ENZYMES

Enzymes Enzymes are protein catalysts A catalyst is a substance that speeds up a chemical reaction without being used up in the process Rates of chemical reactions could be increased with the addition of heat, however heat may denature proteins A catalyst can speed up the rate of chemical reactions at moderate temperature decreasing risk of denaturation

Enzymes The substrate is the reactant that an enzyme acts on when it catalyzes a chemical reaction Enzymes are very specific for the substrate to which they bind Name of enzymes ally end in –ase (ex. lactase, polymerase, maltase, etc.)

How Do Enzymes Work? In an enzyme-catalyzed reaction, the substrate binds to a very small portion of the enzyme The location where the substrate binds is called the active site The substrate must fit into a pocket or groove of the 3D structure of the protein Once the substrate binds to the enzyme, the structure is called the enzyme/substrate complex

Induced-Fit Model As the substrate enters the active site, its functional groups come close to the functional groups of a number of amino acids in the enzyme This causes the enzyme to change shape slightly to accommodate the substrate This slight change in shape causes the substrate to become “stressed” (distorting a particular bond) and, therefore, lowers the activation energy needed to break the bonds

When the bonds are broken the conformation of the substrate changes and the enzyme loses its affinity for the products releasing them back into the environment The active site now becomes available for another substrate to attach The recycling of enzyme molecules causes cells to catalyze many reactions with relatively small numbers of enzymes

Co-factors & Co-enzymes Some enzymes require additional assistance to function These co-factors and co-enzymes are not proteins Co-factors: Inorganic metal ions such as Zn2+ and Mn2+ They help draw out electrons from the substrate where bonds need to be broken

Co-enzymes Organic Shuttle molecules from one enzyme to another Examples: NAD+ and FAD+ These usually get reduced to harvest energy Act as electron carriers

Membrane Structure & Function The “Fluid Mosaic Model”

Eukaryotic Cell Every eukaryotic cell has three main parts: Plasma (cell) membrane: separates inside of cell from external environment Nucleus: organelle that contains the cell’s DNA and is surrounded by a double membrane Cytosol: gel-like fluid from the nucleus to the plasma membrane

Plasma membrane Separates the protoplasm of a cell from the non-living environment Living structure that regulates what enters and leaves the cell Cells must always be in contact with their environment in order to survive (to obtain nutrients and to excrete waste)

Fluid Mosaic Model Scientists have inferred that the cell membrane contains a mosaic of different components scattered throughout it (much like raisins in a slice of bread) Proteins and lipids molecules can drift sideways in the bilayer, supporting the idea that the bilayer has a fluid consistency  fluid- mosaic membrane model

Most of the surface area of the cell membrane is made of phospholipids, but accounts for only 42% of the weight of the membrane. The phosphoslipid is an amphipathic molecule – phosphate heads face the outside and inside, and fatty acid tails are in the middle. The phosphate heads are hydrophilic (water loving) which are soluble in water The fatty acid tails are hydrophobic (water hating) which are not soluble in water

Pg. 83 Fig 3a

Hydrophilic Hydrophobic

Phospholipid Bilayer Since there are two sets of phosphate heads and fatty acid tails (lipids) the membrane is called the phospholipid bilayer The membrane is selectively permeable Fat soluble substances and some other small, uncharged molecules can pass through Cholesterol is a large molecule, and helps to stabilize the membrane. At high temperatures: it helps maintain rigidity At low temperatures: keeps the membrane fluid, flexible and functional – preventing cell death from a frozen membrane

assymmetrical Pg. 82 Fig 2

Protein Molecules Embedded within the lipid bilayer (peripheral or integral) and carry out a number of functions: Attachment & Recognition Most carry a special carbohydrate molecule and are known as glycoproteins: provide the cell with a unique identity Basis for the function of white blood cells and for organ transplant rejection Some attach to cytoskeleton filaments to help stabilize Enzymatic activity Some membranes proteins are enzymes eg. those involved in respiration and photosynthesis

Protein Molecules Triggering signals Transport Some act as receptor sites for hormones which allow cells to communicate with one another Triggers cascade of events within the cell eg. insulin  regulates blood sugar levels eg. serotonin  insufficiency may result in depression Transport Gatekeepers: opening and closing paths (shape-shifting) through which materials might move eg. sodium-potassium pump  nerve impulses Channels: allow hydrophilic molecules and certain ions to move in/out eg. Cl- channel (malfunctioned in CF patients)