Protein Flashcards
Elemental composition of proteins
Carbon, Hydrogen, Oxygen, Nitrogen, some sulphur
Protein structure generally
Amino group, variable group, carboxyl group
20 different amino acids, vary by R group
Protein behaviour in water
Amino acids dissolve in water, carboxyl dissociates freeing a hydrogen atom, -ve charged
Amino acid acquires H+
Amino acid has +ve and -ve charge, zwitterion
Dipeptide formation
Condensation reaction form dipeptide and water
Primary structure
Sequence of amino acids in polypeptide chain
Created by condensation reactions between many amino acids
Due to variation in chemical makeup of R group, interactions between amino acids in chain
Secondary structure
Amino acid chain folds into alpha helix or beta pleated sheets
Due to hydrogen bonds between amino (+ve) and carboxyl group (-ve)
Tertiary structure
Secondary structure folds even more
Unique 3D structure formed
Held together by disulphides bonds, ionic bonds, hydrogen bonds
Not all proteins fold into tertiary shape
Strength of disulphide bonds
Fairly strong
Not easily broken
Strength of ionic bonds
Weaker than disulphide bonds and easily broken by pH changes
Strength of hydrogen bonds
Weakest bond
But numerous
Quaternary structure
Large proteins formed from no of polypeptide chains
Can include prosthetic groups
Types of proteins
Fibrous
Globular
Fibrous protein properties
Structural Large insoluble, strong, flexible Form long parallel chains like cellulose 2ndary level of folding Often have repeating amino acid sequences Do not denature as easily as globular
Examples of fibrous proteins
Collagen- in tendons, cartilage
Keratin- in nails, skin, horns, claws
Elastin- ligaments
Actin, Myosin- muscle fibres
Globular proteins properties
Tertiary structure, resembles globule, compact shape
Often involved in controlling, cellular metabolism
Specific shape
Water soluble, colloidal solutions
Altered easily, not stable, denature more easily
Examples of fibrous proteins
Enzymes
Carriers and receptors in membranes
Antibodies
Haemoglobin
Chemical reactions
When reactants collide, enter transition state
Molecule become strained, molecules activated
In transition state, more chance of strained bonds breaking, new bonds forming
Under normal conditions, very few have enough KE to enter transition state
Catalysts, activation energy
Catalyst provides lower energy pathway
Reacts more rapidly, bind to catalyst, speed up biological reactions
Structure of enzymes
Globular
Specific 3D shape, result of amino acid sequence, R groups
Small region involved in catalysing reactions, active site
Active site made up of small no of amino acids
No change to nature of products, energy change during reaction, catalyst
Lock and key theory
Specific shaped substrate fits in specific complementary
Enzyme substrate complex formed
Products formed, no change to enzyme shape
Advantages and disadvantages of the lock and key theory
Explains enzyme specificity
Assumes enzyme is rigid
Not supported by observation that molecules bind to allosteric site that can alter active site shape
Induced fit model
As substrate approaches enzyme, shape changes
Reaction proceeds as enzyme, substrate binds
Products released, enzyme returns to original shape
Causes conformational change in enzyme
Advantages of the induced fit theory
Explains how other molecules affect enzyme activity
Explains how activation energy is lowered
Effect of temperature on enzyme activity at optimum
High temperature, high KE of molecules
Greater no of successful collisions, producing enzyme substrate complexes
Greater rate of reaction
Effect of temperature beyond optimum
Increased temperature cause atoms within molecule to vibrate more energetically
Increased vibrations strain bonds, bonds break, active site changes shape
Effect of pH on enzyme activity
Change in pH alters charges on amino acids make up active site, enzyme substrate complex cannot be formed
Changes tertiary structure
Denatures enzyme
Effect of substrate concentration on enzyme activity
Activity of enzyme depends on no of substrate molecules/sec, bind to form enzyme substrate complexes
More substrate molecules=more successful collisions, more enzyme substrate formed, more product
Coenzymes
Small organic non protein molecules, carry chemical groups between enzymes
Consumed in reactions in which they are substrates
Regenerated, con maintained in the cell
Prosthetic group
Special subset of coenzymes, non protein parts of conjugated proteins
Involved in active site of enzymes
Control of metabolic pathways
Series of reactions, each step catalysed by enzymes
Highly structured
Non competitive
Level of any chemical maintained at constant level with end product inhibition
Enzyme inhibitors
Directly, indirectly interferes with AS function
Reversible, irreversible
Competitive, non competitive
Competitive
Complementary to AS shape
Bind to AS, prevent substrate binding
Temporary, quickly released
Effect of competitive inhibitors on rates of reaction
Increase substrate concentration, more substrate binds to AS in preference to inhibitor
Non competitive inhibitors
Attach to allosteric site
Produce conformational change in enzyme molecule including AS
Substrate will not fit completely
Effect of non competitive inhibitors on rate of reaction
Increasing substrate no will not affect rate of reaction
