psi is the angle at Cα to C=O. Free rotation can occur at 180° in these bonds.

Enzymes
Introduction
An enzyme is a biomolecule that increase the rate of a chemical reaction. Most enzymes are proteins. But they can also be made of RNA (Ribozymes) or be RNA-Protein complexes (Ribosome). They often catalyze reactions with cofactors and coenzymes. Enzymes work by reducing the activation energy barrier, without affecting the overally change in free energy.

……

Active site
The 3D structure of the protein is critical for its function. Catalysis occurs in a specific site of the enzyme, called the active site. The side chains of the amino acids in the proteins associate with this active site by making and breaking bonds. Active site binding involves many weak, non-covalent interactions (Electrostatics, Hydrogen bonding, Van der waals forces and hydrophobic interactions). These side chains are called Catalytic groups.

Classification of enzymes

……

Cofactors
A cofactor is a non-protein chemical compound that is bound (either tightly or loosely) to a protein and is required for the protein's biological activity. Cofactors are also often further classified depending on how tightly they bind to the protein, with loosely-bound cofactors termed coenzymes(E.g. NAD, NADP+) and tightly-bound cofactors termed prosthetic groups (E.g. FAD, Metal ions).

Catalysts
The four criteria for a catalyst are:
1. Bold text Catalysts increase he rate of reaction.
2. Catalysts are not consumed by the reaction.
3. A small quantity of catalyst should be able to affect the rate of reaction for a large amount of reactant.
4. Catalysts do not chard the equilibrium constant for reactions.
Examples of catalysts include FeCl3(An inorganic catalyst) which can increase the conversion of Hydrogen Peroxide to water and oxygen by about a 1000x and Catalase (A biological catalyst) which increase the reaction rate by about a 1,000,000x.

Factors affecting enzymic activity
pH
pH affects the degree to which the substrate is ionized. This alters the binding of the enzyme to the substrate. Extreme pHs can denature enzymes.
Temperature
Most enzymic driven reactions increase in rate, when temperature is increased. A temperature of about 40° usually denatures enzymes.
Measuring enzymic activity
Enzymic activity can be measured in a unit called a Katal (Kat). 1 Kat is equal to the activity which transforms 1 mole of substrate per second under standard conditions.

Enzyme inhibitors
Enzyme inhibitors can be divided into four groups.

• Irreversible inhibitors - Form of break covalent bonds with or in the enzyme. May be substrate analogues. Effect cannot be reversed by increasing substrate concentration.
• Reversible inhibitors - Interact with the enzyme via non-covalent interactions. Effect can be reversed by increasing substrate concentration.
• Competitive inhibitors - Inhibitor binds only to enzyme, not enzyme-substrate complex.
• Noncompetitive inhibitor - Inhibitor binds either to enzyme and/or to Enzyme-Substrate complex.

Examples of enzyme inhibition in drugs
Penicillin - Inhibits enzymes responsible for the construction of the bacterial cell wall.
Sulfa drugs (E.g. Sulfanilamide) - Inhibits the first enzyme in the metabolic pathway that bacteria use to make folic acid (Vitamin B9.)

（全文请下载）