Tag: biomolecules: chemical constituents of living cells

Enzymes works precisely at certain narrowly optimum conditions, such as appropriate temperature, pH, and ion concentration. Deviation from the optimal conditions adversely affects enzyme activity.  Most enzymes have an optimal temperature, at which the rate of reaction is fastest. For human enzymes, the temperature optima are near the human body temperature (35-40$^o$C). Enzymatic reactions occur slowly or not at all at low temperatures. As the temperature increases, molecular motion increases, resulting in more molecular collisions. The rates of most enzyme-controlled reactions, therefore, increase as the temperature increases, within limits. High temperatures rapidly denature most enzymes. The molecular conformation (3-D shape) of the protein becomes altered as the hydrogen bonds responsible for its secondary, tertiary, and quaternary structures are broken. Because this inactivation is usually not reversible, activity is not regained when the enzyme is cooled.

Enzymes are biological catalyst which catalyze biological reactions. Most of the enzymes are proteins in nature. Some of the enzymes have non protein part which helps them to carry out reaction. Enzymes change the rate of chemical reaction by bringing about a decrease in free energy requirement. Thus, changing the free energy of the reaction. Because enzymes are proteins, they are highly sensitive to heat. Thus, enzymes catalyze reactions only at physiological conditions of temperature and pH.

Reactions do not undergo chemical changes automatically. They do so in transition states. Transition state has more free energy than reactants or products. Higher free energy helps in collision of transition state reactants. The enzymes binds to substrate molecules and form an unstable transient state called as transition state. The transition state rapidly undergoes degeneration into products and unchanged enzyme molecule.

When there is a drop in body temperature due to a drop in environmental temperature. The sensory receptors on the skin sends signal to the hypothalamus, which increases the body temperature by causing vasoconstriction and prevents blood from flowing closer to the skin, this also causes piloerection or goosebumps, this is to conserve energy and raise the body temperature. Once the swimmer has dried himself  the thermosensors of the skin send a feed back of temperature returning to normal to the hypothalamus and the shivering is stopped. This is called negative feedback mechanism.

• Coenzymes are small organic molecules that can be loosely or tightly bound to an enzyme. 
• Tightly bound coenzymes can be called as prosthetic groups. Coenzymes were discovered by Fritz Lipmann. 
• His work on the coenzyme was awarded Nobel Prize in Physiology or Medicine, in 1953, for his discovery of coenzyme A and its importance for intermediary metabolism. 
  Hence, option B is the answer.

Any chemical reaction includes the formation of transition state between reactant and product. The energy required for formation of the transition state is termed as activation energy. Enzymes enhance the rate of reaction by lowering down the activation energy of transition state but themselves remain unchanged. Thus, the correct answer is option B.

Hemes are the iron containing porphyrins. They are extensively found in nature. These are also present in proteins like haemoglobin and myoglobin. Cytochromes are also a haemoproteins. Cytochrome also act as an integral part of membrane protein.