Cellular energetics and the role of enzymes as biological catalysts

===Lecture Details===
Janet Macaulay; Week 2 MED1011; Biochemistry
===Lecture Content===
Potential energy is that of state or position, can be stored in chemical bonds; kinetic energy is energy of movement. Potential energy can be converted to kinetic energy, which does 'work'. Energy cannot be created or destroyed; when it is converted from one form to another the conversion is not 100% efficient and therefore some becomes unable to do work. Muscles convert chemical to mechanical energy but a bit less than 20% actually does work with the rest lost as heat.
Living things obey the laws of thermodynamics. Total energy = useable energy + unusable energy
: H = G + TS G = H - TS
H = total energy (enthalpy); G = free energy (useable energy which cells require for all chemical reactions; S = entropy (unusable energy); T = absolute temperature
Entropy can be a measure of the disorder of a system (energy not able to be used). Changes in free energy can be measured by
: deltaG = deltaH - T deltaS
In a chemical reaction, delta H is the total amount of energy added or released from the system. If a chemical reaction increases entropy its products are more disordered or random than its reactants; eg protein to amino acids has a positive deltaS.
: deltaG reaction = G products - G reactants
Spontaneous, exergonic reactions release free energy and have a negative G overall (may need activation energy added). Non-spontaneous reactions require free energy and have a positive G, and proceed only if free energy is provided.
ATP serves as an energy currency in cells, hydrolysis of which releases a large amount of free energy. The tightly packed four negative charges makes it very unstable and hence predisposes to a large energy release when hydrolysed. Initial reaction is catabolism of glucose, and releases a large amount of energy when hydrolysed. This energy can be used to phosphorylate many different molecules. ATP contains phosphate, adenine, nitrogen and ribose. The ATP cycle (from ATP to ADP and vice versa) couples endergonic and exergonic reactions. Respiration results in reformation of ATP from ADP and inorganic phosphate with an input of chemical energy.
The speed of a chemical reaction is independent of delta G but is determined by the size of the activation energy barrier. Catalysts decrease this barrier. Enzymes are biological catalysts highly specific for their substrated. Catalysis takes place at an active site defined by its tertiary structure determining the specificity of an enzyme, where substrates bind. On binding to the substrate the enzymes change shape. Enzymes such as rubisco in plants can catalyse more than one reaction, though this is unusual.
Concentration of enzymes affects speed of reaction. Increases occur with higher concentration of enzyme up to a maximum rate where all enzyme molecules are occupied with substrate. Both pH and temperature affect enzymatic acitvity (pepsin needs low pH, alcohol dehydrogenase has neutral pH). There is an optimal pH and temperature for all reactions where conversion is quickest. Some enzymes require cofactors for catalysis (often allosteric regulators), cofactors are not permenently bound but enter the reaction as a cosubstrate, as they are changed by the reaction and released from the enzyme. Cofactors can control the rate of reaction by activating different amounts of enzyme.
Prosthetic groups are permanently bound to the enzyme.
Enzyme activity is subject to regulation (usually allosteric), with some compounds binding irreversibly reducing their catalytic activity. Others act reversibly, inhibiting enzyme action temporarily. Inhibition can be competitive (where receptor sites are bound) or non-competitive (substrate binds to enzyme, receptor site is modified). Non-competitive inhibtion is an example of allosteric regulation.
===Readings===
====Medical Sciences; 26-30====
====Life; Chapter 8====
====Metabolic challenges program====