Outline:
Control of Metabolic Reactions
Energy for Metabolic Reactions
Nucleic Acids and Protein Synthesis
Metabolic reactions are simply all the chemical reactions that take place in cells of the body. Proteins called enzymes control these metabolic reactions. There are two broad categories of metabolic reactions; they are anabolic reactions and catabolic reactions.
Anabolism refers to all anabolic reactions that take place in body cells. Anabolic reactions are those that build larger molecules. You can also think of them as synthesis reactions. In general, anabolic reactions require energy.
One very important type of anabolic reaction is called dehydration synthesis. This reaction is used by cells for the synthesis of carbohydrates, proteins, and triglycerides. In this type of synthesis reaction water is also formed. The following are examples of dehydration synthesis reactions:
monosaccharide + monosaccharide + monosaccharide --> carbohydrate + water
amino acid + amino acid + amino acid + amino acid --> protein + water
3 fatty acids + glycerol --> triglyceride + water
Catabolism refers to all catabolic reactions that take place in body cells. Catabolic reactions are reactions that break down large molecules into small molecules. You can also think of them as decomposition reactions. In general, catabolic reactions release energy.
One very important type of catabolic reaction is called hydrolysis. This reaction is used by cells to break down carbohydrates, proteins, and triglycerides. In this reaction, water is a reactant. The following are examples of hydrolysis reactions:
carbohydrate + water --> monosaccharide + monosaccharide + monosaccharide
protein + water --> amino acid + amino acid + amino acid + amino acid
triglyceride + water --> 3 fatty acids + glycerol
Metabolic reactions are controlled by enzymes. Enzymes are proteins that act as catalysts. Catalysts speed up chemical reactions.
In a reaction involving an enzyme the following rules are true:
- The enzyme is not altered in the reaction.
- The enzyme is substrate specific (it only recognizes and changes one type of molecule). An enzyme recognizes its substrate because the two fit together like two puzzle pieces.
- The substrate is changed in the reaction
An enzyme controlled reaction can be summarized as follows:
substrate + enzyme --> enzyme-substrate complex --> product (altered substrate) + enzyme
In an enzyme controlled reaction, the enzyme temporarily binds to the substrate. The substrate then changes into the new product and the enzyme is released. The enzyme can now change another substrate molecule.
The following is an example of an enzyme controlled reaction:
hydrogen peroxide + catalase --> water + oxygen + catalase
What reactant was changed in this reaction? Hydrogen peroxide. Therefore, hydrogen peroxide is the substrate. Notice that catalase was not changed. It is the enzyme. A very common ending for enzymes is (-ase). What was hydrogen peroxde changed to? Water and oxygen. Catalase will only change hydrogen peroxide. Remember enzymes will recognize and change only one type of substrate.
Here is another example of an enzyme controlled reaction:
sucrose + sucrase --> glucose + fructose + sucrase
Sucrose is the substrate. Sucrase is the enzyme (notice the -ase ending). Sucrose was changed into glucose and fructose. The enzyme sucrase will only change sucrose. Remember enzymes are substrate specific.
Click here to see a figure showing how enzymes work
Since enzymes are mostly protein, they are susceptible to being destroyed or denatured by heat, radiation, acids, bases, and various other chemicals.
Energy is the ability to do work and cells need energy to drive chemical reactions. Cells primarily use chemical energy called ATP.
Click here for the structure of ATP
Cells can generate chemical energy through a process called the oxidation of glucose. The oxidation of glucose occurs in two steps: 1) glycolysis and 2) aerobic respiration.
In glycolysis, glucose is split into two molecules of pyruvic acid. This process does not require oxygen and is therefore termed an anaerobic process. It occurs in the cytoplasm of cells and generates a little ATP (chemical energy). Now the pyruvic acid molecules enter a process known as aerobic respiration.
In aerobic respiration, each pyruvic acid molecule is converted into a molecule called Acetyl coenzyme A. Acetyl coenzyme A then enters a series of chemical reactions known as the Citric Acid Cycle. In this cycle, carbon dioxide and water are generated. A lot of ATP is also generated! This cycle requires oxygen and is therefore called an aerobic process. It takes place within mitochondria.
As a glucose molecule is oxidized, about 38 molecules of ATP are generated. ATP stands for adenosine triphospate. It is a molecule that contains three phosphate groups in a chain. The bonds between the phosphate groups contain energy. When these phosphate bonds are broken energy is released and can be used by a cell for various processes. When a phosphate bond is broken and ATP loses one phosphate group, it is now called ADP or adenosine diphospate as it now only has two phosphate groups. An ADP molecule will later gain a phosphate group to become ATP again.
Click here to see the regeneration of ATP
A series of chemical reactions that are controlled by enzymes is called a metabolic pathway. The Citric Acid Cycle is an example of a metabolic pathway. More metabolic pathways are describe below.
Carbohydrates are a good source of energy for cells. When a person eats carbohydrates, the digestive system breaks down the carbohydrate molecules into monosaccharides. Glucose is the most common monosaccharide. When glucose is absorbed into the bloodstream, it is delivered to cells throughout the body. The cells take in the glucose and use it to make energy. Glucose goes through glycolysis to make pyruvic acid. Pyruvic acid is converted to acetyl coenzyme A and this molecule enters the citric acid cycle to generate a lot of ATP.
Lipids or fats are also good sources of energy for cells. When a person eats fat, some of the fat is broken down into triglycerides (glycerol + three fatty acids) by the digestive system. The triglycerides are then transported to cells and the cells use the fatty acid chains of the triglycerides to generate energy. A fatty acid chain can be converted into acetyl coenzyme A by a process known as beta oxidation. The resulting acetyl coenzyme A molecule can then enter the citric acid cycle to generate lots of ATP.
When a person eats protein, the digestive system breaks the proteins down into amino acids. The amino acids are then transported to cells through the bloodstream. The cells can then use the amino acids to rebuild proteins. The cell can also use the amino acids to generate energy. When a cell wants to use an amino acid for energy, the amino acid can be converted to acetyl coenzyme A through a process called deamination. The resulting acetyl coenzyme A molecule then enters the citric acid cycle to generate energy.
Click here to see a summary of these metabolic pathways
Remember that each chemical reaction in a metabolic pathway is controlled by an enzyme. If an enzyme is not available for one of the chemical reactions in the pathway, the metabolic pathway will stop. Thus metabolic pathways are regulated by the amount of enzymes present to drive the chemical reactions.
Genes are defined as segments of DNA that code for one protein. Genes are passed to you from your parents.
DNA is a molecule that contains two chains of nucleotides. These two chains of nucleotides are held together with hydrogen bonds formed between the nitrogen bases of the nucleotides. The nitrogen bases bond very specifically to each other. Thymine (T) will only bind with adenine (A) and cytosine (C) will only bind with guanine (G). Therefore, if one chain of nucleotides in DNA has the nitrogen base sequence ATGC, the sequence of nitrogen bases on the opposite chain of nucleotides will be TACG. The nitrogen bases are always complementary to each other. DNA is called a double helix because it has two (double) chains of nucleotides and the chains are coiled (helical).
Click here to review the structure of a nucleotide and to see a DNA molecule
Click here to see the structure of the DNA molecule
Click here to see the genetic code
RNA is another major type of nucleic acid. Its nucleotides always contain the sugar ribose and on of the following nitrogen bases - adenine, uracil, guanine, or cytosine. RNA is generally found outside the nucleus of a cell and carries out instructions from DNA in protein synthesis. RNA is a molecule with a single strand of nucleotides
Click here to see the structure of an RNA molecule
DNA contains the genes of the body. A gene is defined as a segment of DNA that has the instructions for the making of one polypeptide chain of a protein. The base sequence of a gene is the instructions for the making of a polypeptide. In this section, we will see how the base sequence of a gene is used to make a polypeptide.
DNA is divided up into genes and the genes are divided up into triplets. A triplet is a sequence of three bases. For example, if a gene contains the following base sequence - AAATTTGGGCCCATC, it has how many triplets? The answer is 5. AAA is the first triplet, TTT is the second and so forth.
When a gene is activated, it is used to make a different nucleotide strand called mRNA.
There are three types of RNA that are involved in protein synthesis. One type is called mRNA (messenger RNA). mRNA is made from genes. Another type is called tRNA (transfer RNA). tRNA can be thought of as a molecule shaped like a t. One end of the molecule carries an amino acid and the other end carries a sequence of three bases called an anticodon. rRNA is a type of RNA used to make ribosomes.
Transcription takes place in the nucleus of the cell. It is the making of mRNA from a gene (DNA). Transcription is regulated by an enzyme called RNA polymerase. The mRNA that is made corresponds to the base sequence of DNA. For example, if the DNA sequence of a gene is TTTCAC, the corresponding mRNA base sequence would be AAAGUG. T (of DNA) corresponds with A (of mRNA), C (of DNA) corresponds with G (of mRNA), and in this case A (of DNA) corresponds with U (of mRNA). U stands for the base uracil. Molecules of RNA do not contain the base thymine, so A will always code for U if a RNA molecule is being made. Recall that DNA is divided up into triples. mRNA is divided up into codons. A codon is a sequence of three bases on mRNA. So, the corresponding codon to the triplet AGC would be UCG. Once the mRNA molecule is made, it must be edited. Some of the sequences of bases cannot be used in the process of translation. Therefore, the sequences of bases are removed by enzymes.
In translation, the sequence of bases of mRNA is translated into a sequence of amino acids. This process takes place on a ribosome in the cytoplasm of a cell. In translation, the mRNA molecule associates with the ribosome and is translated one codon at a time. The molecule that translates a codon is tRNA. tRNA carries an amino acid and a sequence of three bases called an anti-codon. The codon is translated by the tRNA molecule with its corresponding anti-codon. For example, if a codon on the mRNA molecule has the sequence UAG, the anti-codon that corresponds with this is AUC. A tRNA molecule that carries the anti-codon AUC will carry a specific amino acid to the ribosome. That amino acid will bond with the neighboring amino acids and a polypeptide chain is formed.
The amino acid in the chain can be predicted by using the genetic code (Click here to see the genetic code). This code give you the CODONS and the amino acids they are translated into.
Consider the following example.
If the gene sequence is TATGGCTAA, what would the corresponding mRNA sequence be? The answer is AUACCGAUU. How many codons are in the mRNA? The answer is 3. What amino acid does the first codon translate into? Using the genetic code look up the codon AUA. It translates into the amino acid called isoleucine. What is the anti-codon that corresponds with this codon? The answer is UAU. What would the second amino acid be in the polypeptide chain? You would determine that the second codon in mRNA is CCG. This codon translates into the amino acid proline. What would the third amino acid be in the polypeptide chain? The answer is isoleucine.
Once a polypeptide chain is made, it floats off into the cytoplasm to become functional by taking on its secondary, tertiary, or possible quaternary structure.
Click here to see transcription and translation
Click here for summary of events in protein synthesis
Click here for a figure of protein synthesis
DNA replication occurs in the nucleus during interphase. It ensures that all daughter cells have identical copies of DNA. During this process, the DNA molecule uncoils and each DNA nucleotide strand is used as a template for the making of a new DNA nucleotide strand. The new nucleotide strand is complementary to the old nucleotide strand. For example, if the old DNA nucleotide strand has a base sequence of adenine (A), thymine (T), cytosine(C), and guanine (G), the new DNA nucleotide strand will have the base sequence of thymine, adenine, guanine and cytosine. This is so because adenine is complementary to thymine and cytosine is complementary to guanine. If a DNA nucleotide strand has the base of TTAGGCC, what would be the base sequence of its complementary strand? The answer is AATCCGG.
Click here to see DNA replication