KATABOLISME KARBOHIDRAT Almost all organisms, whether they reside on land or in the water, carry on cellular respiration, which is most often glucose breakdown coupled to ATP synthesis.

TUJUAN: Setelah mempelajari bab ini diharapkan mahasiswa mampu: Menjelaskan tahap-tahap reaksi yang terjadi dalam glikolisis Menjelaskan energi yang digunakan maupun yang dihasilkan pada proses glikolisis Menjelaskan tahap-tahap reaksi pada siklus asam sitrat (daur Krebs) Menjelaskan energi yang dihasilkan dalam siklus asam sitrat Menjelaskan rantai transport elektron pada respirasi/ proses fosforilasi oksidatif Menjelaskan jalur pentosa fospat Menjelaskan glukoneogenesis Menjelaskan jalur glioksilat

Glycolysis takes place in the cytoplasm of almost all cells.

KATABOLISME KH (RESPIRASI SELLULER) Terjadi pada tiga seri reaksi: 1. Glikolisis 2. Siklus asam sitrat 3. Rantai transport elektron Menghasilkan: 1. CO2 2. H2O 3. ATP Termasuk: Reaksi anaerob (tanpa O2)- menghasilkan sedikit ATP Reaksi aerob (memerlukan O2) – menghasilkan sgn besar ATP

Tahapan Oksidasi sempurna Glukosa Oksidasi glukosa dengan melepaskan H terjadi 4 tahap: Glycolysis – Pemecahan glukosa menjadi 2 molekul as. pyruvat di sitoplasma, tidak membutuhkan oksigen, menghasilkan 2 ATP Reaksi transisi- pyruvate dioksidasi menjadi 2- acetyl group yang dibawa CoA, dan CO2 dilepas; terjadi dua kali per /tiap molekul glukosa.

Glycolysis 10 tahapan reaksi Memecah gluksosa menjadi 2 asam piruvat Terjadi di sitoplasma fase/tahap respirasi selluler anaerob Menghasilkan 2 ATP Terjadi 3 aspek: fosforilasi Penguraian produksi NADH dan ATP 4-13

Langkah – Menginvestasikan Energi Pada awal glycolysis, 2 ATP digunakan untuk mengaktifkan glukosa (6C), yang akan dipecah menjadi 2 molekul berkarbon 3, yaitu PGAL. PGAL membawa fosfat dari ATP. Dimulai dari sini, masing-masing molekul berkarbon 3 (3C) mengalami seri reaksi yang sama. PGAL is glyceraldehyde-3-phosphate.

Glycolysis Tahap 1 - Fosporilasi 2 fospat ditambahkan ke glukosa memerlukan ATP Tahap 2 – penguraian 6-carbon glukosa dipecah menjadi 2 molekul berkarbon 3 yaitu: Dihidroxiaceton dan glyseraldehid 4-14

Langkah – pemanenan energi Terjadi oksidasi PGAL dgn melepaskan elektron (H) yang kemudian ditangkap oleh NAD+: 2 NAD+ + 4H → 2 NADH + 2 H+ Oksidasi PGAL dan substrat berikutnya menghasilkan 4 grup fosfat berenergi tinggi yang digunakan untuk mensintesis ATP pada fosforilasi substrat.

Glycolysis Langkah ke 3- Produksi NADH dan ATP atom hydrogen dilepas atom hidrogen berikatan dengan NAD+ untuk menghasilkan NADH NADH membawa atom H ke rantai transport elektron jika oksigen tersedia ADP difosforilasi menjadi ATP Dua molekul asam pyruvat dihasilkan 4-15

Ringkasan Glycolysis Masukan: Glukosa 2 NAD+ 2 ATP 4 ADP + 2 P Hasil: 2 pyruvat 2 NADH 2 ADP 2 ATP (net gain)

Reaksi Anaerob Jika oksigen tidak tersedia - Rantai transport elektron tidak dapat menerima NADH Asam piruvat dikonversi menjadi asam laktat glikolisis terhamabat Produksi ATP menurun 4-16

Reaksi Aerob Jika oksigen tersedia: asam piruvat digunakan untuk mengahsilkan acetyl CoA Siklus Asam sitrat mulai Rantai transpor elektron berfungsi CO2 dan H2O terbentuk 36 molekul ATP dihasilkan per mol glukosa 4-17

Di Dalam Mitokondria mitochondria adalah organel sel yang memiliki dua lapis membran, yaitu membran internal dan membran luar , terdapat ruang antara kedua membran. Cristae adalah lipatan membran dalam ke arah matrix matrix, The transition reaction and citic acid cycle occur in the matrix; the electron transport system is located in the cristae. Most of the ATP produced during cellular respiration is produced in mitochondria; therefore, mitochondria are often called the powerhouses of the cell.

Transition Reaction The transition reaction connects glycolysis to the citric acid cycle, and is thus the transition between these two pathways. Pyruvate is converted to a C2 acetyl group attached to coenzyme A (CoA), and CO2 is released. During this oxidation reaction, NAD+ is converted to NADH + H+; the transition reaction occurs twice per glucose molecule.

Citric Acid Cycle The citric acid cycle is a cyclical metabolic pathway located in the matrix of the mitochondria. At the start of the citric acid cycle, CoA carries the C2 acetyl group to join a C4 molecule, and C6 citrate results. Each acetyl group received from the transition reaction is oxidized to 2 CO2 molecules. The citric acid cycle was originally called the Krebs cycle in honor of the man who first studied it.

Siklus Asam Sitrat 3. Siklus asam sitrat – satu seri reaksi oksidasi yang melepaskan CO2 dan mengahsilkan ATP; terjadi 2 kali per glukosa. 4. Rantai Transport Elektron – Satu seri senyawa organik yang menerima dan melepas elektron yang dilepas dari glukosa dan melewatkannya dari satu karier ke karier berikutnya hingga sampai pada elektron receptor terakhir yaitu O2. Dihasilkan air; Energi dilepas untuk mensintesis 32 – 34 ATP. Jika oksigen tidak tersedia, terjadi fermentasi di sitoplasma. During fermentation, glucose is incompletely metabolized to lactate or to carbon dioxide and alcohol, depending on the organism. Fermentation results in a net gain of only two ATP per glucose molecule.

During the cycle, oxidation occurs when NAD+ accepts electrons in three sites and FAD accepts electrons once. Substrate-level phosphorylation results in a gain of one ATP per every turn of the cycle; it turns twice per glucose. During the citric acid cycle, the six carbon atoms in glucose become CO2. The transition reaction produces two CO2, and the citric acid cycle produces four CO2 per molecule of glucose. On each occasion, NAD+ accepts two electrons and one hydrogen to become NADH. FAD accepts two electrons and two hydrogen ions to become FADH2.

Citric Acid Cycle begins when acetyl CoA combines with oxaloacetic acid to produce citric acid citric acid is changed into oxaloacetic acid through a series of reactions cycle repeats as long as pyruvic acid and oxygen are available for each citric acid molecule: one ATP is produced eight hydrogen atoms are transferred to NAD+ and FAD two CO2 produced 4-18

Citric acid cycle At the first step, the cycle begins when an acetyl group carried by CoA combines with a C4 molecule to form citrate. Twice over, substrates are oxidized, NAD+ is reduced to NADH, and CO2 is released. ADP becomes ATP as a high-energy phosphate is removed from a substrate. Next, another substrate is oxidized, but this time with FAD reduced to FADH2. Between fumarate and oxaloacetate, a substrate is oxidized and NAD+ is reduced to NADH. The net result of this cycle of reactions is the oxidation of an acetyl group to two molecules of CO2, along with a transfer of electrons to NAD+ and FAD and a gain of one ATP. The citric acid cycle turns twice per glucose molecule.

SIKLUS ASAM SITRAT/ SIKLUS ASAM TRIKARBOKSILAT

Citric acid cycle inputs and outputs per glucose molecule 2 acetyl groups 6 NAD+ 2 FAD 2 ADP + 2 P Outputs: 4 CO2 6 NADH 2 FADH2 2 ATP

Next, oxygen combines with hydrogen, and water forms: Oxygen receives energy-spent electrons at the end of the electron transport system. Next, oxygen combines with hydrogen, and water forms: ½ O2 + 2 e- + 2 H+ → H2O When NADH carries electrons to the first carrier, enough energy is released by the time electrons are accepted by O2 to produce three ATP; two ATP are produced when FADH2 delivers electrons to the carriers. Once NADH has delivered electrons to the electron transport system, it is “free” to return and pick up more hydrogen atoms. In the same way, the components of ATP (ADP + P) are recycled in cells. The recycling of coenzymes and ADP increases cellular efficiency since it does away with the necessity to synthesize NAD+, FAD, and ADP anew.

Overview of the electron transport system NADH and FADH2 bring electrons to the electron transport system. As the electrons move down the system, energy is released and used to form ATP. For every pair of electrons that enters by way of NADH, three ATP result. For every pair of electrons that enters by way of FADH2, two ATP result. Oxygen, the final acceptor of the electrons, becomes a part of water.

Organization of Cristae The electron transport system is located in the cristae of the mitochondria and consists of three protein complexes and two mobile carriers. The mobile carriers transport electrons between the complexes, which also contain electron carriers. The carriers use the energy released by electrons as they move down the carriers to pump H+ from the matrix into the intermembrane space of the mitochondrion.

A very strong electrochemical gradient is established with few H+ in the matrix and many in the intermembrane space. The cristae also contain an ATP synthase complex through which hydrogen ions flow down their gradient from the intermembrane space into the matrix. The flow of three H+ through an ATP synthase complex causes a conformational change, which causes the ATP synthase to synthesize ATP from ADP + P.

Mitochondria produce ATP by chemiosmosis, so called because ATP production is tied to an electrochemical gradient, namely an H+ gradient. Once formed, ATP molecules are transported out of the mitochondrial matrix.

Organization of cristae The electron transport system is located in the cristae. As electrons (e-) move from one complex to the other, hydrogen ions (H+) are pumped from the matrix into the intermembrane space. As hydrogen ions flow down their concentration gradient from the intermembrane space into the matrix, ATP is synthesized by an ATP synthase. ATP leaves the matrix by way of a channel protein.

Electron Transport Chain NADH and FADH2 carry electrons to the ETC ETC series of electron carriers located in cristae of mitochondria energy from electrons transferred to ATP synthase ATP synthase catalyzes the phosphorylation of ADP to ATP water is formed 4-19

Energy Yield from Glucose Metabolism Per glucose molecule, there is a net gain of two ATP from glycolysis, which occurs in the cytoplasm by substrate-level phosphorylation. The citric acid cycle, occurring in the matrix of mitochondria, adds two more ATP, also by substrate-level phosphorylation.

Most ATP is produced by the electron transport system and chemiosmosis. Per glucose molecule, ten NADH and two FADH2 take electrons to the electron transport system; three ATP are formed per NADH and two ATP per FADH2. Electrons carried by NADH produced during glycolysis are shuttled to the electron transport chain by an organic molecule.

Accounting of energy yield per glucose molecule breakdown Substrate-level phosphorylation during glycolysis and the citric acid cycle accounts for four ATP. Oxidative phosphorylation accounts for 32 or 34 ATP, and the grand total of ATP is therefore 36 or 38 ATP. Cells differ as to the delivery of the electrons from NADH generated outside the mitochondria. If they are delivered by a shuttle mechanism to the start of the electron transport system, 6 ATP result; otherwise, 4 ATP result.

Summary of Cellular Respiration 4-20

Rantai Transpor Elektron/ Fosforilasi oksidatif