B.1 Metabolism Requires Enzymes

-  Cell metabolism depends on the efficient functioning of a multitude of enzymes.

• -  Each of these proteins has a characteristic 3-D configuration (shape, or conformation).

• -  Their tertiary or quaternary structure results from the unique sequence of amino acids and allows them to participate in the metabolic activities by reacting with other molecules known as substrates.

• -  The ability of any enzyme to function depends on the precision by which it combines with its substrate to form a temporary structure called an enzyme substrate complex (E-S complex)

• -  Maltose, for example, is the substrate for the enzyme maltase. The molecular shapes for these two molecules allow them to bind together.

• -  When this association occurs, maltose breaks apart into its two component glucose molecules. The exact same pattern exists for sucrase (enzyme) when it reacts with sucrose (substrate) to produce glucose and fructose.

• -  Note that enzymes are generally named after their substrates and their names normally end with “ase”.

o Enzymes are large molecules held in their unique configurations by hydrogen ionic

and covalent bonds.
o Within their structures, they have regions with certain chemical natures that result

from particular sequences of amino acids.
o For example, several amino acids with acidic R-groups may lie close together

creating an acidic area.
o Attributes like this mean that the exact conformation of the molecular complexes is

somewhat at the mercy of the chemistry of their environments.
o In cells, this environment is the cytoplasm.Varying conditions including the

presence of chemical substances can (slightly) alter the shapes of enzymes and

perhaps even some of the larger substrate molecules.
o These changes may affect the chances that E-S.complexes will form efficiently,

negatively influencing the potential for reactions.

• -  Another feature of many enzymatic reactions is their reversibility. Using the example of the

  maltose/maltase reaction, hydrolysis results in the cleaving of the disaccharide maltose (substrate) into two glucose molecules (products). This is an example of catabolism.

• -  The opposite reaction, condensation , occurs when two glucose molecules bond together to form maltose.

• -  This reaction is anabolic (anabolism).

• -  Maltase not only catalyzes the hydrolysis of the disaccharide, it also catalyzes the reverse

  reaction between glucose molecules to form maltose.

• -  Metabolism is the combination of enzymatically controlled catabolic and

  anabolic reactions.

- Cells store energy as bond energy between the terminal phosphates of ATP. The forming and breaking of these bonds is the role of the enzyme AtPase. Metabolism requires accessing this energy.

o To start each metabolic reaction requires an input of energy called activation energy.

o Enzymes effectively lower this energy requirement allowing the reactions to proceed more easily and more quickly.

o At the molecular level, an input of energy is required to form the E-S complex. o The reaction that follows involves a smooth transition in energy from that of the

substrate molecules to the product molecules.
o If the overall reaction requires a net input of energy, it is termed

endothermic.
o If it releases energy, it is exothermic ,and the released energy is

available to reform the P~P bonds of ATP or is given off as heat.
o In either case, when the reactions are enzyme-controlled, the energy changes occur

in such small amounts that other cell functions are not affected.
o This is extremely significant for organisms. With the use of enzymes, the millions of reactions required for life processes take place without large energy requirements or releases.

• Enzymatically controlled reactions contribute to an organism’s overall well-being. Homeostasis, the maintenance of a relatively constant set of internal conditions, allows these reactions to continue in a life-supporting manner.

• -  Food molecules, once ingested by an organism, are digested by enzymes into monomers, which are then distributed to individual cells.

• Enzymatic reactions convert these smaller molecules into other substances or assemble them into the specific polymers they require.

• -  The environmental conditions in which these metabolic reactions take place determine their efficiency and rate, or if they will take place at all. Without this dynamic and successful enzyme function, an organism would not survive.